SIZE, SHAPE AND SHELL MORPHOLOGY IN THE ANTARCTIC LIMPET NACELLA CONCINNA AT SIGNY ISLAND, SOUTH ORKNEY ISLANDS

Comparison of the size, shape and shell morphology in littoraland sub-littoral morphs of the Antarctic limpet Nacella concinnareveal differences in shell morphology which are enhanced bystructural anomalies within the shells of the two types. Infestationof sub-littoral shells by the conchocelis phase of an endolithicalga significantly affects shell density and total chlorophylllevels in the two shell morphs. The surface sculpture of sub-littoralshells is characterised by a series of grooves, the configurationof which closely resembles that of the radular teeth in N. concinna.Limpets utilise the available food supply within the shell matrixof other limpets by grazing the shell material. Epibiotic growthof calcareous algae prevent erosion and preserve underlyingshell layers. In severe cases, where protection is lacking,intraspecific shell grazing may remove parts of the shell exposingthe internal tissues. The Dominican Gull, Larus dominicanus, is a major shore predatorof both shell morphs. Gull middens contain both shell typesbut are dominated by the more accessible littoral shells. Comparisonof living populations and midden assemblages indicates thatsize and shape selection of prey occurs, with pear-shaped limpetsbetween 21 mm and 29 mm in length being taken preferentially. Apparent differences in shell form are induced by physical,biological and behavioural influences. Littoral animals arerobust in nature, resist avian pre-dation and are not extensivelygrazed whereas those of the sub-littoral are not subject tothe same degree of predatory attention but suffer a gradualdepletion of their shallower shell form through a combinationof algal infection and intraspecific shell grazing. (Received 21 February 1990; accepted 5 July 1990)     

Crushing behavior of tropical and temperate crabs

The Guamunian xanthids Carpilius maculatus (L.), C. convexus (Forskal), and Eriphia sebana (Shaw & Nodder), and the parthenopid Daldorfia horrida (L.), possess large master claws with molariform teeth than are used to crush the shells of hermit crabs and snails. These crabs typically sever the spire of their prey, or make a gash in the body whorl. They tend to employ sustained pressure on the prey shell, and, except for Eriphia, rarely attack the outer lip, so that the outer lip of the shell typically remains undamaged, except in shells near the critical size, i.e., the maximum size of vulnerability to predation. Temperate species of Cancer (C. productus Randall and C. oregonensi Rathbun) may also crush shelled prey in the larger of their two claws, but more commonly they use both claws together in breaking open their victims. Sustained pressure is applied for only short periods by these crabs.Gastropod adaptations conferring resistance to crushing by crabs include a thick shell, narrow or otherwise small aperture, thickened outer lip, strong sculpture, and a low spire. Emphasis on these traits lowers the critical size of the prey, i.e., permits escape from cushing at a smaller size. An equatorward increase in the expression of the characteristics of crushing-resistance parallels an increase in crushing power of the crabs.     

INTRA-POPULATION VARIATION OF THE SHELL OF LITTORINA RUDIS (MATON) (MOLLUSCA: PROSOBRANCHIA)

Shell variation within a single population of Litlorina rudis(Maton), collected near the Biological Station at Trondheim,Norway, was studied. The variation of the shell features studied, which includedthe shape of the basal part of the outer lip, and the relativeheight of the spire, width of the shell and of the aperture,were found to be at least partially related to shell height.Furthermore, the shell shape in the population investigated,besides varying with age, also varies due to shell damage. Nosignificant differences were found between the shapes of theshells of males and females. The shell characteristics of different populations of L. rudisalso vary greatly, one of the causes being differences in theenvironmental conditions. When studying this variation betweendifferent populations, however, it must be emphasized that onlyundamaged shells of individuals of about the same age shouldbe compared, in order to obviate the effects of the variabilitydue to age differences and shell damage within each individualpopulation. ^*Contribution from Trondhjem Biological Station no. 199 (Received 18 February 1980;     

Predation by juvenile Polinices duplicatvs (Say) on Gemma gemma (Totten)

Feeding by juvenile Polinices duplicatus (Say) on Gemma gemma (Totten) at Barnstable Harbor, Massachusetts, was examined using laboratory experiments and collections of naturally occurring bored shells. Snails < 19 mm fed on Gemma in the laboratory and borehole diameter was directly related to predator size. Field collections of drilled shells showed that Gemma was an important prey of 0-year-class Polinices. Boreholes in Gemma shells from field collections were made primarily by snails of 1– mm, and < 3% were made by snails of > 10 mm. Most bored Gemma were large 1- to 2-yr-old individuals. The proportion of empty Gemma shells containing boreholes ranged from 3.7–14.6%, indicating that naticid prédation was a minor source of total Gemma mortality. Collections of bored shells closely reflected both the size range and relative abundance of natural predators of Gemma.     

Patterns of shell penetration by Chorus giganteus juveniles (Gastropoda: Muricidae) on the mussel Semimytilus algosus

Patterns are described for shell penetration by the sublittoral muricid snail Chorus giganteus during predatory attacks on the mussel Semimytilus algosus. Location, form and size of shell penetrations were observed in relation to the size of the predator. The results suggested that positions of the perforations on the mussel shells were related to size of the attacking snail. Smaller snails perforated areas near the shell ligament and in the central zone of the shell, while larger snails more frequently attacked shell borders, principally on the ventral side. These observations may be related to: (a) changes in the process of manipulation of the prey during development of the foot and the shell tooth of the predator, (b) changes in internal structure of the snails related to the shell perforation mechanism, or (c) learned behavior acquired experientially by the snails during early growth. Although in other studies of muricid penetration patterns larger boreholes made in shells of the prey were positively correlated with increasing predator size, this relation did not appear to hold with C. giganteus, as larger specimens often made relatively small shell perforations. Areas of boreholes made in the mussel shells by this snail varied from 0.01 to 1.1 mm(2), and were unusually variable in size and shape, especially when compared with literature results on bores characteristic of other muricid species.     

PREDATION ON THE BIVALVE CHOROMYTILUS MERIDIONALIS (KR.) BY THE GASTROPOD NATICA (TECTONATICA) TECTA ANTON

The effects of a population of the boring gastropod Natica tectaon the bivalve Choromytilus meridionalis were investigated atBailey's Cottage, False Bay, South Africa. In July 1979 theN. tecta density on the mussel bed averaged 69 m^–2 andthe population consisted mainly of reproductively mature individualsbetween 20–33 mm shell width. Laboratory experiments on N. tecta showed that prey size selectionis an increasing function of predator size. The prey size rangetaken by large N. tecta is also greater than that taken by smallindividuals. The position of the borehole on the mussel shellis a function of the way in which the shell is held by the footduring the boring process. Consumption rates measured in thelaboratory showed an increase from approximately 1 kJ per weekin 18 mm N. tecta to 4.5 kJ per week in 28 mm individuals. Populationconsumption in the field was calculated as 663 kJ m^–2month^–1. It was estimated that at this rate the standingcrop of mussels in the pool would be eliminated within 10 months.Field measurements showed significant depletion after 6 months. New spat settlement of mussels occur every 4–6 years.The growth curve shows that after one year the population meansize exceeds 30 mm shell length, which is beyond the prey selectionsize range of small N. tecta. It was concluded that at the timeof a new mussel settlement a niche is provided for the simultaneoussettlement and growth of juvenile N. tecta in high densities.However, within one year the increase in prey size, togetherwith depletion due to over-exploitation, limits population growthand density in N. tecta. (Received 14 March 1980;     

Predation on freshwater snails in Miocene Lake Steinheim: a trigger for intralacustrine evolution?

Shells of the freshwater gastropods Gyraulus trochiformis (Planorbidae or ramshorn snails) and Bania pseudoglobula (formerly Pseudamnicola, Hydrobiidae) from Miocene Steinheim Basin in SW Germany contain small holes with a mean diameter of 0.8 mm. Analyses of comparable holes are so far unknown from fossil or extant assemblages of freshwater shells. This analysis of the perforated shells suggests that the holes were not formed by post‐depositional or pure taphonomic processes. Instead, they were most likely produced by predators. This analysis widens the means for identification of predation on freshwater snail shells that can be used in other palaeolimnological studies. The co‐occurrence of fish teeth and perforated shells in the studied samples as well as the perforation features suggest that the predator was either barbel or tench fish. The correlation between shell sizes and hole diameters suggests a size relationship between predator and prey that may generally be related to gape‐limited fish predators. The co‐occurrence of perforated shells in these lake sediments with a dominance of large and armoured shells suggest that these larger shells with protuberances and knobs could not be crushed by the gape‐limited fish predators. This analysis is the basis for a hypothesis that the endemic evolution of Gyraulus in Lake Steinheim, with some varied forms of shell thickness and morphology, was triggered by a predator–prey relationship based on adaptations to avoid shell‐breaking predators.     

SHAPE DIFFERENCES AMONG BOREHOLES DRILLED BY THREE SPECIES OF NATICID GASTROPODS

Naticid gastropods leave a record of predation in the form ofcharacteristic countersunk circular boreholes. We fed bivalveprey to naticids to test whether the ratio between the innerand outer borehole diameter differed among three Recent naticidspecies: Euspira heros, E. lewisii and Neverita duplicata. Fossilshells from a Miocene-aged assemblage, containing naticids previouslyidentified as E. heros and N. duplicata, were also includedin our analyses. Ratios of the inner to outer diameters of naticidboreholes in bivalve shells showed that there were significantdifferences in borehole shape between E. heros and the two otherRecent species. We found no significant differences betweenthe ratios of E. lewisii and N. duplicata; however, a principalcomponent analysis showed that boreholes distinctly clusteraccording to species. The Miocene boreholes were also distinctfrom the Recent, indicating that the Miocene naticids may bea separate species from E. heros and N. duplicata. We have shownthat there is a species-specific component to borehole geometry.This may have important implications for palaeobiological studies,because the index of borehole functionality, previously describedas an inner to outer borehole ratio of 0.5 for all naticids,may differ between species. (Received 23 March 2004; accepted 10 January 2005)     

PREDATOR-PREY INTERACTIONS BETWEEN LEPSIELLA (BEDEVA) PAIVAE (GASTROPODA: MURICIDAE) AND KATELYSIA SCALARINA (BIVALVIA: VENERIDAE) IN PRINCESS ROYAL HARBOUR, WESTERN AUSTRALIA

On a sandy beach at Shoal Bay in Princess Royal Harbour, Albany,southwestern Western Australia, lives a small muricid gastropodthat feeds virtually monotonically on the overwhelmingly dominantresident bivalve Katelysia scalarina. Lepsiella paivae livesburied in the sand and attacks its prey within it. Because ofits small size (<13 mm shell height), bivalve prey isalso small and this study demonstrates a preference for K. scalarinaof 5 mm shell length, i.e. juveniles. Laboratory experimentsalso suggested a possible preference for attack of the rightvalve. Lepsiella paivae can and does, however, attack largerprey (up to 15 mm shell length), but cannot consume themcompletely. A second visit to Princess Royal Harbour in theAustral winter, when there was no juvenile K. scalarina present,showed L. paivae to be attacking at the sand surface, also bydrilling, the small (<4 mm) gastropod Hydrococcus brazieri(Hydrococcidae). SEM studies of experimentally determined drillholes of L. paivae show them to be of variable form, some straightsided, others bevelled (like a naticid) and <500 µmin diameter. On this sheltered Southern Ocean beach, therefore,L. paivae has specialized to attack juvenile bivalves by burrowingafter them. It can, however, attack other species opportunisticallyon the sand surface when seasonally favoured juvenile bivalveprey are not present. (Received 8 January 2005; accepted 16 March 2005)     

Trophodynamics of the scyphomedusae Aurelia aurita. Predation rate in relation to abundance, size and type of prey organism

Ephyra larvae and small medusae (1.7–95 mm diameter, 0.01–350mg ash-free dry wt, AFDW) of the scyphozoan jellyfish Aureliaaurita were used in predation experiments with phytoplankton(the flagellate Isochrysis galbana, 4 µm diameter, {smalltilde}6 x 10^–6 µg AFDW cell^–1), ciliates (theoligotrich Strombidium sulcatum, 28 µm diameter, {smalltilde}2 x 10^–3 µg AFDW), rotifers (Synchaeta sp.,0.5 µg AFDW individual^–1) and mixed zooplankton(mainly copepods and cladocerans, 2.1–3.1 µg AFDWindividual^–1). Phytoplankton in natural concentrations(50–200 µg C I^–1) were not utilized by largemedusae (44–95 mm diameter). Ciliates in concentrationsfrom 0.5 to 50 individuals ml^"1 were consumed by ephyra larvaeand small medusae (3–14 mm diameter) at a maximum predationrate of 171 prey day^–1, corresponding to a daily rationof 0.42%. The rotifer Synchaeta sp., offered in concentrationsof 100–600 prey I^–1, resulted in daily rations ofephyra larvae (2–5 mm diameter) between 1 and 13%. Mixedzooplankton allowed the highest daily rations, usually in therange 5–40%. Large medusae (>45 mm diameter) consumedbetween 2000 and 3500 prey organisms day^"1 in prey concentrationsexceeding 100 I^–1. Predation rate and daily ration werepositively correlated with prey abundance. Seen over a broadsize spectrum, the daily ration decreased with increased medusasize. The daily rations observed in high abundance of mixedzooplankton suggest a potential ‘scope for growth’that exceeds the growth rate observed in field populations,and this, in turn, suggests that the natural populations areusually food limited. The predicted predation rate at averageprey concentrations that are characteristic of neritic environmentscannot explain the maximum growth rates observed in field populations.It is therefore suggested that exploitation of patches of preyin high abundance is an important component in the trophodynamicsof this species. ¹Present address: University of Bergen, Department of MarineBiology, N-5065 Blomsterdalen, Norway     

A new species of Gyraulus (Gastropoda: Planorbidae) from Ancient Lake Lugu,Yunnan-Guizhou Plateau,Southwest China

A new aberrant species of the planorbid genus Gyraulus, Gyraulus luguhuensis n. sp., is described from Lake Lugu (Lugu-hu, in Chinese), Southwest China. The generic assignment with Gyraulus is based on features of the genital anatomy that are characteristic for members of that genus, in particular the presence of a chitinized penial stylet. Gyraulus luguhuensis n. sp. differs from most other congeners by its large, thick shell with an elevated spire. Similarly, aberrant shells are known from congeners in other Ancient Lakes worldwide indicating a potentially convergent evolution of shell characteristics in exclusively lacustrine species. Gyraulus luguhuensis differs from other lacustrine Gyraulus species with similarly large shells in having a sub-terminal penis pore and an unkeeled shell.     

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

Feeding behavior,ration and size structure of a population of the predatory gastropod <Emphasis Type="Italic">Cryptonatica janthostoma</Emphasis> in Vostok Bay,Sea of Japan

The relationships between the shell height of the predatory gastropod Cryptonatica janthostoma and the shell length of its typical prey, the bivalve Ruditapes philippinarum, and the diameter of the borehole on the prey shell resulting from a successful attack of the predator were experimentally found and assessed statistically. The shell height of C. janthostoma calculated retrospectively from the borehole diameter using the obtained relationships was 17–52 mm. The prey of C. janthostoma are burrowing bivalves, whose populations are affected by the predator to a varying degree. In populations of medium-sized mollusks (R. philippinarum, Protothaca euglypta, P. jedoensis, and others), C. janthostoma feeds on mollusks larger than 7–10 mm; in species with a shell length greater than 100 mm (Callista brevisiphonata, Saxidomus purpuratus), it eats specimens of 10–58 mm. C. janthostoma apparently has no effect on populations of small-sized mollusks (Anisocorbula venusta) and mollusks with an active avoidance response (Clinocardium californiense).     

Predatory boreholes in Tournaisian (Lower Carboniferous) spiriferid brachiopods

A brachiopod fauna from the uppermost part of the Tournaisian Tournai Formation (Belgium) contains an undetermined species of Crurithyris (Spiriferida, Ambocoeliidae), which displays numerous bored shells. About 8% of the 432 specimens with conjoined valves display single, small (≤ 1 mm) boreholes, which are smooth-sided, cylindrical or weakly conical, circular to slightly elliptical in plan view, perpendicular to the shell surface and generally complete. Of the 35 bored articulated specimens, 27 were drilled on the ventral valve. Most of the boreholes are located in the posterior half of the shell, and no case of edge-drilling has been observed. The boreholes were drilled by a predator, or possibly a parasite, which selected individuals greater than 2.5 mm long. Crurithyris sp. may have represented an attractive (in terms of energy cost) and easy target for a small-sized predator because of its thin shell and ornament of minute spines.     

CAUSE OF BIMODAL DISTRIBUTION IN THE SHAPE OF A TERRESTRIAL GASTROPOD

The distribution of a phenotypic state is often discontinuous and dispersed. An example of such a distribution can be found in the shell shapes of terrestrial gastropods, which exhibit a bimodal distribution whereby species possess either a tall shell or a flat shell. Here we propose a simple model to test the hypothesis that the bimodal distribution relates to the optimum shape for shell balance on the substrates. This model calculates the theoretical shell balance by moment and obtains empirical distribution of shell shape by compiling published data and performing a new analysis. The solution of the model supports one part of the hypothesis, showing that a low-spired shell is the best balanced and is better suited for locomotion on horizontal surface. Additionally, the model shows that both high- and low-spired shells are well balanced and suited on vertical surfaces. The shell with a spire index (shell height divided by diameter) of 1.4 is the least well balanced as a whole. Thus, spire index is expected to show a bimodal distribution with a valley at 1.4. This expectation was supported by empirical distribution of a spire index, suggesting that the bimodality of shell shape in terrestrial gastropods is related to shell balance.     

Predation of cockles (Cerastoderma edule) by the whelk (Buccinum undatum) under laboratory conditions

The feeding rate and behaviour of whelks (Buccinum undatum)offered cockles (Cerastoderma edule) in laboratory experimentswere examined. When presented with cockles in a range of sizes(10–40 mm), 14 B. undatum (34.6–88.3 mm),held individually in aquaria, consumed a wide size range ofcockles. Small whelks (<40 mm) consumed cockles (<23 mm),whereas large whelks, (>60 mm) ate a greater numberof larger cockles (>30 mm) and a wider size range ofcockles (12–40 mm) than smaller whelks. The majority(90%) of the shells of the predated cockles were undamaged andthe few (<10%) that were damaged showed only slight abrasionsto the anterior and posterior shell margin. Filmed observationsof B. undatum feeding on C. edule showed a method of attackthat has not previously been reported and involved the use ofthe whelk's foot to asphyxiate the cockle or to pull the shellvalves apart. No filmed evidence was found for the previouslyreported shell ‘wedging’ technique for prising openthe closed shell valves of C. edule, although 10% of the shellsof consumed cockles in feeding experiments had damaged shellmargins. (Received 4 April 2007; accepted 30 June 2007)     

Variation in the spire index of some coiled gastropod shells, and its evolutionary significance.

The spire index (height/maximum diameter of shell) is a fairly adequate measure of the shape of the coiled shell of most terrestrial and freshwater gastropod shells but less so in complex marine shells with thorns, flanges and spouts. In this study, only adult free-crawling forms with several whorls, able to retract completely into the shell, are considered. In the Stylommatophora of the Western European terrestrial fauna the distribution of the spire index is markedly bimodal, the modes, with values of about 3 and about 0.5, corresponding respectively to shells with a high to very high spire (and small spire angle) and those varying from more or less globular or trochoid to very flattened and disk-like (spire angle from 60 degrees to 180 degrees). The same two modes are found in the taxonomically different terrestrial stylommatophorans of the U.S.A., and in the faunas of Puerto Rico (Caribbean) and New Caledonia (southwest Pacific). Basommatophorans also show two, rather different, modes. North American marine archaeogastropods are mainly equidimensional but with a few disk-like forms and a very few high-spired ones, marine mesogastropods are mainly high-spired but with disk-like forms, neogastropods high-spired, and relevant euthyneurans sharply bimodal, like the stylommatophorans. Fossil archaeogastropods of the Palaeozoic were much more various at first than modern forms. There is some indication that they became restricted in variety as caenogastropods became abundant, but also that the proportion of marine disk-like shells has decreased markedly since the Palaeozoic. Modes of h/d are characteristic of large taxonomic groups but not taxonomically restricted since given values may appear as specific, generic or subfamilial variants from a mode, and appear sporadically in unrelated forms. There is also no broad association between modal value and broad ecological characters. Since nearly all values do occur in some group or other, no mechanical requirement can be invoked to explain such variation. In the land Stylommatophora enough is known of the broad ecology to suggest that in extreme habitats species with very different size or shell-shape may occur together, and that generalized feeders with similar shells may show separation, ecological or geographical (but in that case, also ecological). Since different shapes of shell will have different mechanical characteristics when considered as burdens to be carried, it is suggested tentatively that they may be related to the positions in which different species normally walk and hence to their preferred feeding places. This would explain an apparent tendency for different taxonomic groups to occupy the same part of the scatter of h/d in different regions of the world, for many groups in the same region to occupy different portions of the scatter, and perhaps the apparent exclusion by caenogastropods of archaeogastropods from part of the scatter since the Palaeozoic. It is argued that the distributions discovered are explicable only by natural selection.     

How much can Conus swallow? observations on molluscivorous species

Feeding of three species of molluscivorous Conus, C. textile,C. bandanus and C. omaria, was studied in aquaria. Conus spp.are able to kill and remove from the shell prey larger thanthemselves. Also, Conus swallowed prey with weight up to halfthat of the predator. Estimates suggest that molluscivorousspecies of Conus are probably able to swallow prey with a shellvolume reaching 85% of that of the predator, depending on theshape of the prey's body. It is confirmed that the thinningof the inner shell walls in Conus is connected with the abilityto swallow voluminous prey. Digestion of prey occurs in boththe oesophagus and stomach. (Received 9 August 2006; accepted 4 January 2007)     

Characterization of boreholes by Octopus dofleini in the bivalve Saxidomus giganteus

Octopus dofleini (Wulker) drills holes in mollusc shells, enabling it to introduce venom. Boreholes in the bivalve Saxidomus giganteus (Deshayes) were generally irregular. Thick shells were more likely to have incomplete boreholes, and the diameter of completed boreholes was larger than in thin shells. A comparison of boreholes to fractured and artificially drilled surfaces suggests that O. dofleini employs chemical dissolution of the shell during drilling.     

ALTITUDINAL VARIATION AND ADAPTIVE ZONES IN ARIANTA ARBUSTORUM: A NEW LOOK AT A WIDESPREAD SPECIES

Arianta arbustorum is a European land snail, occurring in bothlowland and mountainous regions. Its shell is usually globular,which makes the species aberrant among the Campylaeinae in shellshape, habitat, and wide, not exclusively montane range. Withincreasing altitudes, the shells usually decrease in size andbecome more conical. However, in some mountain stocks thereare populations with depressed shells, provided with an openumbilicus. This form (subspecies) is only known from formerPleistocene refugia. It is hypothesized that in the Pliocene, the primary montaneA. arbustorum, with a depressed shell, entered an additional,adaptive zone, while conquering damp lowland habitats. Thisresulted in an enormous range extension. In the new habitata different (globular) shell shape evolved and the species becamepolytypic. The subsequent glaciations wiped out most of theoriginal montane A. arbustorum with depressed shells. Wherethis had occurred, post-Pleistocene re-colonizing conspecificsnails with globular shells, found vacant territory and coulddevelop the dines mentioned before. The invading populationsdid not regain the original, montane, depressed shell shape.This shape was only retained in a few refugia, where the populationssurvived amidst the glaciers. (Received 9 February 1990; accepted 31 May 1990)