145Bollettino della Società Paleontologica Italiana, 44 (2), 2005, 145-154. Modena, 30 settembre 2005

ISSN 0375-7633

INTRODUCTION

The boring bivalve Gastrochaena produces flask-shaped holes in carbonate substrata (rocks, shells,echinid tests, live and dead corals) (Soliman, 1973;Carter, 1978; Freneix & Roman, 1979; Belokrys, 1991a;Schiaparelli et al., 2003). As trace fossils, these boringsare referred to the ichnogenus Gastrochaenolites (Kelly& Bromley, 1984), which is known since the LateTriassic, even though the documented stratigraphicdistribution of gastrochaenids dates back to the Early-Middle Jurassic (Carter & Stanley, 2002, 2004). OlderGastrochaenolites-type borings, recently reported fromthe Ordovician (Benner et al., 2004), cannot be referredto gastrochaenids.

The Atlantic-Mediterranean Gastrochaena dubia(Pennant, 1777) can live as a facultative free tube-dweller on soft bottoms and a similar life habit is knownfor other gastrochaenid genera (Keen, 1969; Carter,1978; Morton, 1982, 1983a, b, 1985; Savazzi, 1982,

1999a). Such a versatile change in the mode of life ispossible due to the thick aragonitic lining secreted onthe inner wall of the borehole, that can be used also toconstruct a free tube.

One of the oldest reports about the tube-dwellinghabit of G. dubia is by Sowerby (1887, pl. 1, fig. 14)who illustrated the “bottle-shaped case” of this species.Fossil “claviform tubes” were reported upon by Cerulli-Irelli (1909) from the Pleistocene of Rome (see below).Savazzi (1982) studied the functional morphology ofthe “crypt” built by G. dubia and other gastrochaenids,in a review of the tube-dwelling adaptations among theBivalvia. Recently, Albano (2003) remarked on thedifferences between the crypts of the two gastrochaenidspecies living in the Mediterranean, G. dubia andCucurbitula cymbium (Spengler, 1783), a Lessepsianmigrant. The study of Pleistocene material providesnew information about the ecological requirements ofthe tube-dwelling habit of G. dubia and some functionaland constructional aspects of the crypt.

Tube-dwelling in Gastrochaena dubia (Bivalvia): ecological requirements,functional morphology and structure of the crypt

Rafael LA PERNA

Rafael La Perna, Dipartimento di Geologia e Geofisica, Università di Bari, Via Orabona 4, I-70125 Bari, Italy; r.laperna@geo.uniba.it

KEY WORDS - Bivalvia, Gastrochaena, Gastrochaenidae, Tube-dwelling, Functional morphology, Pleistocene.

ABSTRACT - The rock boring bivalve Gastrochaena dubia (Pennant, 1777) can also live freely on soft substrata, within a claviformcrypt consisting of a thick, coarsely agglutinated external layer and an inner lining. Examination of Pleistocene material from MonteMario, Rome (Cerulli-Irelli collection), allows an understanding of the ecological requirements necessary for this life habit and providesnew information about crypt functional morphology and construction. The crypt of G. dubia rests subhorizontally, partly buried and withthe siphonal tip emerging from the substratum. A low-energy substratum, with a sandy-muddy texture rich in a coarse biogenic fraction,is the main ecological requirement. Through a process of dissolution and re-secretion, G. dubia enlarges the crypt during growth,particularly anteriorly. The agglutinated, blistered structure of the external layer seems to be formed by means of mucous “bubbles”which mineralise, incorporating sand grains and bioclasts. An elongate, thin appendage of the foot seems to be involved in this process.The same foot appendage produces thin tubules which penetrate the crypt wall. They have a probing function in the boring life-habit, butit is not known if they have any function in the free tube-dwelling habit.

RIASSUNTO - [Il modo di vita tubicolo in Gastrochaena dubia (Bivalvia): necessità ecologiche, morfologia funzionale e strutturadel tubo] - Il bivalve Gastrochaena dubia (Pennant, 1777), un noto perforatore di substrati carbonatici, può vivere anche su substratimobili, all’interno di un tubo (cripta). La cripta è claviforme, leggermente incurvata ed è formata da uno strato esterno grossolanamenteagglutinato e da un rivestimento interno di natura aragonitica. Lo studio di materiale pleistocenico proveniente dall’area di MonteMario (Roma), appartenente alla collezione Cerulli-Irelli, ha permesso di trarre delle considerazione sulle necessità ecologiche relativea questo modo di vita e di raccogliere nuovi dati sugli aspetti funzionali e costruttivi della cripta. In vita, la cripta di G. dubia èposizionata suborizzontalmente, leggermente infossata e con la porzione sifonale proiettata verso l’alto. Le necessità ecologiche di untale modo di vita sembrano principalmente rappresentate da un ambiente a bassa energia e da un substrato sabbioso-fangoso, ricco dielementi organogeni grossolani. Attraverso un processo di dissoluzione e ricostruzione, G. dubia è in grado di ampliare la criptadurante la crescita. Tale processo interessa soprattutto la parte anteriore, attorno all’area pedale. La costruzione di una nuovaporzione di strato esterno agglutinato sembra avvenire attraverso la deposizione di “bolle” di muco che mineralizzano velocemente,cementando granuli sabbiosi e bioclasti. Questo processo sembra controllato da un’appendice allungata e sottile, presente nella parteanteriore del piede, la quale “manipolerebbe” il muco prodotto dal mantello. La stessa appendice produce delle sottili perforazioni(tubuli) nella parete della cripta. Nel modo di vita perforante, i tubuli hanno funzione di sondaggio e guida nella perforazione delsubstrato duro, che è tipicamente di forma irregolare e struttura disomogenea. Non è chiaro se tali tubuli abbiano una funzione anchenel modo vita tubicolo. La possibilità di Gastrochaena di passare da un modo di vita endolitico ad uno libero su substrato mobile, può

realizzarsi attraverso diversi caratteri preadattativi, provenienti dal modo di vita endolitico.

146 Bollettino della Società Paleontologica Italiana, 44 (2), 2005

TERMINOLOGY AND MATERIAL

Two specific names have been used to describe thetube built by the Gastrochaenidae: Carter (1978) andMorton (1982) termed it “igloo”, with reference to thedome-shaped partial envelope produced by semi-endolithic species of Cucurbitula and Gastrochaena toprotect exposed tissues, while Savazzi (1980, 1982)termed it “crypt” and used this name also for othertube-dwelling bivalves. In the present work, crypt isused to define the free or partially free tube, excludingborings.

The study material is from the Cerulli-Irelli collection(Museo di Paleontologia dell’Università di Roma “LaSapienza”) and consists of 13 crypts and 8 loose valves.This material, originally reported upon by Cerulli-Irelli(1909, p. 154, pl. 18, figs. 24, 28), is from the EarlyPleistocene deposits of Farnesina (Rome), one of thelocalities of the Monte Mario area (La Perna et al., 2004).A crypt from Monte Mario was also illustrated bySavazzi (1982, fig. 8f). No data are available about thelithology from where this material comes, except forthe mainly sandy texture of the Monte Mario deposits(Bonadonna, 1968; Marra et al., 1994).

THE SHELL

The shell of Gastrochaena dubia (Fig. 1) is thin-walled, brittle, elongate, with a strongly anterior umbo,an edentulous hinge and a wide anteroventral gape. The

sculpture is weak, consisting of commarginal striaeand deep incremental scars, giving a somewhat undulateappearance to the outer shell surface (and to the innersurface, due to the thinness of the shell wall). Theposterior adductor muscle scar is close to theposterodorsal margin, whereas the anterior adductormuscle is attached to the extreme anterior end of theshell (Fig. 1f). Within the pallial line, there are a numberof muscle scars (Figs. 1b, e-f; see also Purchon, 1954,fig. 2; Carter, 1978, figs. 14-16). Other main musclescars are left by the pedo-byssal retractor muscles(Purchon, 1954; Carter, 1978): the posterior muscle isattached near the posterior adductor muscle, whereasthe anterior one is inserted on the hinge plate, sometimeson a short myophoric apophysis (Fig. 1g).

ECOLOGICAL REQUIREMENTS ANDFUNCTIONAL MORPHOLOGY

According to a morphofunctional interpretation bySavazzi (1982, 1999a), the claviform crypt ofGastrochaena dubia rests on the substratumsubhorizontally, partly buried and with the siphonal tipemerging (Fig. 2). Clear evidence of such a life positionis provided by the bryozoan colonies encrusting oneside of the largest crypt (Pl. 1, figs. 1-3). A large colonyencrusts most of the anterior part and a smaller colonyis on the siphonal tip. They are Calpensia nobilis (Esper,1796), a bryozoan usually forming unilaminar colonieson a variety of hard substrata on shelf bottoms (Rosso

Fig. 1 - Gastrochaena dubia (Farnesina, Rome, Early Pleistocene. Cerulli-Irelli coll., Museo di Paleontologia dell’Università di Roma“La Sapienza”). a, b: 22 mm. c: 14.5 mm. d-f: 20 mm (arrow: anterior adductor muscle scar). g: 22 mm (arrow: anterior apophysis of thepedo-byssal muscle). Size of valves as antero-posterior length.

147R. La Perna - Tube-dwelling in Gastrochaena dubia

A., pers. comm.). These colonies are believed to havegrown while the bivalve was living and thus theiroccurrence and position testify to the exposure of thisside of the crypt. Savazzi (1999a) added that the cryptsof Gastrochaena can be also vertically embedded inthe sediment. In this case, the crypt should be straight,but its siphonal extremity is gently bent (rarely moremarkedly bent, Pl. 1, fig. 4), either in the examinedmaterial and in published pictures (e.g. Carter, 1978,fig. 47; Savazzi, 1982, figs. 8e-f; Albano, 2003, fig.10), in accordance with the subhorizontal position.

Savazzi (1982) noted that the crypts of Cucurbitulaand Gastrochaena are reminiscent of the shape ofseveral invertebrates living on soft bottoms, such asGryphaea (“horn shaped recliners” of Seilacher, 1984).However, the gastrochaenid crypts lack the weight ofthe convex side and their stability must rely on a passivereorientation, as experimentally shown in a flumeapparatus by Savazzi (1982, 1999b). Crypts with agreater curvature (like the Cucurbitula crypts) are moreeasily reoriented than straighter ones (like those ofGastrochaena), thanks to sediment being eroded belowthe crypt side facing into the current. The almost straightcrypts of Gastrochaena would then suffer instability(rolling), unless they live on a quiet bottom. Theabsence of regenerated siphonal regions, i.e. a newsiphonal extremity built as a response to the cryptoverturning, in the material examined by Savazzi (1982),and the single crypt with a regenerated syphonal opening(Pl. 1, fig. 7) in the present material, support thisconclusion about the low-energy environments requiredby G. dubia to live as a free tube-dweller.

The agglutinated material is mostly coarse, as inGastrochaena crypts reported upon in the literature.The need of this species to construct such a coarsecoating implies that the substratum must be particularlyrich in a coarse (biogenic) fraction. In the presentcrypts, the hard parts of three species are morefrequently present in the agglutinated material, namelythe bivalves Saccella commutata and Timoclea ovataand the polychaete Ditrupa arietina, all typical of shelfsandy-muddy bottoms, rich in coarse material.However, an exceedingly coarse substratum wouldprevent the crypt from resting firmly on it, whilst amuddy, soupy sediment would cause the crypt to sink.G. dubia has a marked rugophilic behaviour, i.e.preferential settlement by larvae within microcavities,particularly those produced by clionid bioerosion(Schiaparelli et al., 2003 and pers. comm.). A stable,low-energy substratum, with a low rate of bioturbationand a heterogeneous sandy-muddy texture, rich in acoarse and bioeroded fraction seems to be the mainecological requirements for the tube-dwelling habit ofG. dubia.

The function of the thick, agglutinated coating hasnever been investigated. The crypt of Cucurbitula hasa rather homogeneous sandy coating (Carter, 1978, fig.58; Savazzi, 1982, figs. 3-4), whereas the crypt ofEufistulana bears scattered sand grains and smallbiogenic fragments, not forming a true coating (Carter,1978, fig. 62; Morton, 1983b, pl. 1; Belokrys, 1991b,p. 44). Gastrochaena, Cucurbitula and Eufistulanahave different tube-dwelling behaviours (Fig. 3): resting

on the substratum, mainly semi-endolithic and verticallyburied, respectively (Carter, 1978; Savazzi, 1982,1999a; Morton, 1983a, b; Kleemann, 1998). Themarkedly different external texture of the crypt isprobably related to these differences in life habit. Thethick, agglutinated coating of the Gastrochaena cryptmay fulfil several functions: thickening andstrengthening of the crypt (against microborers anddurophagous predators), weighting and anchorage(thanks to the projecting, larger bioclasts) to increasestability (Fig. 2) and, possibly, camouflage against visualpredators. Similar conclusions were reported upon byTaylor et al. (1999) about two infaunal veneroid bivalvespossessing a thick arenaceous shell coating, whichwould serve two main purposes: resistance to predation(by increasing the effective diameter of the animal andthe thickness of the shell, and by providing camouflage)and resistance to environmental stresses (includingresistance to boring organisms and improved anchoragein the sediment).

As documented by Savazzi (1982), the bivalveseems to have a preferred orientation within the crypt.This is supported by two considerations: theoccurrence of a pedal scar on the anterior inner wall ofthe crypt (Savazzi, 1982, fig. 6) and the occurrence ofa siphonal diaphragm (Pl.1, figs. 11-12), into whichthe posterior part of the shell fits. However, shell rotationmust be possible (to contribute mechanically to theboring process) by contraction of the foot andwithdrawal of the posterior part from the diaphragm.The illustrations of Cucurbitula by Savazzi (1982, fig.6) indicate that the bivalve rests with the ventral side

Fig. 2 - Postulated life position of the crypt of Gastrochaenadubia. Note the encrusting bryozoan colonies (open arrows) andthe anchoring effect of the large, projecting bioclasts (solidarrows).

148 Bollettino della Società Paleontologica Italiana, 44 (2), 2005

towards the convex side of the crypt (lower side, inlife position). Accordingly, concave and convex cryptsides are referred to as dorsal and ventral, respectively(see captions of Pl. 1). The bivalve in the sectionedcrypt illustrated in the present work (Pl. 1, fig. 10) isclearly not in life position.

CRYPT CONSTRUCTION

The old hypothesis by Sowerby (1854, p. 269) abouta single episode of construction of the crypt by the“Tubicolae” (=Gastrochaenacea, Pholadacea and

Clavagellacea), when maturity is attained, is in part validfor the Clavagellacea, who seem to be able to lengthenand repair the tube, but not to enlarge it in diameter(Harper & Morton, 2004; Morton, 2004). The processof crypt construction in the Gastrochaenidae impliesdissolution and reconstruction of a new enlargedportion, as postulated by Morton (1982, 1983a, b, 1985)for Cucurbitula and Eufistulana. In these genera, thewide anteroventral shell gape permits reflection overthe shell of the middle and inner mantle folds,anteroventrally and posteroventrally respectively.Specialised glands in these folds would be responsiblefor dissolution and re-secretion of the aragonitic lining.

EXPLANATION OF PLATE 1

Crypts of Gastrochaena dubia (Farnesina, Rome, Early Pleistocene. Cerulli-Irelli coll., Museo di Paleontologia dell’Università di Roma“La Sapienza”).

Figs. 1-3 - A large crypt, 45 mm long, in dorsal (1) and lateral (2) views. It lacks the anterior end through which the inner lining (largearrow) and the shell (small arrow) are seen. Note the bryozoan Calpensia nobilis encrusting the dorsal surface (3, scale bar= 1 mm) and the siphonal tip (1, arrow).

Fig. 4 - A 30 mm long crypt in dorsal view. The shell is partly exposed anteriorly.Fig. 5 - A 32 mm long crypt in ventral view. The shell is fully exposed anteriorly.Fig. 6 - A 30 mm long crypt in lateral view. Note the strong curvature, the siphonal elongation and the large, projecting bioclasts.Fig. 7 - A 22 mm long crypt in dorsal view. Note the regenerated siphonal tube, at right angle to the original position.Figs. 8-9 - Details of the outer coating. Note the aragonitic blisters with agglutinated sand grains and the small pit on the top. Scale

bars = 1 mm.Figs. 10-14 - A 30 mm long crypt sectioned longitudinally. Note the upturned, non physiological position of the bivalve (10), the

siphonal diaphragm (11-12), the small, incomplete boring (11, 13) and the key-hole perforation, probably produced by ajuvenile of the same species (12, 14). Scale bars of figs. 13-14 = 1 mm.

Figs.15-17 - Tubules on the anteroventral region of a 28 mm long crypt (15) and details (16-17) of the perforated area. Scale bars: fig.16 = 3 mm, fig. 17 = 1 mm.

Fig. 3 - Life-habit ranges of the gastrochaenid genera (see text). Gastrochaenopsis Chavan, 1952 and Kummelia Stephenson, 1937 areextinct. Illustration sources: http://dominique.millet2.free.fr/div5.html (bored rock), http://www.ciesm.org/atlas/Gastrochaenacymbium.html (Cucurbitula cymbium), http://shell.kwansei.ac.jp/~shell/pic_book/data18/r001754.html (Eufistulanagrandis).

149Pl. 1R. La Perna - Tube-dwelling in Gastrochaena dubia

150 Bollettino della Società Paleontologica Italiana, 44 (2), 2005

The siphonal tube is secreted by the siphonal epithelium,while the foot seems to contribute mainly to dissolution.A wide shell gape is also present in Gastrochaena, butthe mantle is not reflected over the shell, as reported inthe anatomical description and illustrations of G. dubia(Deshayes, 1846) and allied species (Purchon, 1954;Soliman, 1973; Carter, 1978). In these species themantle lobes are fused ventrally, leaving a pedal aperture.The mantle was said to be “muscular and highlycontractile … expanded well beyond the shell margin,coming in contact with adjacent anteroventral walls ofthe burrow shell chamber” (Carter, 1978, p. 16). Thegastrochaenid foot has been described as a stout,columnar organ, with a roundish sole longitudinallycrossed by a byssal groove (Purchon, 1954; Soliman,1973, Carter, 1978). There is a small, elongateappendage extending from the anterior margin of thefoot sole, called the “foot tongue” (also “papilla”, “toe”or “pedal organ”). Purchon (1954) found this organ tobe relatively well developed “in those species possessingunited siphons”, i.e. species of Gastrochaena, while itis reduced or absent in Spengleria (which has separatesiphons producing two distinct siphonal openings inbored substrata).

In the present material, some crypts (including thelargest one) lack the anterior end or part of it (Pl. 1,figs. 2, 4-5), leaving the shell partly exposed. Thesecases might be due to accidental breakage before orafter death (or during fossilization), but they mayrepresent specimens that died during crypt enlargement,like the crypts of Cucurbitula illustrated by Savazzi(1982, p. 282, figs. 6a-d). It is worth noting that in thepresent cases the shell gape is fully exposed, suggestingthat dissolution starts from, or is more rapid aroundthe pedal area. This is in accordance with the typicalboring life habit of Gastrochaena, which needs aprogressive “downward” expansion of the borehole intoa hard substratum. The marked incremental scars seenon the valves (Fig. 1), may reflect periodic enlargementof the crypt.

The process employed by gastrochaenids to build anew portion of crypt seems to be mostly controlled bymantle, which is extensively supplied with mucus glands(Purchon, 1954; Carter, 1978). Carter (1978, p. 44)observed a burrow repair in Gastrochaena hians bymeans of overlapping aragonitic laminae, accompaniedby secretion of a mucous sheet and inflation of the

anteroventral mantle. A similar process was observedin Gastrochaena stimpsoni while constructing a freecrypt (Carter, pers. comm.). In the case of the long,slender crypt of Eufistulana, Morton (1983b, p. 400)suggested that inflation of the mantle, caused by bloodbeing pumped into it, “compacts the sediment aroundthe animal forming a solid surface against which newtube secretions can be laid down”.

The crypts illustrated in the literature (see above)and those examined in the present work exhibit amarkedly rough surface, not only due to the coarseagglutinated material, but also because of the presenceof “blisters”, sometimes bearing a small apical pit (Pl.1, figs. 8-9). Even though most blisters are actuallyeroded, the pits are not caused by erosion, as indicatedby their regular shape and sharp rim. These crater-likestructure have never been observed before, but similarstructures occur in the crypt wall of the teredinidKuphus (Savazzi, 1982, fig. 11e), whose life habit issimilar to that of Eufistulana. They may represent thesmall portions by means of which the external,agglutinated part of the crypt wall is built. In thin section,the crypt wall consists of the layered aragonitic lining(Pl. 2, figs. 1, 5-6) and a thick (up to 2 mm) agglutinatedcoating, the structure of which proves to be mostlyformed by aragonitic blisters incorporating sand grainsand bioclasts (Pl. 2, figs. 1-7). The “plastic” appearanceof the blisters (Pl. 2, figs. 2, 4, 6) suggests an originfrom mucous “bubbles”, which become calcified afterdeposition. The mantle secreted mucus is probably“manipulated” by the foot tongue which leaves a pitwhen calcification is attained, while the aragoniticlaminae forming the inner lining provide a support tothe outer layer. Sand grains and bioclasts seem to bepassively incorporated, either on and within the blisterwalls (Pl. 2, fig. 4), and among the blistered structure(Pl. 2, fig. 6). Most blisters are empty or in part filledwith sand grains (probably entering from the pit), butthey sometimes appear to be filled with aragoniticmaterial (Pl. 2, fig. 5). The layered structure of thecrypt lining confirms the observations of Carter (1978)and Savazzi (1982, fig. 6f).

A relation between mucous secretion andcementation of sediment grains to form an agglutinatedcoating is recurrent in the malacological literature(Taylor et al., 1999, with references). The occurrenceof bacteria in the mucous secreted by the mantle and

EXPLANATION OF PLATE 2

Longitudinal thin section of the sectioned crypt of Pl. 1, figs. 10-12.

Figs. 1-2 - A fragment of gastropod shell agglutinated in the outer coating. Note the layered internal lining of the crypt (1) and theempty aragonitic blister (2, arrows) occupying the inner part of the bioclast. Scale bars: fig. 1 = 1 mm, fig. 2 = 0.5 mm.

Figs. 3-4 - Two aragonitic blisters, the lower flattened and with a thicker wall, the upper incorporating bioclasts in the wall and withsandy grains inside (probably entering from the pit). Scale bars: fig. 3 = 1 mm, fig. 4 = 0.5 mm.

Fig. 5 - A filled aragonitic blister. Note the thin, poorly distinct wall (arrow), the many agglutinated particles on the outside andthe layered internal lining of the crypt. Scale bar = 1 mm.

Fig. 6 - Three irregularly shaped aragonitic blisters containing sediment particles. Note the layered internal lining of the crypt andthe bioclast perpendicular to the crypt surface. Scale bar = 0.5 mm.

151Pl. 2R. La Perna - Tube-dwelling in Gastrochaena dubia

152 Bollettino della Società Paleontologica Italiana, 44 (2), 2005

deposited on the outer shell margin in two bivalves witharenaceous coatings (see above) was documented byTaylor et al. (1999). Bacteria probably mediate the initialnucleation of the calcium carbonate which will cementthe sediment grains together.

TUBULES

Carter (1978) remarked that the occurrence inGastrochaena of short tubules which penetrate theanterior burrow lining, around the pedal area, areproduced by the foot appendage. They most probablyhave a probing function, guiding the boring direction,since the substrata commonly bored by gastrochaenidsare irregular in shape, thickness and internal structure(Carter, 1978, p. 49). This function was documentedby Schiaparelli et al. (2003) for G. dubia boringlimestones with quartz veins which cannot be perforatedby the bivalve.

The occurrence of tubules in a Gastrochaena cryptwas first reported upon by Carter (1978) and was furtherdiscussed by Savazzi (1982), who attempted tocompare these structures to the perforated basal(anterior) plate (“watering pot”) of the crypt built bythe Clavagellidae. As shown by Savazzi (1982), aprimary function of the clavagellid anterior tubules isrelated to an hydraulic burrowing process, through aflow of water pumped out from the mantle cavity (seealso Morton, 1985, fig. 16). This hypothesis issupported by the lack or poor development of tubulesin endolithic clavagellids (Savazzi, 2000). However,Harper & Morton (2004) and Morton (2004) alsoreported upon the ability of clavagellids to pumpinterstitial water into the tube through the basal plate,probably to supply the bivalve with bacteria and otherorganic material as an additional trophic source.

All of the examined crypts bear tubules on theanterior half of the crypt. They can be easily confusedwith the many pits present on the crypt surface (seeabove), but they differ by having mostly an ellipticalopening, with a narrow, raised rim (Pl. 1, figs. 16, 17).In a single case, a crypt surface devoid of anagglutinated coating bears several tubules, all close toeach other (Pl. 1, fig. 15). Probably, not all of the tubulesseen on the crypt surface actually penetrate the cryptwall, since the tubules can be produced at differenttimes and sealed by the internal lining. The sectionedcrypt exhibits an incomplete perforation (Pl. 1, figs.11, 13) which penetrates the crypt lining but not theagglutinated coating. Another crypt (Pl. 1, fig. 5) bearsfour small perforations on the inner surface, close tothe “broken” margin, from the ventral to the dorsolateralside. Two of these penetrate the crypt wall.

A “key-hole” perforation is present on the other sideof the sectioned crypt (Pl. 1, figs. 12, 14). It differsfrom other tubules in lacking an external rim and bybeing notably larger. Rather than a tubule, this perforationseems to have been produced by a juvenile of the samespecies (e.g. see the juvenile perforations reported uponby Schiaparelli et al., 2003, figs. 1e, g-h) whichattempted to settle on and perforate the host crypt.

The occurrence of tubules in the free crypts ofGastrochaena is puzzling. Probably they represent thenormal boring activity of the foot appendage,unfunctional in the tube-dwelling habit. As suggestedby Carter (pers. comm.), tubules are too small to createa jet of water. On the other hand, no tubules are presentin Eufistulana, the gastrochaenid genus which exhibitsthe more specialised adaptation to soft bottoms andthe strongest convergence with the clavagellid life-habit.

The bifurcated tubules (“additional openings”) onthe siphonal tip of the crypt of Cucurbitula, reportedon and discussed by Savazzi (1982 and pers. comm.),are even more puzzling. They were tentatively thoughtto be related to tentacle-like organs, employed by thebivalve to maintain a stable semi-buried position(Savazzi, 1982, 1999a). However, Carter (pers. comm.)found no siphonal accessory organs in C. cymbium.Clearly, these siphonal tubules cannot be produced bythe foot appendage, but their origin and function remainunknown.

TAXONOMIC NOTE

Freneix & Roman (1979) proposed a reclassificationof Gastrochaena into three subgenera, namelyGastrochaena s.s. Spengler, 1783, Rocellaria Blainville,1828 and Lamychaena Freneix, 1979, mainly based onshell shape and the development of the anteriormyophoric apophysis. Gastrochaena and Rocellaria aresimilar in shape but the anterior apophysis is absent inthe former and poorly developed in the latter.Lamychaena has a terminal umbo, a pyriform shape, adeeper pallial sinus and a well developed apophysis inthe form a triangular plate. Whereas Lamychaena isclearly worthy of distinction, probably to full genusrank, Gastrochaena s.s. and Rocellaria seem to be lesswell distinguished (e.g. within the examined materialthe anterior apophysis is absent to poorly developed,mostly in relation with the growth stage). The shellmorphology of Lamychaena, including the large anteriorapophysis, points to a more efficient and deeper boringhabit, compared with that of Gastrochaena s.s. andRocellaria. These conclusions agree with the datareported upon by Carter (1978) for Gastrochaena hiansand G. ovata: the former is the type species ofLamychaena, the latter is a representative of Rocellaria,whose type species is G. dubia.

DISCUSSION

Within the extraordinary evolutionary pathways tothe re-colonization of soft bottoms by hard bottomdwelling bivalves (Seilacher, 1984), the Gastrochaenidaeand the Clavagellidae represent successful andfascinating families. Savazzi (1982, 1999a) stressed theexaptive (= pre-adaptive, Gould & Vrba, 1982) aspectsof tube-dwelling, of which the aragonitic lining secretedonto the borehole to smooth and seal the poroussubstratum (often coral skeletons) is the most obvious.Also the ability to enlarge the crypt during growth,through dissolution and reconstruction, as well as the

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repair of the crypt and elongation and regeneration ofthe siphonal tip, all constitute exaptive traits, inheritedfrom the endolithic life habit and employed in the tube-dwelling habit. The mucous produced by the mantleseems to be employed to bind and expel the particlesproduced by perforation (Purchon, 1954; Carter, 1978),while in the tube-dwelling habit it provides a calcificationand agglutination medium. Another exaptive functionis played by the foot, particularly by its appendage:used to produce probing tubules in the typical, boringlife-habit, it seems to contribute to the construction ofthe crypt, “manipulating” the mucous to form theagglutinated crypt coating.

Within the Gastrochaenidae, the genusGastrochaena s.l. exhibits the widest range in life habit,from fully endolithic to free tube-dwelling (Fig. 3), eventhough Lamychaena is kept as a distinct genus. Clearly,the adaptive-exaptive traits allow Gastrochaena,particularly species of the subgenus Rocellaria, toadopt a wide range of life habits. Conversely, the“primitive” Spengleria, whose boring habit seems tobe mainly based on mechanical abrasion (Carter, 1978),with separated siphons and probably with a poor abilityto line its burrows (Morton, 1983a; Belokrys, 1992),is an obligatory fully-endolithic form. At the oppositeend of the gastrochaenid spectrum are members ofKummelia and Eufistulana, highly specialised to a tube-dwelling buried habit (Savazzi, 1982, 1999a; Morton,1983a, b). An intermediate semi-endolithic condition isrepresented by Cucurbitula which perforates small, thinsubstrata, destroyed by later growth and replaced by acomplete crypt (Carter, 1978; Morton, 1982; Savazzi,1982, 1999a).

ACKNOWLEDGEMENTS

I am grateful to Riccardo Manni (Università “La Sapienza”,Roma) for his kind assistance in accessing the Cerulli-Irellicollection, to Antonietta Rosso (Università di Catania) for usefulinformation about bryozoans, to Stefano Schiaparelli (Universitàdi Genova) and Joseph C. Carter (University of North Carolinaat Chapel Hill) for reading and improving an early draft ofmanuscript. The referees Enrico Savazzi (Kyoto University)and Brian Morton (Western Australian Museum, Perth) arethanked for their helpful comments and suggestions. Worksupported by Fondi d’Ateneo 2004.

REFERENCES

Albano P.G. (2003). Mediterranean Gastrochaenidae (Mollusca:Bivalvia). Bollettino Malacologico, 38 (9-12): 135-138.

Belokrys L.S. (1991a). New Eocene gastrochaenids (Bivalvia)from the Ukraine. Paleontological Journal, 25 (2): 9-20.

Belokrys L.S. (1991b). Tube-dwelling bivalves from the Eoceneof the Ukraine. Paleontological Journal, 25 (4): 40-53.

Belokrys L.S. (1992). The genus Spengleria (Gastrochaenidae,Bivalvia) from the Eocene of Ukraine. PaleontologicalJournal, 26 (2): 154-158.

Benner J.S., Ekdale A.A. & De Gibert J.M. (2004). Macroborings(Gastrochaenolites) in Lower Ordovician hardgrounds ofUtah: sedimentologic, paleoecologic, and evolutionaryimplications. Palaios, 19: 543–550.

Bonadonna P. (1968). Studi sul Pleistocene del Lazio. V. Labiostratigrafia di Monte Mario e la “Fauna Malacologica

Mariana” di Cerulli-Irelli. Memorie della Società GeologicaItaliana, 7: 261-321.

Carter J.G. (1978). Ecology and evolution of the Gastrochaenacea(Mollusca: Bivalvia) with notes on the evolution of theendolithic habit. Yale Peabody Museum Bulletin, 41: 1-91.

Carter J.G. & Stanley G.D. (2002). Origin and early evolution ofthe endolithic and tube dwelling superfamily Gastrochaeno-idea (Mollusca, Bivalvia, Autolamellibranchiata). FirstInternational Palaeontological Congress, Sydney, 2002,Abstracts: 31.

Carter J.G. & Stanley G.D. (2004). Late Triassic gastrochaenidand lithophaginid borings (Mollusca: Bivalvia) from Nevada(USA) and Austria. Journal of Paleontology, 78 (1): 230-234.

Cerulli-Irelli S. (1909). Fauna Malacologica Mariana.Palaeontographia Italica, 15: 125-214.

Desahyes M. (1846). Examen anatomique du Gastrochène de laMéditerranée (Gastrochaena dubia). Comptes Rendus desséances de l’Académie des Sciences, 22: 37-38.

Freneix S. & Roman J. (1979). Gastrochaenidae endobiotesd’Échinides cénozoïques (Clypeaster et autres). Nouvelleclassification de ces Bivalves. Bulletin du Muséum d’HistoireNaturelle, 4e sér., 1, section C, 4: 287-313.

Gould S.J. & Vrba E.S. (1982). Exaptation - a missing term in thescience of form. Paleobiology, 8 (1): 4-15.

Harper E.M. & Morton B. (2004). Tube construction in thewatering pot shell Brechites vaginiferus (Bivalvia;Anomalodesmata; Clavagelloidea). Acta Zoologica, 85: 149-161.

Keen A.M. (1969). Superfamily Gastrochaenacea. In Moore R.C.(ed.), Treatise on Invertebrate Paleontology, N, Mollusca 6(2), N857-N859. Geological Society of America andUniversity of Kansas Press.

Kelly S.R.A. & Bromley R.G. (1984). Ichnological nomenclatureof clavate borings. Palaeontology, 27: 793–807.

Kleemann K.H. (1998). Superfamily Gastrochaenoidea. InBeesley P.L., Ross G.J.B. & Wells A. (eds), Mollusca: TheSouthern Synthesis. Fauna of Australia. Vol. 5, part B. CsiroPublishing, Melbourne: 367-370.

La Perna R., Ruggiero A.F. & Manni R. (2004). La collezioneCerulli-Irelli e la diversità malacofaunistica del Pleistoceneinferiore mediterraneo. IV Giornate di Paleontologia,Bolzano, 21-23 maggio 2004, Abstracts: 37.

Marra F., Carboni G., Di Bella L., Faccenna R., Funicello R. &Rosa C. (1994). Il substrato plio-pleistocenico nell’arearomana. Bollettino della Società Geologica Italiana, 114: 195-214.

Morton B. (1982). Pallial specializations in Gastrochaena(Cucurbitula) cymbium Spengler, 1783 (Bivalvia:Gastrochaenacea). In Morton B.S. & Tseng C.K. (eds),Proceedings of the First International Marine BiologicalWorkshop: The Marine Flora and Fauna of Hong Kong andSouthern China. Hong Kong University Press, Hong Kong:859-873.

Morton B. (1983a). Evolution and adaptive radiation in theGastrochaenacea (Bivalvia). Journal of Molluscan Studies,Suppl. 12A: 117-121.

Morton B. (1983b). The biology and functional morphology ofEufistulana mumia (Bivalvia: Gastrochaenacea). Journal ofZoology, 200: 381-404.

Morton B. (1985). Tube formation in the Bivalvia. Soosiana,13: 11-26.

Morton B. (2004). The biology and functional morphology ofFoegia novaezelandiae (Bivalvia: Anomalodesmata:Clavagelloidea) from Western Australia. Malacologia, 46 (1):37-55.

Purchon R.D. (1954). A note on the biology of the LamellibranchRocellaria (Gastrochaena) cuneiformis Spengler. Proceedingsof the Zoological Society of London, 124 (1): 17-33.

Savazzi E. (1980). New records of fossil Gastrochaenacea(Pelecypoda) from the Venetian region (NE Italy). Memoriedi Scienze Geologiche, 34: 171-182.

R. La Perna - Tube-dwelling in Gastrochaena dubia

154 Bollettino della Società Paleontologica Italiana, 44 (2), 2005

Savazzi E. (1982). Adaptations to tube dwelling in the Bivalvia.Lethaia, 15: 275-297.

Savazzi E. (1999a). Boring, nestling and tube-dwelling bivalves.In Savazzi E. (ed.), Functional Morphology of theInvertebrate Skeleton, John Wiley & Sons: 205-237.

Savazzi E. (1999b). Soft-bottom dwellers and the leaning Towerof Pisa: adaptive exploitation of unstable life positions. InSavazzi E. (ed.), Functional Morphology of the InvertebrateSkeleton, John Wiley & Sons: 123-128.

Savazzi E. (2000). Morphodynamics of Bryopa and the evolutionof clavagellids. In Harper E.M., Taylor J.D. & Crame J.A.(eds), The Evolutionary Biology of the Bivalvia. GeologicalSociety, London, Special Publication, 177: 313-327.

Schiaparelli S., Franci G., Albertelli G. & Cattaneo-Vietti R.(2003). Autoecologia del bivalve perforatore Gastrochaenadubia lungo la falesia del promontorio di Portofino. Attidell’Associazione Italiana di Oceanologia e Limnologia, 16:29-36.

Seilacher A. (1984). Constructional morphology of bivalves:evolutionary pathways in primary versus secondary soft-bottom dwellers. Palaeontology, 27 (2): 207-237.

Soliman G.N. (1973). On the structure and behaviour of therock-boring bivalve Rocellaria retzii (Deshayes) from theRed Sea. Proceedings of the Malacological Society of London,40: 313-318.

Sowerby G.B. (1854). Popular British Conchology. 304 pp.,John Edward Taylor, London.

Sowerby G.B. (1887). Illustrated Index of British Shells, 2nd

edition. 16 pp., 26 pls. Simpkin, Marshall & Co., London.Taylor J.D., Glover E.A. & Braithwaite C.J.R. (1999). Bivalves

with “concrete overcoats”: Granicorium and Samarangia.Acta Zoologica, 80: 285-300.

Manuscript received 18 July 2005Revised manuscript accepted 06 September 2005