predation and intertidal zonation of barnacles at leigh, new zealand

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Predation and intertidalzonation of barnacles atLeigh, New ZealandPenelope A. Luckens aa Department of Scientific and IndustrialResearch , New Zealand OceanographicInstitute , P.O. Box 8009, Wellington, NewZealandPublished online: 30 Mar 2010.

To cite this article: Penelope A. Luckens (1975) Predation and intertidalzonation of barnacles at Leigh, New Zealand, New Zealand Journal of Marineand Freshwater Research, 9:3, 355-378, DOI: 10.1080/00288330.1975.9515573

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1975] 355

PREDATIQN AND INTERTIDAL ZONATION OFBARNACLES AT LEIGH, NEW ZEALAND

PENELOPE A. LUCKENS

New Zealand Oceanographic Institute, Department of Scientific andIndustrial Research, P.O. Box 8009, Wellington, New Zealand

(Received 10 March 1972; revision received 20 September 1974)

ABSTRACT

Experiments using cages at four shore levels were carried out to determine thepart played by the feeding of the gastropods Neothais scalaris and Lepsiellascobina on the zonation of three species of intertidal barnacles: Chamaesiphobrunnea, C. columna, and Epopella plicata. The lower limit of the two largerbarnacle species was determined by predation, but C. columna was affected onlyin the absence of the other two barnacles.

INTRODUCTION

The zonation of barnacles on the shore at Goat Island Bay, Leigh,results from the interaction of many factors, as on shores in other partsof the world (Connell 1961a, b). The effects on zonation of breedingand settlement behaviour, emersion and submersion times (Luckens •„1970a), and temperature (Foster 1969, 1971), and competition (Luckens1975) have been published already for barnacles at Leigh. Furtherdetails of the location, the experimental reef, the four experimental levels,the height distribution of the main species, and the experimental methodsare available in Luckens (1970a) (see also Figs 11 & 12). The zonationof the area is outlined in Morton & Chapman (1968).

Experiments are described here on the effects of predation by thegastropods Lepsiella scobina and Neothais scalaris on barnacle zonationat Goat Island Bay, Leigh, carried out during 1964-66. The only workavailable on the gastropod predators was that of Fearon (unpublished1962) which deals with the thaid L. scobina, its biology and behaviourparticularly in the South Island (Lyttelton, Christchurch, Nelson, andKaiteriteri). This was supplemented by my observations at Goat IslandBay, and at West Tamaki Head and Piha, Auckland. The Lepsiellafound at West Tamaki Head, Leigh, and Piha were all regarded asbelonging to the one species designated here as L. scobina.

At Leigh the common thaid gastropods which feed on barnacles areL. scobina and N. scalaris (Figs 1 & 2). Haustrum haustorium, anotherthaid, was also present and although it had not been seen eating barnacleson the shore, it was caged with them to see if it would eat them.

N.Z. Journal of Marine and Freshwater Research 9 (3): 355-78

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356 N.Z. JOURNAL OF MARINE & FRESHWATER RESEARCH [SEPT.

FIG. 1—Lepsiella scobina feeding on Chamaesipho brunnea cemented within theC. columna zone near the lower middle level on the experiment reef, GoatIsland Bay, Leigh, New Zealand.

METHODS

The study area was on the seaward side of a reef at Goat Island Bay,Leigh, on the east coast 96 km (60 miles) north of Auckland. Concreteframes 20 cm (8 in.) square internally and 5 cm (2 in.) deep werecemented to the shore at the four experimental levels (see Figs. 11 &12). Rocks covered with the appropriate barnacles for the experimentwere cemented inside, and fibreglass screening was attached across thetop of each frame.

The four levels where the frames were attached were:A. Highest Level, in the Chamaesipho brunnea (barnacle) zone

at the top of the reef about 0.5 m above high water neaps;B. Upper Middle Level, at the top of the Epopella plicata

(barnacle) zone at about the level of high water neaps;C. Lower Middle Level, in the Chamaesipho columna (barnacle)

zone about 0.3 m above the top of the brown algal zone; andD. Lowest Level, about 0.3 m below the top of the brown algal

zone, between low water neap level and extreme low waterspring tide level.

Before the start of each experiment the number of dead and livebarnacles was carefully noted, then the weighed and counted predatorygastropods added. Within any experiment every effort was made toensure that all the barnacles used as prey at each of the four levels wereof a similar size and present in similar numbers, similarly distributedacross the floor of the frame. For each experiment the barnacleswere cemented at each experimental level at the same time, while the

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1975] LUCKENS—PREDAT10N ON BARNACLES 357

.-* iifc'v

FIG. 2—Neothais scalaris feeding on Epopella plicata at the lowest level on theexperimental reef, Goat Island Bay, Leigh, New Zealand.

predators were added, again simultaneously, 12-36 h later, when thecement had set satisfactorily and had been submerged for at least onetide cycle.

A control frame containing barnacles but no predators was also setup at each level. All frames were inspected every few days, and thenumber of dead and live barnacles counted. Experiments were stopped3-4 weeks after the predators were added, well before all the barnacleshad been eaten, to avoid scarcity of prey affecting feeding rates. Therocks were removed from the frames, with as much cement as pos-sible. Another set of barnacle-covered rocks was then cemented in,ready for the next experiment. The removed rocks with their attachedbarnacles were taken back to the laboratory for further examination.

A list of all the animals eaten by the three gastropod predators,whether as part of an experiment or incidentally either in the frames oron the shore at Leigh, is given in Table 1.

RESULTS

BREEDING BIOLOGY OF THE GASTROPOD PREDATORS

At Christchurch, Lepsiella albomarginata copulated earlier in springthan L. scobina, and had another breeding season in autumn. Spawning

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358 N.Z, JOURNAL OF M A R I N E & FRESHWATER RESEARCH [SEPT.

TABLE 1—Species of animals eaten at Leigh by the three predators Lepsiella scobina, Neothaisscalaris, and Hauslrum haustorium both on the shore and in the experimental frames.

rREDATOR

Lepsiellascobina

Neothaisscalaris

Haustrumhaustorium

PREY

Xenostrobus pulexCrassostrea glomerataMelagraphia aelhiopsLunella smaragdaNerita melanotragusCellana spp.Siphonaria zealandicaPomatoceros cariniferusChamaesipho columnaChamaesipho brunneaBalanus trigonusElminius modestusEpopella plicata (small)Tetraclita purpurascensMelagraphia aethiopsLunella smaragdaCrassostrea glomerataCellana ornataCellana radiansChamaesipho brunneaEpopella plicataElminius modestusBalanus decorusMelagraphia aethiopsLunella smaragdaSiphonaria zealandicaCellana ornataCellana radiansChamaesipho brunneaEpopella plicata

ON SHORE

When availableYesYesY e s •

Not ObservedOccasionallyOccasionally

YesYes

OccasionallyYesYesYesYes

OccasionallyOccasionally

YesYesYes

Not observedYes

RarelyRarely

YesYes

Usual preyYesYes

Not observedNot observed

IN FRAMES

YesNot tested

YesYes

OccasionallyIncidentallyNot testedNot tested

YesYes

Not testedYesYesYesYesYes

Not testedNot testedNot tested

YesYes

DoubtfulNot tested

YesYes

Not testedYesYesYesYes

BORED

YesYesYesYesN o

SometimesSometimes

YesNoYesYesYesYesYesNoYesYesN oNoNoNoNoYesNoYesN oNoNoNoNo

occurred more than a month after copulation. At certain times of theyear, all females of both species showed evidence of fertilisation, butthe frequency of spawning was reduced because of the lack of suitableshelter for egg deposition. Initiation of copulation in Lepsiella wascorrelated with mean sea temperatures and lunar phases, but initiationof spawning was thought to be dependent on the availability of shelterfor the deposition of egg capsules (Fearon unpublished 1962).

At West Tamaki Head in the Auckland Harbour, recently laid eggcapsules were seen under loose stones at all times of the year. Many ofthe egg capsules had been bored and eaten out by the numerous adultLepsiella congregating under the rocks. At Leigh, where suitable rocks orledges were not generally available, egg capsules were laid in the shellsof dead Epopella plicata, among clusters of living E. plicata, or on themain stems of algae such as Laurencia.

Little is known of the breeding of either Neothais scalaris or Haustrumhaustorium, but at Leigh egg capsules of both species containing livingembryos have been found in June, July, and November.

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GROWTH AND POPULATION STRUCTURE OF THE GASTROPOD PREDATORS

At Christchurch crawling larvae of Lepsiella were 1 mm long athatching; after 15 weeks they reached a length of 5 mm, and at 6months a body whorl diameter of 7.5 mm. Growth was most rapid inthe first 8 months and tailed off over the next 16 months. Shore condi-tions did not favour continuous feeding, and growth rates were lowerthere than in the continuously fed laboratory specimens. It was unlikelythat feeding occurred during June, July, and August, when lowertemperatures prevailed at Christchurch.

On the shores at Auckland and Leigh there was no evidence ofreduced feeding in the winter, nor was there any downward migration asin the South Island at Kaiteriteri, Nelson, and Lyttelton. As egg-layingoccurs over most of the year at Leigh, Lepsiella scobina of all sizeswere usually present at all times. There was, however, a marked differ-ence in maximum size of populations on different parts of the shore,which appeared to be related to available food. Whether the differencein size reflected differences in growth rates or differences in feedingpreferences is not known definitely. Animals found in areas where onlyChamaesipho columna was available as food were the smallest, whilethose found among the mussel Xenostrobus and the rock oyster Crasso-strea had the largest sizes (Fig. 3).

The largest Neothais scalaris at Leigh were much smaller than thoseat Piha, and the frequency of small specimens was greater at Leigh.Nothing is known of the growth rates of this species or of the relatedHaustrum haustorium, which also occurred at Goat Island Bay.

EFFECTS OF STARVATION ON GASTROPOD PREDATORS

Fearon (unpublished 1962) found that Lepsiella would survive for upto 4 months without food. Under experimental conditions Lepsiellacaged without food at the highest experimental level (within the Chamae-sipho brunnea zone) showed no mortality after 1 month, 27% mortalityafter 3 months, and 70% mortality after 4.5 months. Here the effects ofstarvation were reinforced by a reduction in submergence time.

In the laboratory, Lepsiella survived for longer periods without food(more than 6 months) than in the shore experiments. Animals confinedin bare cages on the shore became dislodged in rough weather, and notonly damaged their shells, but also dislodged other Lepsiella. Suchdamage would not occur under normal conditions. No evidence ofcannibalism was ever seen among the Lepsiella at Leigh, although it wasreported by Fearon (unpublished 1962) to occur at Christchurch.

EFFECTS OF PHYSICAL FACTORS ON THE GASTROPOD PREDATORS

On the experimental reef at Leigh Lepsiella scobina occur from justabove the second highest experimental level down to the barnacle limitbetween the lowest two experimental levels (limits of natural Lepsiellapredation in Figs 11 & 12). Neothais scalaris is normally found lower

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on the shore than Lepsiella, with few individuals being found above thelower middle experimental level. Subtidally, Neothais extends down tothe gently sloping sand bottom below the reef at 18-24 m (60-80 ft).

When Lepsiella were enclosed in a frame at the top experimentallevel, with Chamaesipho brunnea, 70% died within 2 months (in Novem-ber-December). The frames offer more shelter from desiccation thanthe open barnacle-covered rock surface, so it is likely that the survivalrate under normal conditions on the shore would be less at this levelthan in the frames. When not restrained on the shore, the Lepsiellaretreat to a lower level. The experiment was repeated at the beginningof February without including barnacles. After 3 months Lepsiellascobina had suffered 27% mortality and after 4.5 months L. scobina,Neothais scalaris, and Haustrum haustorium had suffered 70%, 10%,and 40% mortality respectively.

From these results it appears that Lepsiella would not survive at thehighest experimental level throughout the summer, although survivingthere for short periods with cool, damp weather, or in protected crevices.

Tn this experiment Neothais scalaris seemed surprisingly resistant todesiccation, compared with Lepsiella and contrasted with later predationexperiments at this level when Neothais died. Fearon (unpublished 1962)has shown that the temperature difference between total survival andtotal mortality was only l-2°c. Thus one or two particularly hot dayscoinciding with midday low tides and calm seas may have a greatereffect on mortality than a longer period with temperatures severaldegrees lower.

Fearon (unpublished 1962) noticed the clustering behaviour shown byLepsiella albomarginata when heated or cooled to the limits of itstemperature range for general activity, and supposed that such aggrega-tion reduces evaporation and desiccation. He found a correlationbetween water loss, animal size, and shore height; i.e., larger animalslost proportionately less water than small animals and were found higheron the shore. At Leigh, when comparing the sizes of Lepsiella fromthe upper and lower limits of Chamaesipho oolumna, both the meanand extreme sizes were found to be greater at the higher level. Thissupports Fearon's results.

In laboratory tanks having a constant water level, L. scobina tendedto climb up out of the water, and sometimes to remain there until death.In tanks fitted with a siphoning device to empty the tank periodically,the snails stayed below the upper water line. Lepsiella appear to belimited to their shore positions by these reactions. They move littlewhen the tide is out, but unless feeding intervenes, move upwards whensubmerged. Fearon (unpublished 1962) concluded that negative geo-tropism, negative phototrophism, rugotropism, and gregariousness prob-ably all play some part in preventing desiccation of Lepsiella on theshore.

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30-

20-

10-

0

30

20-

10-

0OLU

§ 30-1LU

cr

10-

o

30-

Crassostrea and large Xenostfobus

Chamaesipho columna and small Xenostrobus

C. columna

C. columna < 1mm diameter

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16Lepsielia BODY WHORL DIAMETER (mm)

FIG. 3—Relationship between size and diet in Lepsielia scobina. The smallestL. scobina feeding on Chamaesipho columna were collected in May, theother samples in July.

SPECIES EATEN BY GASTROPOD PREDATORS

At Leigh most of the Lepsielia were found on shores dominated eitherby barnacles (in particular Chamaesipho columna and Epopella plicata)or by Xenostrobus, and these formed the major part of the diet of theLepsielia. At the upper part of their range, Lepsielia ate C. brunnea andthe oyster Crassostrea glomerata. On the shore Lepsielia also ate tube-worms (Pomatoberos), various gastropods, and other barnacles in-cluding Balanus spp. and Tetraclita.

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When Lepsiella were confined in cages with other animals, they atecertain of them (see Table 1). Of these, some were bored but in othersthere was no apparent damage to the shell of the prey. The Lepsiellawere observed actually feeding on these specimens, not merely restingon or near them. In experiments on feeding rates in relation to size andshore height, the only prey species investigated systematically werebarnacles.

Neothais scalaris is usually found lower on the shore than Lepsiellaand at low tide is rarely found feeding; it extends well below low tidelevel. At Leigh, N. scalaris eats large Epopella plicata; at Piha, wherePerna forms extensive beds, this gastropod is a common predator of thismussel.

Under experimental conditions Haustrum haustorium will eat bothEpopella plicata and Chamaesipho brunnea, but on the shore it wasusually found feeding on molluscs. In a cage it ate Melagraphia, Cellana,and Lunella, but in a shore survey at Leigh Siphonaria zealandica wasfound to be the commonest prey.

SUITABILITY OF BARNACLE SPECIES AS PREY IN EXPERIMENTS

Although the original plan was to use as many sizes and species ofbarnacles as possible at all four experimental levels, it became obviousthat with certain species the results did not justify the efforts involved.

In the experimental frames it proved extremely difficult to tell whetherChamaesipho columna were alive or dead. At the highest levels mortalitydue to physical factors was high, and difficult to distinguish from preda-tion without microscope examination.

Adult Chamaesipho brunnea ( > 3 mm diameter) were the best ex-perimental material, as they survived well in the frames at all levels,were large enough to be seen clearly, and were eaten readily by thepredators. Specimens less than 1 y old were difficult to find in largeenough numbers for experimental purposes, and, like C. columna, itwas hard to distinguish whether they were dead or alive under experi-mental conditions on the shore.

Fully grown Epopella plicata were too large to be eaten by Lepsiellaand most of the frames were too shallow to include many large Neothais.Specimens under 1 y old were not available in sufficient numbers, andinformation on their mortality was restricted to shore observations.

Adult Elminius modest us were readily available at Leigh Cove (3 kmto the south-east) but, like C. columna, they had a high mortality at theupper experimental levels except in the winter.

FEEDING BEHAVIOUR OF THE GASTROPOD PREDATORS

Although there is undoubtedly a certain amount of individuality infeeding behaviour, certain prey species were usually attacked in certaindistinct ways.

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In spite of detailed examinations there was no positive proof thatChamaesipho columna were bored by Lepsiella. The plates appearedundamaged, and whether they were forced apart, before or after smother-ing, is not known. Elminius modestus, Tetraclita, and Epopella plicatawere usually bored through their apical plates, often at their junction,but sometimes the holes were in the parietal plates, particularly if thesewere more accessible (see Luckens unpublished 1964). Xenostrobus,Crassostrea, and Pomatoceros were bored, with no particular prefer-ence for the location of the hole. In both oysters and mussels manydead valves contained more than one hole, and Lepsiella were seendrilling through the valves of dead oysters. Cellana spp. and Siphonariawere sometimes bored, usually near the apex of the shell.

The greatest specificity of boring site was shown when feeding onLunella and Melagraphia. These were bored on the upper half of thebody whorl opposite the operculum. In general, the proportion of preyanimals which were bored by Lepsiella is related to the type and thick-ness of the operculum and shell. No Nerita (thick shell and thick oper-culum), a few Melagraphia (thin, horny operculum and thick perio-stracum), and almost all the Lunella (thick, calcified "cat's eye"operculum and thin shell) in the frames were bored by Lepsiella.

Neothais scalaris did not appear to bore into either Chamaesiphobrunnea or Epopella plicata. At Leigh and Piha, however, many E.plicata were found with their apical plates stained purple, presumablyby secretion from the purple gland of Neothais. Such staining wasfound on both living barnacles and inside the tops of the parietal platesof dead barnacles. In the related American species Urosalpinx cinerea,boring has been shown to consist of two phases. During the first phase,the accessory boring organ is closely applied to the shell. This secretesan acid substance which weakens the shell so it can be more easilyrasped away by the radula in the second phase (Carriker et al. 1967).Neothais is known to bore Crassostrea glomerata, Perna canaliculus,and Lunella smaragda, but no evidence of purple colouration has beenseen on them at Leigh. Both Pope & Federighi (cited in Carriker 1955)found that the oyster drill U. cinerea made perforations all over thesurface of the oyster valves and that there appeared to be no selectionfor depressions, thin areas of shell, or the limits of the adductor muscle.No particular area is favoured by Neothais as a boring site in oystersand mussels. Where mussels are found in dense beds, most holes arebored near the wider posterior end, presumably because it is moreaccessible to the Neothais, as with barnacle predation at West TamakiHead (Luckens unpublished 1964).

In contrast to the lack of site specificity while boring bivalves, Neothaisshows a marked preference for a particular boring site on Lunellasmaragda, a preference shared by the related thaid Haustrum haustorium.Holes were bored near the operculum on, or closely adjacent to, anacreous patch on the parietal wall.

Melagraphia was not bored by either Neothais or Haustrum and itappeared likely that entry was forced.

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BEHAVIOUR OF GASTROPOD PREDATORS IN RELATION TO THEIRDISTRIBUTION

Lepsiella scobina occurs occasionally at the highest experimental level,slightly more commonly at the upper middle level, and abundantly atthe lower middle experimental level. At the lowest experimental level,an occasional Lepsiella was found, but was thought to have been washeddown from a higher level.

Under experimental conditions on the shore at the upper levels,particularly the uppermost, the Lepsiella tended to gather in the lowestcorner of the cage irrespective of whether there were barnacles there ornot. At the lower middle level, Lepsiella could be found in any partof the cage, with no particular aggregations except when feeding on thebarnacles. At the lowest level, there was a marked tendency for theLepsiella to collect as high as possible in the frame, often away from thebarnacles.

Intertidally, maximum densities of Neothais scalaris are found at lowtide level, i.e., at and below the lowest level. Where the shore is brokenby numerous crevices and cracks and is kept moist while the tide is out,Neothais is found higher on the shore. On the reef where most of theexperimental work was done, Neothais were rare above the bottom ofthe Epopella plicata zone except in the dampest and largest crevices.Further along the shore on a sandstone reef rising abruptly from deepwater to just above low tide and then sloping moderately to high tidelevel, Neothais could be found among and feeding on Chamaesiphobrunnea only when the weather was damp and overcast, and when thewaves were breaking well up the reef. In both places, the Neothais wouldhave been wetted for at least 50% of the time.

In the experimental cages, Neothais were seldom found feeding duringlow tide. Usually they would be in any depressions at the bottom of thecage, often away from the barnacles.

Neothais is rarely found feeding when the tide is out, and this mayrestrict it to the lower levels of the shore, where more time is availablefor feeding. Where small Neothais were feeding on young E. plicata,barnacles were found with a depression bored at the junction of theplates, but the barnacles were still alive, and the holes had not penetratedright through the shell. These depressions may have been caused byNeothais which started boring too late in the tide cycle to complete thehole before the tide went out, and which then moved to a more sheltered(damper) place. If the Neothais does not return to the same barnacleand continue boring, all the effort expended in partly boring the hole hasbeen wasted (see also Connell 1970).

FEEDING RATES OF GASTROPOD PREDATORS IN RELATION TO THEIRDISTRIBUTION

If feeding or boring occurs only when the predator is submerged, thenthe number of barnacles eaten may be expected to be proportional tosubmergence time, and hence to their height on the shore.

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lowest

10 20DAYS

FIG. 4—Feeding rate of 10 Neothaisscalaris weighing 5.5 g each onChamaesipho brunnea at the 4experimental levels.

FIG. 5—Feeding rate of 20 Neothaisscalaris weighing 5.0 g each(solid lines) and two Neolhaisscalaris weighing 50 g each(broken lines) on Chamaesiphobrunnea at the two low experi-mental levels.

In species where feeding can occur out of water, the behaviour ofthe predator in response to other factors may alter its feeding rates atcertain levels on the shore. A predator such as Neothais near its upperlimits on the shore is usually found in damp and shaded crevices whenthe tide is out. Thus, although the predator may not require submergenceto feed, it may seek shelter from desiccation when the tide recedes,irrespective of whether it is feeding or not. At high levels, avoidance ofdesiccating conditions restricts the predator to the food available in thedamper "refuges". Under experimental conditions where no escape ispossible, starvation, desiccation, and death will result.

It may be expected, therefore, that feeding rates will be maximal lowon the shore, and decrease progressively at higher levels.

With Neothais, a species which extends at Goat Island Bay from atleast 24 m (80 ft) below low tide level to about mid-tide level, feedingis likely to be curtailed above this level. Lepsiella, on the other hand,occurs most abundantly at the middle experimental levels, and feedingcould be reduced above and below these levels.

Are these theoretical considerations justified by the results actuallyobtained?

Because of the variation in the feeding rate through the experiments,the low feeding rates obtained, and the differences in feeding rates withsize and with species of predator, the results are graphed as total numberof barnacles dead, against time (Figs 4-7).

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In each experiment the barnacles were as uniform in size as possible.Since the different species of barnacles differed in size, experiments usingdifferent barnacles as prey cannot be strictly compared.

Of the experiments with Neothais as predator, that using Chamaesiphobrunnea as prey at all four levels gives results closest to those expected(Fig. 4). Ten Neothais weighing 5.5 g each were used at each level. Theresults shown in Fig. 5 compare the feeding of 20 Neothais weighing5 g each and 2 Neothais weighing 50 g each feeding on C. brunnea atthe two lowest experimental levels. The feeding rate of the smallerNeothais is considerably greater at the lowest level, but for the largerNeothais the feeding rate is similar at both levels.

Since Lepsiella is common at the two middle levels and rare higher orlower, predation should be greatest at these levels. Of five experimentsconducted at all four levels concurrently, three showed maximum pre-dation rates at the lower and two at the upper of the middle levels. Ina further five experiments where results were not obtainable from allfour levels, all except one (Fig. 6) showed maximum predation at theupper middle level (Fig. 7). An experiment using Chamaesipho columna(in which mortality from desiccation is usually higher at the upperlevels) was an exception and was further complicated by the difficultyof distinguishing dead from living barnacles. The low mortality ratesat the two other levels of this experiment also suggest that the previouslystarved Lepsiella used as predators were not feeding normally.

FEEDING RATES AND SIZE OF GASTROPOD PREDATORS

Since, in general, growth rate decreases with increasing size and ageof animals, food intake per gram of body weight should also decreasewith increasing size of predator. According to Fearon (unpublished1962) the growth rate of Lepsiella does decrease with increasing size,and the above statement should hold. To test this an experiment withElminius modestus as prey was set up. Groups of 40, 80, and 160Lepsiella were used, each group having a total weight of 50 g. Thefollowing results were obtained after 3-4 weeks:

Number of predatorsWeight of one predator (g)Number of prey eatenNo. prey/g predator

A further experiment of a similar duration using Chamaesiphobrunnea as prey and Lepsiella as predator gave the following results:

Number of predators 160 40Weight of one predator (g) 0.3 1.25Number of prey eaten 230 124No. prey/g predator 4.6 2.5

In both these experiments a fourfold increase in weight of individualpredator results in the number of prey eaten being almost halved. Thus,food intake per gram of body weight does decrease with increasing sizeof predator.

1600

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<UJzz_ J

on

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0 10 20DAYS

FIG. 6—Feeding rate of 20 Lepsiellascobina weighing a total of 25 g onChamaesipho brunnea at three ex-perimental levels.

highest

10 20DAYS

FIG. 7—Feeding rate of 80 Lepsiellascobina weighing a total of 25 gon Chamaesipho brunnea at thethree highest experimental levels.

RELATIVE SIZES OF PREDATOR AND PREYFrom previous work on other thaids (Connell 1961a, b), Lepsiella

and Neothais were expected to prefer larger barnacles where available.(The term "available" is used here to mean not only the presence of thebarnacles, but the predator's ability to eat them. Thus, some barnaclesmay be too large to be bored or forced open by some smaller predators.Such barnacles, though present, are unavailable to the predator.)

At the other end of the scale, the largest predators were not expectedto eat the smallest barnacles. In both predator and prey the size rangewas wide. Newly settled barnacles 0.5 mm long weigh less than 0.0014 g,large Chamaesipho brunnea may weigh 1 g or more (Fig. 8), and someof the Epopella plicata 25 mm in diameter and 20 mm high weighed6 g or more (Fig. 9). Lepsiella used experimentally ranged in weightfrom less than 0.5 g to 1.5 g while the Neothais weighed up to 60 g(Fig. 10). All animals were prodded and blotted to remove excess waterbefore being weighed in their shells.

The size range of food eaten by any particular predator was foundto be very wide. Upper size limits were related to boring capability andimmersion time, and were more definite than lower limits, which seemedto be altered much more by circumstances, e.g., if only small barnacleswere available these might be eaten, whereas if larger barnacles werepresent too then the small barnacles would not be eaten until all thelarger ones were gone.

Under experimental conditions and on the shore, large Lepsiella(1.25 g total wet weight) ate Chamaesipho columna larger than 1 mmacross, E. plicata up to 10 mm basal diameter, and C. brunnea of all

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1-0-

Li)

<

0-5-

o8®

5 10 15CARINO-ROSTRAL DIAMETER (mm)

t-I

LUI

10

specimens fromfused sheets

high levelisolated specimens

0 5 10 15CARINO-ROSTRAL DIAMETER Cmm)

FIG. 8—Relationship between total wet weight, shell height, andcarino-rostral diameter in Chamaesipho brunnea.

sizes. They did not eat larger E. plicata, and seldom ate any conicalbarnacles of less than 1-2 mm basal diameter.

Small Lepsiella (0.3 g total wet weight) would eat 1-2 mm diameterbarnacles, and almost the same range of larger barnacles as the largeLepsiella. However in experiments using large C. brunnea or E. plicatanear the 10 mm diameter limit, the largest specimens often remaineduneaten at the end of the experiment.

When rocks covered with C. brunnea, E. plicata less than 10 mmdiameter, or adult Elminius modestus were cemented on the shore among

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6-

I

o

12 16 20CARINO-ROSTRAL DIAMETER

24

(mm)

28

FIG. 9—Relationship between total wet weight and carino-rostraldiameter in Epopella plicata.

C. columna below the E. plicata zone on the main reef, the Lepsiellaaggregated to feed on them. The Lepsiella did not move away from thecemented rocks until almost all the larger barnacles were eaten (seeFig. 1).

These reactions show that the Lepsiella had a preference for thelargest barnacles that they were capable of eating, but it could havebeen either a size preference or a preference for a change of food. Sinceit occurred when the food size was increased regardless of the species ofbarnacles used, it was not an inherent preference for a certain speciesof barnacle.

From these results and from the size of Lepsiella round feeding ondifferent species and sizes of prey on the shore, it appears that largeLepsiella eat larger prey when it is available, but the size of food eatenis highly dependent on that available.

With Neothais there was a similar wide range in prey size. At theextremes, large Neothais (weighing 50 g or more) occasionally ate C.columna, but it was doubtful whether small specimens (weighing 1 g orless) could eat the largest adult E. plicata. Certainly 10 g Neothais atethe largest barnacles available during the experiments, including Balanusdecorus more than 20 mm in diameter and 30 mm high.

Although a predator of any given size has a wide range of both speciesand sizes of prey available to it, yet some sizes and species of barnaclesare safe from predation by virtue of their position on the shore. Small,recently-settled barnacles are readily eaten by small Lepsiella found onthe shore at all times of the year. However, small Lepsiella are usually

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found in patches close to suitable egg-laying sites. Barnacle mortality inthe first month after settlement is higher than 90%. The largest E. plicata(i.e., those older than 3 y) are too large to be eaten by Lepsiella and,where Neothais is absent, these are free from thaid predation.

FEEDING RATES DURING AN EXPERIMENT

During the course of each predation experiment, the number of deadbarnacles was noted at intervals. The rate of feeding altered throughoutthe experiment. At the start of an experiment the predators were some-times slow to start feeding.

The feeding pattern of individual predators was not studied in theNew Zealand species, but with the similar Japanese species Ocenebrajaponica the time taken to bore and eat small Mytilus edulis variedwidely (Luckens 1970b). All Mytilus eaten by Ocenebra were bored andeaten through the bored hole. One individually marked Ocenebra wasfound feeding on two different Mytilus at two examinations 12 h apart,but the length of time spent boring and feeding on one mussel could lastup to 60 h. The time spent boring depended partly on shell thickness.Thicker shelled Septifer (mussels) took longer to be bored by Ocenebra.In experiments using Septifer as prey, the number of Ocenebra boringand feeding on the mussels was always higher than when Mytilus wereused.

Besides variation in the time taken to bore and eat either a barnacleor a mussel, there were wide variations in behaviour. The time spentcrawling or resting far exceeded that spent boring and feeding (Luckens1970a).

When the Goat Island Bay results are examined, it is found that lessthan one barnacle was eaten by each predator in a day. If one Lepsiellaeats 0.0625 barnacles a day (and rates as low as this were obtained),this is equivalent to eating one barnacle every 16 days. This rate offeeding seems a little slow when compared with Urosalpinx cinereafeeding on 0.2-33.0 oysters/week (Carriker 1955).

When Ocenebra was fed Chthamalus challengeri (flesh weight 0.01 g)each ate an average of 0.92 barnacles a day, and when fed Mytilus (10-20 mm long, flesh weight 0.0822 g) each ate an average of 0.686-0.1108mussels a day (Luckens 1970b).

If the observation times are closer together, and each animal isindividually marked, a more complete picture of feeding behaviour canbe gained. In this way, day-to-day changes in rate of feeding can bemeasured more accurately, and related to such possible causes as hoursof submergence, or weather. That predators such as Lepsiella andOcenebra do not feed all the time is amply demonstrated both in theexperiments and on the shore by the numbers of predators found in themidst of food but not actively feeding. This could be either the resultof physical factors, such as emersion, which may prevent feeding orboring, or it could be due to physiological repletion. Certainly in

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3 LEPSIELLA SCOBINA

" 0 4 8 12BODY WHORL DIAMETER (mm)

16

BODY WHORL16

DIAMETER (mm)

FIG. 10—Relationship between total weight and body whorldiameter in Lepsiella scobina and Neothais scalaris.

Lepsiella, feeding can at least continue while emersed. With Neothaisfew specimens are found feeding while the tide is out, and in this speciesfeeding may be actually inhibited by emersion. As the predators didnot feed as readily in laboratory experiments as when caged on theshore, the laboratory experiments were discontinued.

DISCUSSION

Both on the shore and under experimental conditions the gastropodsLepsiella scobina and Neothais scalaris ate a wide range of prey includ-ing barnacles, bivalves, and other gastropods, but Haustrum haustoriumfed only on gastropods.

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FIG. 11—Relative effect of each major factor affecting zonation of Chamaesipho colurnna on the experimental reef, GoatIsland Bay, Leigh. Horizontal broken lines indicate the four experimental levels. For 'Settlement', inner line representsnatural conditions, and outer line represents settlement on recently cleared rock, for 'Lepsiella predation', inner linerepresents that occurring naturally, and outer line shows predation in experimental frames.

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The ideal prey species for such experimental work should survive atall experimental levels, dead specimens should be readily distinguishedfrom living ones, and if possible the cause of death should be easilydetermined. In practice, almost all the prey species were adverselyaffected by increased emersion times at the highest level.

The method of gaining entry to the prey varies. The sites used byLepsiella and Neothais when boring the gastropod Lunella were onopposite sides of the shell and were species specific, but when bivalvesor barnacles were bored, the hole was usually located on the most easilyaccessible part of the prey shell. Some species were eaten without beingbored.

Predation was maximal towards the centre of the shore range of eachpredator (Figs 11 & 12). The reaction of the predators which normallyrestrict them to certain levels on the shore resulted in decreased feedingrates outside these levels in shore experiments. The predators tended toaggregate in the part of the frame nearest their normal shore height,irrespective of whether there were prey there or not. Thus predatorbehaviour and feeding rates were closely related to the normal shoredistribution of the predator. Where a predator has its centre of distribu-tion below the intertidal zone, intertidal predation rates may appearproportional to submergence time.

Food intake per gram of body weight decreases as predator sizeincreases in Lepsiella. Both Lepsiella and Neothais prefer to eat thelargest prey that they can successfully attack. The largest Epopellaplicata are not attacked by Lepsiella, but are readily eaten by Neothais.In general, small predators eat small prey, but while maximum prey sizeis dictated by the ability of the predator to gain entry to it, the smallestbarnacles are usually not attacked by large predators even in the absenceof other food.

Feeding rates are not constant during an experiment, feeding isdiscontinuous, and for a large proportion of the time the predators donot feed although food is available.

PATTERNS OF PREDATION ON BARNACLES

To simplify discussion, postulate that the area under consideration iscovered evenly with barnacles of a suitable size for predation and ofsimilar age. The predators, all of a similar size, are at present groupedin crevices and on feeding areas just outside the area under consideration.

Gradually the predators will encroach on the area, feeding as theymove. Since they were aggregated before they entered the area, they willprobably remain in groups. There is a tendency for a predator such asLepsiella to eat almost all the barnacles as they move slowly into thearea, avoiding areas which have already been eaten out. This behaviourresults in:

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Desiccation

Adult Settlementdistribution

Predation Competition Growth PotentialLepsiella Neothais C.columna ' range

Algalcompetition

FIG. 1 2 — i 5 f I °{ ma,]-°r f̂ Ct,°rS aff^tmg nation of Epopella plicata on the experimental reef at Goat Island BaVLeigh. Horizontal broken lines indicate the four experimental levels. For 'Settlement', inner line represents natural con-Jitions, and outer line represents settlement on recently cleared rock; for 'Lepsiella predation' outer line shows predationin experimental frames, and inner line shows that occurring naturally (Lepsiella predation is usually restricted to Epopella<10mm dmmeter). Potential range' is range which could be occupied by Epopella in absence of competitor Indpredators.

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(a) Patches containing mainly dead barnacles;(b) Patches where the Lepsiella are actually feeding on the

barnacles; and(c) Patches of live barnacles untouched by the predators.

With removal by the sea of the dead barnacle plates, the rock surfaceis once more available for settlement by barnacles. Since the predatorhas a preference for larger barnacles, newly settled specimens growunmolested by predators, until they reach "edible size", determinedpartly by the actual size of the predator, but also affected by the generalavailability of food in the area.

If the area is large enough it will develop into a mosaic of patches ofthe following types:

(a) Patches of mainly dead barnacles;(b) Patches where the predator is feeding actively;(c) Patches of undersized barnacles, including newly settled

specimens;(d) Patches of adult "edible size" barnacles which have not yet

been eaten; and(e) Patches of rock available for settlement.

The predators, in moving through the area, will avoid the areas ofundersized and dead barnacles and tend to remain close to areas ofpotential food.

When predators of different sizes are found in the area, the size rangeof barnacles suitable for food is widened, but at no time are any speci-mens unavailable to the predator because of large size. The situationprobably becomes more complex, and the patches become less distinct.The larger barnacles are still eaten preferentially, allowing newly settledones to grow unmolested.

For a species or population to have continuity it must produceprogeny. Since older, larger barnacles produce more larvae, thisdifferential predation might be regarded as harmful in the long run.Howeyer, as long as a certain percentage of the population is able tobreed, it is likely that the species will survive. With barnacles the plank-tonic phase adds one further complication, but as these species arewidely distributed, and as the same conditions are likely to occur atthese different places, this complication can be disregarded in thisdiscussion.

The species survives because of the spatial separation of predator andprey, brought about by preferential feeding by the predator, and con-tinuous reproduction allowing rapid resettlement of bare areas by theprey. This situation occurs in sheltered waters where extensive areas arecovered with Elminius modestus preyed upon by Lepsiella.

If the discussion is extended to include a species such as Epopellaplicala which can live for several years, in contrast to the usual life spanof a year or less in Elminus modestus and Chamaesipho columna, certainother factors must be considered. When Lepsiella is the predator, those

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E. plicata that survive to a size of about 10 mm basal diameter are nolonger available to the predator. Instead of the oldest barnacles beingsystematically removed from the population by the predators, there isan accumulation of large barnacles. As long as these barnacles are closeenough for cross-fertilisation the continuity of the species is assured.

The final result depends on the intensity of predation. If predatorsare abundant, few if any barnacles will survive to a size where theybecome immune to predation. Where predation is light many barnaclesreach this large size. Subsequent settlements on and around the largebarnacles result in clumps of E. plicata.

Where large predators such as Neothais occur, even the largestbarnacles will be available to the predator providing that they are lowenough on the shore to allow sufficient feeding time for the predator.When this predation is continual, few barnacles will survive long enoughto reproduce. Where both Lepsiella and Neothais are present, thesituation is similar to that obtaining where C. columna or E. modestusare found with Lepsiella, discussed in the previous section.

On areas such as this where predation is heavy, and the rate ofbarnacle removal is rapid, further cyprids are stimulated to settle onareas of bare rock surface by the presence of already settled specimensnearby.

EFFECTS OF PREDATORS ON SETTLED BARNACLES

Lepsiella scobina

Since predation on Chamaesipho brunnea occurs only in the lowestparts of the zone, the effects on the breeding stock are limited.

Where the growth rate of Chamaesipho columna is rapid, Lepsiellaprobably do little more than "prune" the oldest specimens. Wheregrowth is slow and conditions generally adverse, the entire breedingpopulation may be removed. Breeding may occur when C, columnahas a carino-rostral diameter of 1 mm, although embryo productionmay be as low as 12 per adult. Where the predators are small, thesesmallest breeding specimens may be eaten, and thus the breeding popula-tion removed.

Of the number of juvenile Epopella plicata settling, a high proportion(over 90%) will be eaten by Lepsiella in the first 6 months. This maybe considered as a pre-breeding population removal.

Where there is a lower shore population of dense but young specimensand breeding occurs early, there is a similar reduction of breedingpopulation. However, the breeding contribution of all the E. plicatasmall enough to be eaten by Lepsiella is slight.

Here is an example of a predator/prey situation where adults areactually immune to predation, and recruitment to mature populationsis slight. If areas in the E. plicata zone are cleared of all barnacles and

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predators, a moderate settlement of E. plicata will occur (at 2-3 mmdiameter, 0.1 animal/cm2) and of these 25-50% will survive to maturity.The absence of C. columna and the low density of the E. plicata reducesthe number of Lepsiella moving on to the area, and increases thepercentage of E. plicata surviving to adult size.

Among populations of mature E. plicata, Lepsiella are usually com-mon to abundant feeding on C. columna. Epopella plicata also provideshelter and suitable places for the Lepsiella to lay their egg capsules.

Neothais scalarisPredation on Chamaesipho brunnea is usually limited to the lowest

parts of the zone and has little effect on the population as a whole.The smallest Neothais, which were not used in the experiments,

probably eat Chamaesipho columna, but as this would occur only at thelowest parts of the barnacle's distribution, its effect would be small(Fig. 11). Larger Neothais (all sizes used in the experiments) will noteat C. columna from choice, and usually starve rather than eat it.

The extent of the Epopella plicata zone on a shore seems to be mainlydetermined by the abundance of Neothais (Fig. 12). The largest E.plicata can still be eaten by Neothais. Where these predators are com-mon, adult E. plicata are restricted to those parts of the shore abovethe range of the predators.

POTENTIAL RANGES OF BARNACLES IN ISOLATION

In the absence of other barnacles and predators Chamaesipho brunneasurvives well at lower levels of the shore. As it can feed only in turbulentwater,, and since silting is detrimental (Luckens 1970a), it is found onlyon exposed coasts. Where C. columna is absent, C. brunnea occurs down.to the brown algal zone.

Except for competition by algae at the lowest part of its range C.columna already occupies all possible levels on the shore at Leigh. Itcan take advantage of bared rock areas at all levels initially (even belowthe top of the brown algal zone), although at the highest levels it maydie of desiccation.

In the absence of predators and other barnacles, the upper limit ofEpopella plicata would be little altered. The lower limit would then bethe top of the brown algal zone. If algae were absent, E. plicata wouldsettle lower still.

ACKNOWLEDGMENTS

I am indebted to all those members of the Zoology Department at Aucklandand at the Leigh Laboratory who have helped me willingly in so many ways,in particular my supervisor, Professor J. E. Morton, Dr W. J. Ballantine, DrB. A. Foster, Dr J. B. Gilpin-Brown, Mr C. Aldridge, and Mr G. Batt.

This work was carried out at the Leigh Marine Laboratory during the tenureof a University Grants Committee Research Fund Fellowship.

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LITERATURE CITED

CARRIKER, M. R. 1955: Critical review of biology and control of oyster drillsUrosalpinx and Eupleura. U.S. Fish and Wildlife Service, SpecialScientific Report, Fisheries 148. 150 pp.

CARRIKER, M. R., VAN ZANDT, D. & CHARLTON, G. 1967: Gastropod Urosalpinx:pH of accessory boring organ while boring. Science 158 (3803) :920-2.

CONNELL, J. H. 1961a: The influence of interspecific competition and otherfactors on the distribution of the barnacle Chthamalus stellatus.Ecology 42: 710-23.1961b: Effects of competition, predation by Thais lapillus, and otherfactors on natural populations of the barnacle Balanus balanoides.Ecological Monographs 31 (1) : 61-104.

1970: A predator-prey system in the marine intertidal region. I.Balanus glandula and several predatory species of Thais. EcologicalMonographs 40 (1) : 49-78.

FEARON, C. unpublished 1962: Studies in the biology and taxonomy of thethaisid gastropod Lepsiella Iredale. M.Sc. thesis lodged in CanterburyUniversity Library, Christchurch. 249 pp.

FOSTER, B. A. 1969: Tolerance of high temperatures by some intertidal barnacles.Marine Biology 4 (4) : 326-32.

1971: On the determinants of the upper limit of intertidal distributionof barnacles (Crustacea: Cirripedia). Journal of Animal Ecology40 (1) : 33-48.

LUCKENS, P. A. unpublished 1964: Settlement and succession on rocky shores atAuckland. M.Sc. thesis lodged in the University of Auckland Library,Auckland. 285 pp.

1970a: Breeding, settlement and survival of barnacles at artificiallymodified shore levels at Leigh, New Zealand. N.Z. Journal of Marineand Freshwater Research 4 (4): 497-514.

1970b: Predation and intertidal zonation at Asamushi. Bulletin of theMarine Biological Station of Asamushi, Tohoku University 14: 33-52.

1975: Competition and intertidal zonation of barnacles at Leigh, NewZealand. N.Z. Journal of Marine and Freshwater Research 9 (3) :379-94 (this issue).

MORTON, J. E. & CHAPMAN, V. J. 1968: "Rocky Shore Ecology of the LeighArea, North Auckland". University of Auckland, Auckland. 44 pp.

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predation and intertidal zonation of barnacles at leigh, new zealand

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