taxonomic problems in the genus oliva · 2018-02-07 · phuket marine biological center special...
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Phuket Marine Biological Center Sp ecial Publication 18(2): 263-284. (1998 ) 263
TAXONOMIC PROBLEMS IN THE GENUS OLIVA
Bernard TurschLaboratoire de Bio -Ecologie, Faculte des Sciences, Unioersite Libre de Bruxelles,
50 av. RD. Roosevelt, 1170-Brussels, Belgium.
ABSTRACTThe limitations and the implications of the objective morphospecies approach are brieflyreviewed. Attention is called to frequent unwarranted interpretations of data. The morphological continuum method is advocated for taxonomic decisions on allopatric samples.Solutions are presented for some problems particular to the genus Oliva , such as the choiceof characters and the necessary reduction of the scale of sympatry.
THE OLIVA PROBLEMThe common and widespread gastropod genus Oliva is quite easy to recognise, becauseit is homogeneous. Yet, we know nearly nothing about the natural history of Oliva andover 95 % ofthe literature deals exclusivelywith nomenclature.This is probably becausethe formal naming of species is the branchof science that requires the least science.Theproblem with Oliva is continual shuttlingofthe status of names between those of spe cies, subspecies, forma and synonym.
Within the genus, relatively few speciesare easy to recognise. Many more 'species'are not easy at all and pose frustrating problems, familiar to tropical malacologists (seeAbbott 1991; Tursch 1992). The classicalcomplaint is : "My specimen stands right inthe middle between two species". The frequency of identification problems certainlymeans that many (most?) Oliva species havenot been properly delimited.
I will here confine myself to this problem of species differentation. This is not onlyan academic exercise: in the tropics, Olivaoften constitute a major element of soft substratum communities (see for instance Kohn1997). The questions of phylogeny, identification or nomenclature will make sense onlyafter the species problem has first beensolved. Difficulties in species delimitationalso exist in other groups of molluscs, sosome of my problems might also be yours. Iwill attempt to show that many taxonomicobstacles can be surmounted by using a logi-
cal, objective approach.Most Oliva species are notoriously vari
able in some characters, such as size andcolour patterns. The combination ofthe twoingredients, homogeneity of the genus andvariability ofthe species, is a sure recipe forproducing taxonomic chaos. This has indeedhappened: hundreds of nominal taxa arenow considered to be species, subspecies,forms or synonyms, apparently at the whimof the authors. Of course many of thesenames are genuine synonyms. In addition,misidentifications are very common. Interestingly, for some species there are very fewor no problems. So, from the start, it wouldseem that one is dealing with a few 'goodspecies' and a large number of 'bad species'.
There is no hope of cleaning up this taxonomic mess unless we understand whatwent wrong in the first place. One maincause of problems is certainly that the definition of simple, reliable taxonomic characters is very difficult because the shells ofmany Oliva species are not separated byeasy 'yes or no' characters. The shells aredevoid of any sculpture and mainly differby rather subtle variations of shape. Thesedifferences are hard to describe in words andat first sight may even appear to be gradual.
VARIABILITY OF OLIVA ANDVARIABILITY OF TAXONOMISTS
The blame for the difficulties in Oliva taxonomy have invariably been laid upon the
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'extreme variability' of the shells (which isa biological problem). But could it be thatpart of the problem is due to the variabilityof the taxonomists?
Just for fun , this hypothesis was te stedat a meeting, many years ago, by asking 29experienced shell collectors to say how manyspecies there were in the same sample of 12Oli va shells. The fact that the panel consisted of hobbyists matters little, as mostOliva taxa have been described by non-professionals . The distribution of answers wasvery irregular, with a large standard deviation. The correct answer (which I learnedmyself many years later ) is different fromthe mean, the median and the mode (illustr ating, by the way, that taxonomic mattersshould not be settled by a majority vote ).Thecoefficient of variation was over 50 %, waymore than what could be measured on theactual specimens. So, in this particular case,malacologists were more variable than theOliva .
This variability reflects methodologicalproblem s and there are many. The first iscommon to all of malacology: many of thenames are due to the descriptive excess ofsome authors. It is true that taxonomists aresupposed to describe and classify variation,but they are certainly not supposed anymoreto name all variants in terms of arbitraryspecies and subspecies. This attitude is, ofcourse, re lated to an antiquated typo logicalconcept of species. This concept unfortunately still thrives today, albeit often masquerading under the vocab ulary of evolutionary biology.
The second methodological problem is theuse of a vague vocabulary to describe veryvague taxonomic 'character s'. Here is, forinstance, the description of an Oliva species:"Shell variable in shape, us ually fusiform,with rounded sides and low spire; colorranges from white, yellow, orange, cream orwhite, to brown and black, overlaid withdark brown or black zig-zags, triangles,blotches, and/or dots; aperture dark brown;columella white with dist inct , although
poorly-developed plications." The text (author deliberately not cited) calls for severalremarks:
* The description hardly contains anyone character that is well -defined and useful. All Oliva are "usually fusiform" and have"rounded sides" to some extent. The colourpattern is so general that it could also describe my cat. The only clear indications "aperture dark brown; columella white" are simply misleading: there are many exceptions.
* This text dates from the mid-eightiesbut could also be 150 years old. It gives onethe feeling that nothing has happened in biology dur ing the last two centuries.
* This is not the language of the scientist, but more that of the art criti c. Thi s canonly lead to a 'semi-aesthetical' classification. I have nothing against aesthetics (wellon the contrary) but such a classification isnot refutable as scientific statements shouldbe.
So, in practice, it is im possible to recognise most Oli va species from their wordeddescription without the help of an illustration or of ty pe material.
The third methodological problem concerns the traditional choice of characters.The paramount characters used in descriptions of Oliva species are: absolute size , theshape of the body whorl, the relative heightof the spire, the presence of a callus on thespire, the number and the aspect of columellar plica tions and the colour pattern of thebody whorl. The choice of these 'classicalcharacters' is most unfortunate: it so happens that they are precisely the most variable she ll attributes in the genus Oliva.
* Size is highly variable. Small and largethick-lipped specimens can be found withinthe same population (see Fig . 1). Mean sizeof popu lations is generally not centred onlarge specimens (see Tursch et al. 1995) andthere is no way of determining from the shellif a specimen is 'adult' . For writers on Oliva ,this word is often used with a rather vaguemeaning.
* The shape of body whorl and the rela-
Phuket Marine Biological Center Special Publication 18(2): 263-284. (1998 ) 265
Figure 1. Variability in size (sse text p. 2). Example: two thick-lipped specimens of O. miniacea(Roding, 1798) from Kwajalein Atoll, Marshall Is. Scale bar: 10 mm.Figure 2. Variability in relative spire height and shape of body whorl (see text this page). Example:three thick-lipped specimens of 0. flammulata Lamarck, 1811 from Luanda, Angola.
tive height of spire are very variable in manyspecies (see Fig. 2). The relative height ofthe spire is possibly the most variable measurement in Oliva (see also Figs. 4, 5, 6).
* The presence of a callus on the spire isnot always a reliable character. In some species, one can find all intergrades betweenfully callused and completely open spires,sometimes even in the same population (seeFig. 3).
* Columellar plications can vary verymuch in importance, even amongst speci-
Figure 3. Variability in spire callosity (see textthis page). Example: two thick-lipped specimensofo. irisans Lamarck, 1811 from the same population, Pulau Hantu, Singapore.
mens of similar size within the same population (see Fig. 4). For some species they dohave a rather characteristic aspect, but inmost cases, the plications are difficult tocount because they get progressively smallerand indistinct. In general, small specimenshave sharper, more distinct columellarplaits.
* Colour and patterns on the body whorlmay in most species be extremely variable(see Fig. 5). This aspect will be discussedmore in detail later.
Except for the size, these features havealways been reported in subjective terms. Sothe variability of these 'classical characters'is further amplified, because their description is often as variable as the character itself. For example, for a long time there wasonly one known specimen of Oliva esiodinaDuclos. The spire of this specimen was described by three authors, each in a very different way (see Tursch 1992).
METHODOLOGYThe species conceptIfmuch of the 'Oliva problem' derives frominadequate methodology (or no methodologyat all) , the first task is to define an operational system for detecting the limits of species (identification, a different question, can
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Figure 4.Variability in columellar plications (seetext p. 3). Example: tw o thick-lipped specimensof O. miniaceaflammeacolorPetuch and Sargent,1986 from the same lot , 20 fms off Madras, India.
only come later).The more complex the problem, the more we have to be strict about principles. This is certainly the case for the genus Oliva.
At this stage, two points are already clear.First, most species have been very poorlydefined and, in any case, the species conceptof many early authors was very differentfrom ours. So we'd better start from scratch,without trusting the Wisdom of the Eldersand admitting any species a priori. Second,the best way to prevent Oliva taxonomy frombeing largely a matter of personal opinionis certainly to use a quantitative approach.
At the species level, a logical classification is a two-step procedure. The first step
consists in grouping similar animals intophena (sets of animals that resemble oneanother). The objective existence of thesephena is only a working hypothesis at thestart. The validity of this hypothesis mustthen be tested by checking if these phenacan or can not be objectively separated fromothers. Phena that cannot be separatedmust, of course, be combined into largerunits.The second step (ranking) consi sts indetermining the taxonomic rank (species,subspecies, form ) of the groups that havebeen separated (or united) in the first step.Only then does naming make any sense.
Before delimiting species, let us first .agree on what we shall consider a species.For practical reasons we are using the morphospecies. This word has different meanings to different authors. It is used here inthe meaning "objective morphospecies"(which can be demonstrated), in oppositionto what we might call the "psychospecies"(which is an act of faith). The "objectivemorphospecies" is not a new concept of thespecies. It is only an indirect approach to thebiological species (which is not operationalin practice: has any species of marine mollusc ever been defined by direct proof of reproductive barriers?).
The morphospecies approach is basedupon the demonstration of gaps in the distribution of phenetic characters. One simply determines the ranges of variability oftwo or several phena; if the ranges do not
Figure 5. Colour patterns. Some species are extremely variable (see text p. 3). Example: six thicklipped specimens of 0. bulbosa (Roding, 1798) from the same lot, Tulear, Madagascar. Scale bar: 10mm.
Phuket Marine Biologi cal Center Sp ecial Publication 18(2): 263-284. (1998) 267
Figure 6. Colour patterns. Some populations are very homogeneous (see text p. 3). Example: three thick-lipped specimens of0. picta Reeve, 1850 from the same lot, 35 m, Hansa Bay, PapuaNew Guinea. This is a case of cryptic colouration: specimensmatch the colour of the sediment (dark sandy mud with smalldebris). Scale bar: 10 mm.
overlap, the phena are separated by gaps.The existence of such morphological gapscan demonstrate the absence of hybrids, butonly under certain conditions (see Tursch1997 ). It will be seen later that these conditions are not always met. The morphospeciesapproach has its limitations. It cannot detect true sibling species (these can be evidenced mainly by molecular or behaviouralstudies) and is thus necessarily minimalistic.
So, instead of lamenting about the variability of Oliva , let us now take full advantage of it and use it as an efficient tool. Variability is indeed the worst enemy of thetypologist but the best friend of the evolutionist.
Detection ofgap sBefore discussing the biological meaning ofgaps, one should first make sure that thesegaps reflect reality. Evidencing gaps in met-
Figure 7. Colour patterns: convergence by crypsis (see text p. 11). Example:O. caerulea (Roding, 1798) (left) and O. concinna Marrat, 1870 (righ t) .Populations living on white coral sand with debris, Hansa Bay, Papua NewGuinea. Scale bars: 10 mm.
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Figure 8. Colour patterns: convergence by crypsis (see text p. 11). Example: O.elegans Lamarck, 1811 (left) and 0. reticulata (Roding, 1798) (right) . Populationsliving on dirty coral sand and rubble, Mandi, Hansa Bay, Papua New Guinea.Scale bars: 10 mm .
ric characters looks very sim ple but raises alot of problems, both in theory and in practice.
One has to keep in mind that the size ofthe gaps does not necessarily have to belarge. In the genus Oliva , most of the observed interspecific gaps are smaller ormuch smaller than the intraspecific variability ranges. This is what gives the impression that some species form a continuum. Italso entails the necessity of very accurateobservations.
Only full separations do constitute gap s.A difference in mean values, even when statistically significant, is no evidence for theabsence of hybrids (it can, at best, mean thathybrids are rare).
Only features (or combination offeatures)that yield separations do constitute taxonomic characters.The others, no matter howinteresting, are just features.Note that a feature that is not a character at the specieslevel might be an excellent character at thegenus level.
By the way, it is often believed that themost interesting taxonomic characters arethose separating all the objects being studied . Such an analysis actually has the sameinform ation content as one that separatesnothing. The highest information content isfound when half the objects are separated
AJlFigure 9. Analysis on one variable (one dimension: frequencies hi stogram). See text this page.
and the other half are not. This does makesense because classifications rest upon bothseparations and combinations.
In biometric studies, each specimen isrepresented by one point in a space havingas many axes as there are characters to beconsidered (t h e attribute hyperspace ).Phena are thus re presented by clusters(clouds') of points. The difficulty in detecting gaps between the phena varies with thenumber of dimensions of the attribute space(this is the number of variables that we wantto observe at the same time). One shouldnotice that groups separated on one, two,three or x dimensions are by that very factalso separated in the attribute hyper space.
Separations on one variable (histogramsof frequencies, see Fig. 9) are very familiarand pose no particular problem. Because thegenus Oliva is very homogeneous, many species are packed into a small range of totalvariation. So it is fully expected that good
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Figure 10.Analysis on two variables x and y (twodimensions: scatter diagram ) can separate entities that are not separable by x or y taken separately. See text this page.
unidimensional separations will be observedonly in lucky cases.
Separation on two variables (scat ter diagrams) are also very familiar. One will notice that the combination of two variables xandy can separate entities that are not separable by x or y taken separately (see Fig. 10).In the same way, the combination of threevariables x, y and z can separate entities thatare not separated by combinations of the
variables two by two (see Fig. 11).Should we then extrapolate these obser
vations and simultaneously use as many dimensions as possible? Use the whole attribute hyperspace? There are several reasons for moderating our taxonomic spatialambitions.
Taxonomists are essentially visual animals and, by increasing the number of dimensions, we are going to meet severe problems of representation. There is no problemwith one or two dimensions (we are used tothese).With just three dimensions, it alreadytakes some fiddling to orient the image inorder to show a convincing separation (as inFig. 11). With more than three axes, we cannot see anything at all :we are in hyperspace.Of course, we can use a whole arsenal ofmathematical tricks such as factor analysis,which reduces the number of axes by making linear combinations of variables. But theresulting image can be severely distortedand much information can be lost. The problem is specially serious if we include variables that are not proven taxonomic characters: they can easily generate enough 'random noise' to obscure real separations. This
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Figure 11. Analysis on three variables x, y and z (three dimensions diagram) can separate enti tiesthat are not separated by combinations of the variables two by two. See text this page.
z
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does not mean that we should not use factoranalysis. On the contrary, it is most useful,mainly for detecting correlations betweenvariables and thus for eliminating taxonomically useless features.
In the attribute hyperspace, one can gethorribly lost. In addition to problems of representation, there is a problem with the density of points. Fifty shells in a two-dimensional representation look like a nice littlecrowd and make us feel quite confident inour analysis . The same fifty specimens in a10-dimensional space can become so scattered as to make interpretations qu ite unsafe. To get the same density of points (andthe same feeling of confidence) we had before , we would have to measure millions ofspecimens. In my opinion, bidimensionalscatter diagrams are the best practical compromise. These graphs are very heuristic,clearly show the 'special covariance sets' inwhich the taxonomist is interested (Gould1984) and large quantities of scatter diagrams, us ing different combinations ofvariables, can be very rapidly computer-produced.
The examples of Figs. 9, 10 and 11 aretoo simple and hide another, very seriousproblem. In practice, we are not dealing withnice surfaces or volumes but with scatteredexperimental points.We draw these reassuring envelopes only after we have decidedwhich points go together and which do not.This is the important problem of clustering.There are many different mathematical clustering algorithms (see Sneath & SokaI1973),mainly aiming at obtaining 'taxonomic distances' (a notion not used here). These different algorithms often give different solutions in complex cases. So, what does simplecommon sense say?
Nobody will hesitate much in admittingthe existence of a gap in the scatter diagramof Fig. 12A. Let us now bring the two groupscloser to each other, as in Fig. 12B. The separation is not so reliable anymore. It couldwell be meaningful, but there is a chancethat any additional specimen could appear
right in the middle of the 'gap'. Our confidence in the reality ofthe gap would greatlyincrease if it persists when the number ofspecimens is augmented.At the species level,our confidence would also increase if thesamples contained specimens from manydifferent localities: more of the variabilitywould then be included in the analysis.
Let us then bring the groups even closer,as in Fig. 12C (it has earlier been arguedthat gaps can be very small).The two groupsare still perfectly separ ated, as statisticalte sts would show. We could even fit a verygood discriminating function somewherealong the dotted line. Such separations, frequently presented as 'proofs' , are really mostunconvincing, for two reasons. First, the uncertainty about the position of additionalpoints is even greater than in the previouscase. Second, the very same distribution ofpoints could also be perfectly separated inanother way, as in Fig. 12D (with equallyconvincing statistical backing and the blessing offactorial discriminant analysis).
There is one easy way out of the 'verysmall gap problem'. In Fig. 12E, the different symbols (full and open circles) indicatethat we have additional information, this isany kind offeature that does not automatically derive from the values of x or y, Forinstance, ifx andy are shell measurements,let us say that the full and open circles represent different colours of eggs (it could bedifferent localities, etc .) Now, the separationpresented in Fig. 12E becomes highly credible because it is supported by facts thatwere not even used in the separation procedure. If one makes a statistical test, the colour of eggs is highly correlated to variablex. This is not a mathematical, but a biological correlation: exactly the kind of information the taxonomist is looking for.
No sacred book says that a discriminating function should be linear. If it is not, asin the case of Fig. 12F, the use of additionalinformation is, here again, the only practical way out.
Evidencing the absence of gaps, ie show-
Ph uket Marine Biological Center Special Publication 18(2): 263-284. (1998) 271
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Figure 12. Problem s of clustering. See text on opposite page.
272 Tropical Marine Mollusc Programme (TMMP)
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CHOICE OF PHENETICCHARACTERS
Figure 13. The problem of overlap. See text thispage.
shell characters will be considered here,mainly because many recent taxa are knownfrom their shell alone. Shell characters oftaxonomic value in the Olividae are of twokinds: colour patterns and shell morphometry.
Colour patternsThese have been the source of so many errors that they deserve special consideration.There are two main problems with colourcharacters : complexity and variability.
Complexity: The patterns of many Olivashells are often very intricate, defying accurate verbal description or even simple coding . Furthermore, all Oliva specimens being different, what should be described is notthe pattern but the 'style' (the program thatleads to the pattern).This is not an easy taskbecause different sets ofparameters can leadto nearly identical patterns. The nature ofwhat we see is also complex : it is the additive effect of different patterns occurring indifferent crystal layers. The final aspect depends on the intensity of colours, the thickness and the transparency of the layers.
Variability: The range of variation is often very great, but always within limits.Even the most variable species of Oliva hasa large, but not unlimited repertoire of colour patterns. Any pattern of the repertoirecan be expressed or not (for reasons generally unknown to us ). So colours and patternsoften appear to be 'optional' features. Thereare some regularities in the variability ofcolours and colour patterns. Colours andpatterns are most probably not even perceived by the Oliva themselves. But theycould be important in the relation betweenOliva and their predators.
All intermediate situations are met between two extreme "colour strategies":crypsis and polymorphism. In many species,all the members of a population are quitehomogeneous in aspect (see Fig. 6). The colours of the shell (and of the soft parts) thenoften match the colour of the sediment (seeVan Osselaer et al. 1993). Most biologists
Overlap OKy
CharactersThe phenetic characters we need for themorphospecies approach could be found either in anatomy or in shell characters. Only
ing an overlap in the distribution of characters, is as important as evidencing presenceof gaps. Demonstrating overlaps is mucheasier than demonstrating separations. Theoverlap illustrated in Fig. 13 is quite convincing, although resting upon a very smallsample. This overlap will indeed persist, nomatter how many additional specimens areincluded in the analysis (only full separations constitute gaps, as previously shown).
The extrapolation of separations observed on limited samples always includesa risk oferror, so the delimitation ofmorphospecies is, to some degree, probabilistic. Butwe certainly can increase our chances of being right. In short:
* Separations with small gaps requirelarge samples.
* Separations with very small gaps require 'additional information'.
* Overlaps can always be trusted, evenbetween small samples.
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then speak of cryptic coloration, supposedto protect from predators. Note that, in suchcases, the general colour describes thesubstrate more than the animal. It followsthat when two different species 'imitat e' thesame substrate, they also necessarily 'imitate' each other (see Figs. 7 and 8). So theynot only fool predators, but they also fool taxonomists into believing they are the samespecies. Note that, in such cases, the ventral faces ofthe shells (normally not seen bypredators) do often differ substantially.
In many other species, every specimen ofa population is different in colour (see Fig.5). Many biologists then speak of polymorphic coloration, supposed to confuse the'search image' of predators (the scepticalreader might notice that we always have agood story ready, whatever the situation). Inthis case again, the Oliva do not only foolpredators; they also fool taxonomists into believing that they deal with different species.The same confusion might occur in the caseof the 'crypt ic species' that can live in verydifferent sediments, each time adopting adifferent aspect.
So, in many cases, colours and generalcolor patterns are unreliable specific characters because they are too sensitive to environmental influences.The nature oftheseinteractions is still far from being clear. Olivaoliva (L., 1758 ) living on blade sand beachesare very largely blackish. We have shownexperimentally (Van Osselaer, Bouillon &Tursch et al. 1993) that black 0. oliva do notchoose between two different sediments;they will go indiscriminately into black orwhite sand. Many Oliva specimens in collections have abruptly switched from onepattern to another. Such changes have beenexperimentally induced (Tursch et al. 1995).When black O. oliva taken from black sandwere kept on white sand, nearly all sharplyswitched from black to white color (one unsolved question amongst many: how do thecolour-blind Oliva 'know' the colour of thesubstratum?).
It is tempting to speculate that patterns
are biologically more important than colours .The in situ colour of an Oliva living below15 m depth is certainly not what we see inour drawers (the reds and yellows are gone ).And, in general, most of the many potentialOliva predators have no colour vision butmany can distinguish patterns. Patternsmay also have adaptive values: strong contrasting colour zones (disruptive patterns)can visually break up the shell outline; others (aposematic patterns) can be warningsignals protecting toxic animals. One shouldalso keep in mind that some patterns canbe cryptic when seen at a distance but conspicuous at close range.
I do not downplay the taxonomic importance of colour patterns. They often constitute our first, immediate clue to identification. But identification only makes sensewhen we already have a good classification.In any case, optional and variable characters such as colour patterns would lead to aclassification based upon polythetic taxa. Insuch taxa, organisms that have the greatestnumber of shared characters are placed together, but no single character is either necessary or sufficient for group membership(see Sneath & Sokal 1973). Such a classification might well be sensible but would befundamentally different in nature from allwe are used to (it would for instance invalidate clustering by synapomorphy, see Panchen 1992 ). In contrast, biometry leads tothe familiar monothetic taxa in which thepossession of a unique set of features is bothsufficient and necessary for membership.
Shell measurementsWe are thus reduced to search for shell characters in morphometry. It has already beensaid that Oliva shells are devoid of meristiccharacters (these are discrete variables thatone can count, such as a number of spines,for instance). So we are restricted to metriccharacters (these are continuous variablesthat one can measure, for instance the diameter of the shell).
Many shell measurements specially
274 Tropical Marine Mollusc Programme (TMMPJ
..---D-'
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Teleoconch,apical view
Teleoconch, latera l view
MPRO.----.
Protoconch,apical view
Protoconch,lateral view
Figure 14. Sketch of some shell measurements. See text this page.
suited for Oliva studies have been definedin this laboratory (Tur sch & Germain 1985 ,1986, 1987;Tursch & Van Osselaer 1987;VanOsselaer & Tursch 1988, 1993; Tursch &Bouillon 1993). Some are shown in Fig . 14.This figure is only a rough sketch: the measurements are accurately defined and manyrequire precise orientation of the shell. Allhave been tested for reproducibility, precision and taxonomic potential. These featurescan also be measured on fossil material.
In theory there is an infinity of possiblemeasurements, but there are practicallimitations. The problem is not with the datathemselves, as modern microcomputers canrapidly handle very large databases. Theobstacle is in measurements: if it takes halfa day to measure one specimen then the lifeexpectancy of taxonomists might constitutea real limitation.
Measuring shells and handling morpho-
metric calculations takes much time andeffort and is not adapted to field work. Oursecret hope was that, once species have beenproperly defined on morphometric criteria,we would find some 'easy character' to tellthem apart at first glance.
The use ofshell measurementsShell measurements fall into two differentcategories: intensive variables (that do notdepend on size) and extensive variables (thatare depend ant on size) .
Protoconch measurements (obtainedfrom camera lucida drawings made undermagnification) are always considered to beindependent ofthe size of the teleoconch. Butis this really true? The fact that the protoconch does not grow further after its completion does not ensure that it's characteristics are not correlated with shell size . Onecould, for instance, imagine that larvae with
Phuket Marine Biological Center Special Pu blication 18(2): 263-284. (1998) 275
NW
4.2
O. mantichora
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O. amethystina
O. parkinsonia
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Figure 16.After objective delimitation of the species by biometry, one can generally find clues forquick identification (see text this page). Example: Oliva am ethystina (Roding, 1798) (a) is re adily separated from the closely related O. mantichora Duclos, 1835 (b) by the colour pattern inthe suprafasciolar zone.
Fig. 15.An example of separation by direct use ofintensive variables (see text this pa ge). Oli vaamethy st ina (Reding, 1798), O. mantichoraDuclos, 1835 , O. bu elowi Sowerby, 1889, O.parkinsoni Prior, 1975. Samples (each n>15) selected over whole distribution ranges. Scatter diagram of pr otoconch measurements NW vs. LPRO(see Fig. 6). Minimum convex polygon s.
a larger protoconch would turn into individuals with a larger adult shell. Massive experimental evidence established that this is notthe case in Oliva. Protoconch measurementsare intensive (size independent) variables.They are also independant of sex . So, we canuse protoconch measurements directly assuch.They do have a great power of discrimination, as shown in the example of Fig. 15.
This example gives a good opportunityto illustrate that highly accurate microsco pemeasurements are not always required inOliva identification . Oliva amethystina(Roding, 1798) and O. mantichora Du clos,1835 that are well separated in the graph ofFig. 15 (and in many other graphs) were forcenturies confused under the name O. annulata Gmelin, a nomen dubium (Tursch et al.1986). Their shape and colour patterns areindeed quite similar and (of course) veryvariable. Now that their separate specific
0.45 0.55 0.65 status has bee n established, the two speciescan be dist inguished at first glance by theircolour pattern in the suprafasciolar region(see Fig. 16). O. am ethystina has squarish,diffuse purplish blots of variable intensity(generally in small number and sometimesabsent); O. mantichora always has thick,sharply defined, curved lines (dark purpleto brown, generally numerous). Note that weare not using general colour pattern but adetail of ornamentation. It concerns a verysmall portion ofthe shell, located on the ventral side (not normally exposed) and seemsunlikely to have any adaptive function.
The protoconch of Oliva shells is oftenbroken and sometimes difficult to observe(for instance if it is hidden und er a callus).So we also nee d to use teleoconch measurements. In contrast to protoconch measurements, all linear measurements of the te leoconch do obviously vary with the size of theshel l. They are extensive variables . Massiveexperience has shown that if one uses thesemeasurements as such in scatt er diagramsor in factor analysis, one will mainly separate big shells from small shells .And we already know that the shell size of Oliva isextremely variable (and lea ds to many problems).
There is a nice way to dodge this problem of size . If in stead of using raw linear
276 Tropical Marine Mollusc Programme (TMMP)
measurements one use ratios ofthese measurements, one now has descriptors of shape,in theory more independent of size . Theseratios not only make more taxonomicalsense; they also have a much greater discriminating power. This might appear to bea rather strong statement, so it requires ademonstration. Let us take the example ofthe linear teleoconch measurements H, Dand R (data in Tab. 1), taken on two groupsof shells (Group 1 and Group 2, from different localities).The classical approach wouldconsist in examining the bivariate diagrams(here: regression lines) of the pairs of measurements Hand D (Fig. 17A), Hand R (Fig.17B) and D and R (Fig . 17C). From thesegraphs, it is far from evident that there isany difference between Groups 1 and 2. Incontrast, their separation is obvious in thescatter diagram of Fig. 17D which uses thevery same measurements, expressed as theratios HIR and HID (shape factors).
INTERPRETATION OF GAPSIt is one thing to observe gaps (and we havenow rather sensitive tools to do this) but itis another to interpret these gaps (or theirabsence) in biological terms.
Biological interpretation of 'no gap 'As long as we are unable to give any objective evidence for the existence of a gap separating two samples then, in all circumstances, these samples must be consideredto belong to the same morphospecies. Species are not a product of our minds, they areobjective, natural entities. We are not supposed to invent a natural order; we are supposed to demonstrate its existence. Weshould therefore admit the existence ofseparate species only when there is no way ofavoiding it .
Let us however keep in mind that taxonomic decisions are always temporary because there is an infinity of possible characters and all possible characters have notbeen tested. New facts may (and will ) bringchanges in our interpretations. Like all other
Table 1. The use of shape factors (see text this
page). Numerical data for example of Fig. 9.
H D R HID HIRGroup 110.00 6.00 5.50 1.67 1.8250.00 29.80 26.00 1.68 1.9235.00 21.00 18.20 1.67 1.9222.00 13.00 12.50 1.69 1.7612.00 7.00 6.50 1.71 1.8531.00 19.00 17.30 1.63 1.79
Group 28.00 5.30 4.00 1.51 2.00
60.00 40.00 29.00 1.50 2.0740.00 26.00 19.50 1.54 2.0537.00 24.00 18.30 1.54 2.0215.00 10.00 7.40 1.50 2.03
sciences, taxonomy is dynamic and mustconstantly adapt to new evidence. So, inshort: "no proof of gap, no morphospecies".This logical restriction results, of course, ina sharp diminution in the number of admitted species. Of course, if some specimensseem 'different ' or 'interesting' enough wecan give them any name below the specieslevel, just to keep in mind that further studymight be necessary.
Anybody who applies the method advocated here will be called a 'lurnper' (oftenwith some commiseration by collectors who'know' their shells). Please note that 'splitters' and 'lumpers ' can exist only when taxonomic decisions are a matter of personalopinion or are based on inadequate data.This can certainly be the case at the supraspecific level: one can disagree on where to'cut the branches' of the supposed phylogenetic tree (your subfamily could becomemy genus). But the species is (or at leastshould be) the most objective of all taxonomiccategories. At the species level, we shouldnot even have the choice between 'splitting'and 'lumping'. These two attitudes can, atbest, be provisional strategies for handlingunsolved cases.
Biological interpretation ofgapsDifficulties in the taxonomic interpretationof morphological gaps may stem from a variety of causes. In many animal species
Phuket Marine Biological Center Special Pu blication 18(2): 263-284. (1998) 277
40
20
o
A
o•
H
20
10
R
o
B
o •
H
20
R
20 40
o •60
1.70
1.60
HID
10
o
20
c
40
o
1.50
1.8
o
1.9
H/R
2.0
Figure 17. Analysis of intensive variables. A, B, C (see text p. 14): Comparison of scatter diagrams ofdirect measurements. D: scatter diagram of reduced measurements (shape factors) . Data from Tab. 1.
adults and juveniles, males and females andeven different populations of a same specieshave very different morphological characteristics. So the customary 'common sense'statement "if two samples are demonstrably different they must be of two separatespecies" is totally unjustifiable on theoretical grounds. Such a 'conclusion' is really onlya gue ss (unless the observed differences are
very important, let us say beyond the variation range within a genus). Of course, ifsubsequently demonstrated by objective research, some guesses can turn out to be right.Many do not.
Morphological gaps do clearly demonstrate the existence of separate species onlyif four conditions (at least) are met (seeTursch 1997 ).
278 Tropical Marine Mollusc Programme (TMMP)
•
••
L (mm)
O. buetowi
••
14 D (mm)
..'
~/ ... / ..12
. ........ .~.ji". ;········D ·
: .( '".
10 Q. ,~~~......~.......
•••...1 •8 ,' ., / . , .
L (mm)
.,'". > , @
.....,;'".
(mm)
70
10
90
30
50
110
5 25 45 65 85 105 14 18 22
Figure 18. Th e growth of many Oliva species isapproximately isometric (see text this pa ge). Example: O. porphyria (Linnaeus, 1758), samplefrom West Panama .
Figure 19. The growth of some Oliva species isnon-isometric (see text this page). Example: O.buelowi Sowerby, 1889 , sa mple from Hansa Bay,Papua New Guinea.
(1) The samples to be compared must bestrictly sympatr ic (living in the same area).Morphological gaps constitute indirect evidence for lack of interbreeding only if thepotential sexual partners have a chance tomeet. Sympatric samples do have anotheradvantage. When we note that two samplesare different , we are looking at their totalphenotype .We do not know how much ofthedifference is genetic and how much is due toenvironmental effects. If the two sampleslive at the same place, in the same environment, any observed difference must, ofcourse, be genetic. This raises the very general and very serious problem of what to dowith allopatric samples (non-sympatric).Thetraditional attitude is to leave such decisionsto the wisdom of the taxo nomist (a most variable attribute). There is no clear rule andthis is most annoying because most Olivasamples in our collections are allopatric.
(2) Separations should rest upon unbiased samples. In practice, it is often difficult to check if this obvious requirement ismet. The field collector, especially when dealing with abundant species, is often inclinedto collect only the most 'int erest ing' speci-
mens (extremes in the distribution of somefeature),neglecting the 'uninterest ing' intermediates. Such biased samples, when studied by unaware researchers, are a frequentcause of taxonomic errors.
(3)The discriminating characters shouldnot be sex-related. In the case of Oliva , nosignificant sexual dimo rphism in she ll characters could ever be evidenced by us , so itfortunately seems that this aspect can be ignored in Oliva taxonomy.
(4) There is another, often overlooked riskof errors: the growth pattern of many species undergoes rather abrupt changes, solarge and small shells have a different shape.If this phenomenon is und etected , there isan obvious danger of splitting cons pecificyoung and old specimens into separate, artificial "species". The risk of error can behigh, because the 'miss ing links' in size canbe quite rare (see Tursch 1997). In mostOliva species growth is approximately isometric (see Fig. 18): large and small shellshave similar shapes. But this is not alwaysthe case (see Fig. 19) and the juveniles ofmany Oliva species have indeed been described as separate species.
Phuket Marine Biological Center Special Publication 18(2): 263-284. (1998 ) 279
To avoid that pitfall, one should exert special caution whenever a morphological gapis correlated to absolute size. Data on obviously juvenile shells should be interpretedwith great caution and one should endeavour to obtain complete, uninterruptedgrowth series. Non-isometric growth is often betrayed by changes in the slope of thespire whorls. Conspecificity can then be established by size-invariant characters (intensive variables such as protoconch measurements) and by the analysis of colour patterns.
THE MORPHOLOGICALCONTINUUM
The analytical tools described here above arevery sensitive. We soon found out that theydo neatly separate a great number of samples, so our first impression was that therewere a very large number of Oliva species,possibly several hundred.
It quickly became evident that this wasnot the case .We were often dealing only withdifferent populations of the same, geographically variable species. There is nothing surprising in this: it is fully expected that different populations of a same species couldbe separated by a suitable combination ofcharacters (see Mayr 1969 ; Futuyma 1986).
It has been shown (Tursch 1994) thatOliva species consist of a mosaic of distinctpopulations, each being quite homogeneous.When a large enough number of such local ,conspecific populations are compared, theyinvariably show considerable character overlap. The species is thus represented by amorphological continuum in the attributehyperspace. This is a set in which no population (or group of populations) can be separated from all the others. Even if two (ormore) of the populations forming the continuum can be easily separated from eachother, the gap is invariably bridged by another conspecific population (or a morphologically unbroken chain ofpopulations).Theboundaries ofthe morphological continuumare the limits of the phenetic variability of
B
Figure 20. The concept of morphological continuum (see text this page). Samples A, B, C andD can be allopatric.
a species..Conversely, all the members of a morpho
logical continuum belong to the samemorphospecies. Ifone cannot detect any gapbetween groups A and B (see Fig. 20), thenit follows that sp. A =sp. B. In the same way,sp. B = sp. C, and sp. C = sp. D. It is thenhard to escape the conclusion that sp. A =sp. D. This is a useful acquisition, becausenow we have a consistent, logical tool fortaking taxonomic decisions on allopatricsamples, a problem hitherto left to personalinterpretation. For an example of the use ofcontinua, see Tursch & Greifeneder (1996 ).It follows that interspecific discriminantsshould not anymore be searched betweenisolated populations but between continua(encompassing large sets of populations).Note that constructing a continuum is fast,because few specimens are needed to demonstrate overlap. Separation is another matter.
THE SCALE OF SYMPATRYWe first ran into trouble during a study ofOliva oliva (Tursch et al. 1992). Two sympatric samples (both from Phuket,Thailand)were easily separable. The samples werelarge and we had no reason to doubt the locality. So they had to be considered different species. But these two samples were alsomembers of the same morphological continuum, linked by an unbroken chain of
280 Tropical Marine Mollusc Programme (TMMP)
(f) DURANGIT ~~: oReet !!!.
~ ~::J ::JlO lOc
~LAI NGc
3 3
Island
2
Figure 21. The scale of syntopy (see text thispage). Example: two strikingly different populations of 0. oliva (Linnaeus, 1758) live in closeproximity at Hansa Bay, Papua New Guinea.Both belong to the same morphological continuum (see Fig. 23).
allopatric populations with intergrading features. So they had to be the same species.This was a real problem: how could theseshells be two species in Phuket and only onespecies in the global distribution? Was theresomething wrong with the morphologicalcontinuum approach?
The same difficulty occurred again formany Oliva species from many places, butwe noticed that it happened only with shellsthat were not collected by ourselves. So theproblem was more probably with the localities on the labels (or, more exactly, with theinterpretation given to the localities). Thishypothesis was checked by carefully comparing cons pecific samples with very accurateand detailed locality data, all collected in
Figure 22. The scale of syntopy (see text p. 19).Example: two slightly different (but fully separable) populations of 0. amethystina (Roding,1798) live in close proximity at Hansa Bay, PapuaNew Guinea. Both belong to the same morphological continuum (see Fig. 24).
Hansa Bay, a small bay on the Northerncoast of Papua New Guinea, where Olivahave been under scrutiny for over 20 years.
It was found (Tursch 1994) that completely separable populations of a same spe cies can occur within very small distances.The difference between neighbouring populations is sometimes very striking, for instance in the case of the extremely variable(see Tursch et al . 1992 ) O. oliva (L., 1758) ,the type species of the genus, which is foun donly on exposed, dynamic beaches.Two verydifferent populations occur nearly side byside (see Fig. 21). One (A, consisting mostlyof"black" shells) lives on the long black sandSisimangum Beach , the other (B, all "white"shells) lives on the small white sand Boro
Phuket Marine Biological Center Special Publication 18(2): 263-284. (1998) 281
RiD PAT17 I PAT 18
0.650.72
0 .60
0 .6 8
0 .55
0 .640 .50
0 .45 0 .60
0 .40UH 0 .56
1.2 1.3 1.4 0.58 0 .62 0.66
Figure 23. Variability of populations of 0. oliva(Linnaeus, 1758) (see text this page ). Scatter diagram of D/L vs. pat15/pat16. Minimum convexpolygon s. Two population s from Hansa Ba y,Papua New Guinea (A: "white" shells from BoroBeach ;B: mixture of"black"(common) and "white"(rare) shells from Sisimangum Beach) are separated by an obvious morphological gap. This gapis bridged by other conspecific populations (C:she lls from Samarai, Papua New Guinea; D:she lls from Phuket; Th ailand, E: shells fromZamboanga , Philippines) (modified from Tursch1994).
Beach. The two populations are readily separable and the separation is not correlated toshell colour (sample A contains both "black"and "whi te" shells). They are the samemorphospecies because they belong to thesame morphological continuum (see Fig. 23).
The difference between neighbouringpopulations can also be very small, (but stilldetectable by sensitive methods).This is thecase for O. amethystina (Widing, 1798 ), aspecies strictly confined to coral sand mixedwith coral rubble (in which the shell is verywell camouflaged). Here again, two separable populations occur within a short distance(see Fig. 22). One (A) lives around a smallwreck C'Davit 's wreck").The other (B) is distributed around Laing Island and DurangitReef Here again, the two populations are
Figure 24. Variability of populations of Oli vaam ethystina (Roding, 1798) (see text this page).Scatter diagram ofD/L vs. pat17/pat18. Minimumconvex polygons. Two populations from HansaBay, Papua New Guinea (A: shells from "Davit'swreck"; B: shells from Durangit Reef) are separated by an obvious morphological gap. This gapis bridged by other conspecific populations (C:shells from "Mast wreck",Hansa Bay, Papua NewGuinea; D: shells from Solomon Is.) (modifiedfrom Tursch 1994).
part of one same morphological continuum(see Fig. 24). Interestingly, one of the habitats is the coral sand surrounding a datedWorld War II wreck, isolated in a zone ofdark muddy sand (unsuited for O. amethyst ina ). This afforded a clue to the tempo ofmorphological changes in Oliva populations(for details and discussion of possible causes,see Tursch 1994).
Such morphological divergence of populations is not a local phenomenon: collecting trips soon detected similar separationsof neighbouring conspecific Oliva populations in many places around the world. Soit ap pears that the Oliva species are a mosaic of populations, fairly reflecting discontinuities in the habitat. Some populationsare very widespread, others are very restricted, according to the size of continuous
282 Tropical Marine Mollusc Programme (TMMP)
tracts of biotope. It follows that, in manyspecies of the genus Oliva, the notion ofsympatry must be reduced to syntopy (in themeaning of 'st rictly living together, in thesame microhabitat'). Conversely, if twoclosely related Oliva forms are never foundtogether, this should be taken as a strongindication (not a proofl ) that they might beonly different populations of the same species (Tursch 1995).We should then look hardfor other populations with bridging characters.
We can now understand what happenedwith the Phuket samples that caused somuch worry, as mentioned above. There aremany disjunct beaches in Phuket, and ifoursamples came from different beaches theywere not sympatric but allopatric. So therewas nothing wrong with the morphologicalcontinuum approach. We simply had no rightto interpret the gap separating the samplesin terms of species.
EPILOGUEIn conclusion, it appears that Oliva are notmore difficult to classify than many othermollusc groups, provided one selects taxonomic characters that are operational andanalyses them with an appropriate methodology. What we first called the 'good species' (in Section 1) are the species with littleor no geographic variation. In the 'bad species' most populations are different, evenover very small distances. Of course, if weneglect the existence of morphological continua and start naming the differentpopulations, we might end up with as manyunfounded 'species' as there are habitats.This is exactly what did happen in the unfortunate history of Oliva taxonomy.
In an apparent paradox, we are now backto using the criteria of colours and colourpatterns that we have so much disparagedpreviously. But we use them only for quickidentification clues (the separation of thespecies was demonstrated on other, safercharacters).
With the use of proper tools, the number
of Oliva species has decreased very muchand is still decreasing. New collecting techniques (SCUBA diving, deep sea trawling)have yielded many interesting deep-waterforms of known taxa but failed to deliver theprofusion of new species expected by some.This is not surprising because the greatmajority of Oliva species are found between1 and 10 m depths (see Van Osselaer, Bouillon, Ouin & Tursch 1993).There are presumably a few species (probably of small size)still lurking around, awaiting to be discovered. The chances of finding them seem better in rarely visited waters, such as muddybottoms with little appeal for tourists. Exploration of such places could undoubtedlyyield as many undescribed forms as one canwish for - but should these really be described? Chances of finding novel speciesmight be higher in the drawers of old collections. Our team has described only two species (Tursch & Greifeneder 1989; Tursch &Greifeneder 1996) and both had alreadybeen collected by Hugh Cuming before 1800.
The time now seems ripe for directingmore efforts towards a more promising (andstill largely unknown) topic : the natural history of Oliva species.
ACKNOWLEDGEMENTSI am most grateful to all the friends whohave made (or still make) Oliva researchsuch a pleasant venture: Ralph Duchamps,Luc Germain, Dietmar Greifeneder, Dominique Huart, Sidney Johnson, Yuri Kantor,Yonous Machbaete, the late Millar Magap,Olivier Missa, Jean-Marc Ouin, Jean Pierretand Christian Van Osselaer. I thank Dr R.N.Kilburn for helpful comments on the manuscript. I am indebted to the Fonds Nationalde la Recherche Scientifique and to BIOTEC,S.A. for unfailing support.
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Tursch, B. & D. Greifeneder. 1996. The "Olivaminiacea comp lex", with the desc ription of afamiliar, unnamed species. (Stu dies on Olividae 25). Apex 11(1): 1-49.
Tursch, B., O. Missa & J. Bouillon. 1992 .Th e taxonomic structure of Oliva oliva (auct. ). (Studies on Olividae 14). - Apex 7(1): 3-22.
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Van Osselaer, C., J . Bouillon , J .M. Ouin & B.Tursch. 1993. The distribution of Oliva species and the vari ation of their colour patternsin Hansa Bay (Papua New Guinea). (Studieson Olividae 18). - Apex 9(2-3): 29-46.
Van Osselaer, C., J . Bouillon & B. Tursch. 1993.Data on depth of burrowing, motion andsubstrate choice of some Oliva species. (Studies on Olividae 17). - Apex 8(4): 151-158.
Van Oss ela er, C. & B.Tursch . 1988.Ten additionalsuture characters for Oliva taxonomy. (Stu dies on Oliv idae 9). - Apex 3(4): 81-87 .
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284 Tropical Marine Mollu sc Programme (TMMP)