Shell-bearing Molluscs Research Articles

Imbricated shell sculpture in benthic bivalves.

Molluscan shells display a high diversity of external sculpture. Sculptural elements may be symmetrical, where both edges of an element are morphologically similar, or asymmetrical, where one edge is steeper than the other. Asymmetrical sculpture can be ratcheted, with the leading edges (those in the direction of locomotion or growth) less steep than the trailing edges, or imbricated (leading edges steeper than trailing edges). While the ratcheted sculpture is better known, the diversity of imbricated sculpture has remained largely unexplored. In a survey of extant benthic shell-bearing molluscs, we document imbricated sculpture primarily in epifaunal bivalves or on the exposed sectors of shells of semi-infaunal bivalves. Imbricated sculpture is particularly widespread in pteriomorphian bivalves, but it is absent in the subclade Mytiloidea as well as in highly mobile Pectinidae. It also occurs in many carditid bivalves (Archiheterodonta) and in phylogenetically scattered euheterodonts. In several infaunal bivalves including species of Cardites (Carditidae), Hecuba (Donacidae), and Chione (Veneridae), comarginal elements on the posterior sector are imbricated whereas anterior comarginal ridges are ratcheted. Imbricated sculpture in bivalves tends to be concentrated on the upper (left) valves of pectinids or on the posterior sector of both valves in archiheterodonts and euheterodonts. Imbricated sculpture is uncommon in gastropods, even in epifaunal species, but does occur in the collabral ridges in some Vasidae and a few other groups. Expression of imbricated sculpture does not depend on shell mineral composition or microstructure. The ecological distribution and within-shell pattern of expression of imbricated sculpture point to the likelihood that this type of asymmetrical sculpture is both widespread and potentially functional. Additionally, we present a potential methodology whereby shell sculpture categories (symmetrical, ratcheted, and imbricated) may be quantified by comparing the lengths of corresponding leading and trailing edges across the shell surface.

Modiolus cimbricus sp. nov. (Bivalvia: Mytilidae) endemic in the Kattegat-Skagerrak

Small usually beautifully coloured mussels from the Kattegat were to date identified as Modiolus adriaticus (Lamarck, 1819) since C.G.Johs. Petersen’s account of the shell-bearing molluscs was published in 1888. However, these mussels from Danish and Swedish waters represent a very distinct new species which also is endemic. The present paper serves to describe this new species, Modiolus cimbricus sp. nov., which belongs to the genus Modiolus Lamarck, 1799. The genus Gibbomodiola Sacco, 1898, to which Modiolus adriaticus has been moved, is here synonymized with Modiolus Lamarck, 1799.

The evolution of molluscan photosymbioses: a critical appraisal

Partnerships between animals and photosynthesizing microbes have evolved repeatedly, although their history, adaptations, and ecology remain controversial and little understood. In a critical review of 17 fossil and living clades of shell-bearing molluscs with photosymbionts (two of them newly inferred), adaptive shell modifications and ecological aspects are discussed in the broader context of photosymbioses in other phyla. Fossil candidates have characteristics that are rare or unknown in living photosymbiotic molluscs, including cementation, porous shell microstructure, and epifaunal habits on carbonate muds. Many ancient photosymbioses may have lived in planktonically more productive environments than are typical of living tropical forms. This may be related to the late appearance (Early Eocene) of the dinoflagellate Symbiodinium, which can thrive under highly oligotrophic conditions. Living photosymbiotic molluscs represent a small and atypical sample of all the photosymbiotic clades that have evolved. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 497–511.

The evolution of gigantism on temperate seashores

The extent to which animal lineages achieve large body size, a trait with broad advantages in competition and defence, varies in space and time according to the supply of (and demand for) resources, as well as the magnitude and effects of extinction. Using the maximum sizes of shallow-water marine shell-bearing molluscs belonging to nineteen guilds (groups of species with similar habits and food sources) in seven temperate regions from the Early Miocene to the Recent, the present study examined the controls on productivity and predation that enable and compel large size to evolve. The North Pacific (especially its eastern sector) has been most favourable to large-bodied species from the Pliocene onward. Large productive kelps (Laminariales) evolved there in conjunction with herbivorous mammals, setting the stage through positive feedbacks between production and consumption for the evolution of large molluscan herbivores and suspension-feeders. The evolution of bottom-feeding predatory mammals together with other large predators created intense selection for large molluscan sizes. Very large molluscs in the Early Miocene were concentrated in the southern hemisphere, especially among metabolically passive species. Extinctions, which preferentially targeted the largest members of guilds in most regions, were more numerous in the southern hemisphere and the North Atlantic than in the North Pacific. Minimal disruption, together with the early evolution of metabolically-active consumers and the positive feedbacks they engendered, accounts for the evolution of molluscan gigantism in the North Pacific. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.

The tropical history and future of the Mediterranean biota and the West African enigma

AbstractAim Since the opening of the Suez Canal in 1869, many tropical taxa from the Indo‐West Pacific (IWP) realm have entered the Mediterranean Sea, which is experiencing rising temperatures. My aims are: (1) to compare biogeographically this tropical transformation of the Mediterranean biota with the tropical faunas of the Mediterranean and adjacent southern European and West African seas during the Late Oligocene to Pliocene interval; (2) to infer the relative contributions of the tropical eastern Atlantic and IWP to the tropical component of the marine biota in southern Europe; and (3) to understand why West Africa is not now a major source of warm‐water species.Location Southern Europe, including the Mediterranean Sea, and the coast of tropical West Africa.Methods I surveyed the literature on fossil and living shell‐bearing molluscs to infer the sources and fates of tropical subgenus‐level taxa living in southern Europe and West Africa during the Late Oligocene to Pliocene interval.Results Ninety‐four taxa disappeared from the tropical eastern Atlantic (including the Mediterranean) but persisted elsewhere in the tropics, mainly in the IWP (81 taxa, 86%) and to a lesser extent in tropical America (36 taxa, 38%). Nine taxa inferred to have arrived in the tropical eastern Atlantic from the west after the Pliocene did not enter the Mediterranean. The modern West African fauna is today isolated from that of other parts of the marine tropics.Main conclusions Taxa now entering the Mediterranean through the Suez Canal are re‐establishing a link with the IWP that last existed 16 million years ago. This IWP element, which evolved under oligotrophic conditions and under a regime of intense anti‐predatory selection, will continue to expand in the increasingly warm and increasingly oligotrophic Mediterranean. The IWP source fauna contrasts with the tropical West African biota, which evolved under productive conditions and in a regime of less anti‐predatory specialization. Until now, the post‐Pliocene West African source area has been isolated from the Mediterranean by cold upwelling. If further warming should reduce this barrier, as occurred during the productive and warm Early Pliocene, the Mediterranean could become a meeting place for two tropical faunas of contrasting source conditions.

Sound reasons for silence: why do molluscs not communicate acoustically?

Many adaptively beneficial states of form, behaviour and physiology are absent in large parts of the evolutionary tree of life. Although the causes of these absences can never be fully known, insights into the possibilities and limitations of adaptive evolution can be gained by examining the conditions that would be necessary for the forbidden phenotypes to evolve. Here, the case of acoustic communication in molluscs is considered. The production of sound as a warning to predators or as a means to attract mates is widespread among arthropods and vertebrates, both on land and in water, but is unknown among molluscs, even though many derived clades of gastropods and cephalopods are characterized by internal fertilization and by the evolution of long-distance visual and chemical signalling. Many molluscs possess suitable hard parts – shell, operculum and jaws – for producing sound, but most shell-bearing molluscs lack the agility or aggression necessary to cope with high-activity enemies attracted to an acoustic beacon. Their evolutionary background, arising from the generally passive adaptations of molluscs and other animals with low metabolic rates, prevents selection favouring communication by sound, and indeed favours silence. Several clades of shell-bearing gastropods and cephalopods were identified in which sound production has the greatest potential to arise or to be discovered. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 485–493.

Fluorescent Pigment Distinguishes Between Sibling Snail Species

Traditional taxonomy of shell-bearing molluscs does not generally use soft-body coloration. However, the land snails Bradybaena pellucida and B. similaris have been distinguished only on the basis of the color of the soft-body visible through the shell. Thus, the taxonomic status of the two species has traditionally been questionable. We found that dense spots of pigments embedded in the dorsal mantle are responsible for the yellow coloration of B. pellucida . Similar spots in B. similaris are white and less densely aggregated in whorls further from the apex, and the brown color of the hepatopancreas is visible through the shell. The yellow pigments of B. pellucida seep out with mucus from the body in natural and laboratory conditions. The two species became externally indistinguishable after 30 days of laboratory feeding, because the yellow spots disappeared in B. pellucida and the color of the hepatopancreas changed from dark brown to pale brown in both species. Irradiation with ultraviolet A demonstrated that the yellow pigment of B. pellucida fluoresces. Adult specimens of the two species were distinct in penial microsculpture, with F(1) hybrids intermediate in form. Populations of the two species differed significantly in allelic frequencies at four allozyme loci. Therefore, B. pellucida and B. similaris are morphologically and genetically distinct. The fluorescent yellow pigment distinguishes B. pellucida from B. similaris under natural conditions despite its environmental dependence.

From Europe to America: Pliocene to Recent trans-Atlantic expansion of cold-water North Atlantic molluscs

Data on the geographical distribution, phylogeny and fossil record of cool-temperate North Atlantic shell-bearing molluscs that live in waters shallower than 100 m depth belong to two biogeographic provinces, one in eastern North America north of Cape Cod, the other in northern Europe. Amphi-Atlantic species, which are found in both provinces, comprise 30.8% of the 402 species in the northeastern Atlantic and 47.3% of the 262 species in the northwestern Atlantic. Some 54.8% of these amphi-Atlantic species have phylogenetic origins in the North Pacific. Comparisons among fossil Atlantic faunas show that amphi-Atlantic distributions became established in the Middle Pliocene (about 3.5 million years ago), and that all represent westward expansions of European taxa to North America. No American taxa spread eastward to Europe without human assistance. These results are in accord with previous phylogeographic studies among populations within several amphi-Atlantic species. Explanations for the unidirectional expansion of species across the Atlantic remain uncertain, but may include smaller size and greater prior extinction of the North American as compared to the European fauna and biased transport mechanisms. Destruction of the European source fauna may jeopardize faunas on both sides of the Atlantic.

Colophon, contents, abstract, introduction Danish sites with marine sediments

Late Quaternary, marine deposits in Denmark have yielded 247 subfossil species of molluscs. The sites are presented, and comparisons are made between the subfossil mollusc assemblages and the 278 shell-bearing mollusc species presently living in the Danish seas. 184 species are common to the two groups. The 63 species no longer occurring around Denmark are used as indicators of changing environmental conditions, including temperature, salinity and depth, throughout the last 130 000 years. Seven modern faunal regional units are defined and considered: the Bælt, the Baltic, the Kattegat, the Limfjord, the North Sea and the Vendsyssel regions, and the Skagen area based on the Skagen III Well DGU File No. 1.287. The Late Quaternary, marine, shell-bearing molluscs, comprising 341 subfossil and recent species, are characterised from the point of view of climatic (i.e. Arctic, Subarctic, Boreal and Lusitanian) affinities and animal–sediment relationships. On this background the faunal and environmental evolution recorded in the 217 m long Skagen Well core is analysed and described. The mollusc assemblages in the Skagen sequence indicate a deeper-water facies during the Eemian, the Weichselian and the older Holocene in contrast to what hitherto was known in other parts of the Danish area during the Late Quaternary. For the Skagen Well the chronozones Preboreal/Boreal, Atlantic, Subboreal and Subatlantic can be identified by 14C dating. The environmental changes within the seven regions through the Late Quaternary are evaluated by depicting the molluscan communities encountered in the seven Late Quaternary stages together with remarks on studies of the neighbouring areas. By following the marine communities through the Late Quaternary in the light of the classical bottom communities sensu C.G.J. Petersen, it is demonstrated how facies have changed both through time and space within the Danish marine realm. The well-established, more temperate Eemian marine fauna was closely associated with shallow-water environments. The inferred climatic changes reflect an interglacial–glacial cycle. However, the climatically induced changes during the Holocene in the marine environment were small and overshadowed by the facies changes. Out of the 341 species recorded in this study, 140 occur in the Eemian, 36 in the Early/Middle Weichselian and 41 in the Late Weichselian. The Holocene fauna is represented by 183 species of shell-bearing molluscs, of which the first recorded occurrence of 148 species has been radiocarbon-dated.

​Late Quaternary environmental changes recorded in the Danish marine molluscan faunas

Late Quaternary, marine deposits in Denmark have yielded 247 subfossil species of molluscs. The sites are presented, and comparisons are made between the subfossil mollusc assemblages and the 278 shell-bearing mollusc species presently living in the Danish seas. 184 species are common to the two groups. The 63 species no longer occurring around Denmark are used as indicators of changing environmental conditions, including temperature, salinity and depth, throughout the last 130 000 years. Seven modern faunal regional units are defined and considered: the Bælt, the Baltic, the Kattegat, the Limfjord, the North Sea and the Vendsyssel regions, and the Skagen area based on the Skagen III Well DGU File No. 1.287. The Late Quaternary, marine, shell-bearing molluscs, comprising 341 subfossil and recent species, are characterised from the point of view of climatic (i.e. Arctic, Subarctic, Boreal and Lusitanian) affinities and animal–sediment relationships. On this background the faunal and environmental evolution recorded in the 217 m long Skagen Well core is analysed and described. The mollusc assemblages in the Skagen sequence indicate a deeper-water facies during the Eemian, the Weichselian and the older Holocene in contrast to what hitherto was known in other parts of the Danish area during the Late Quaternary. For the Skagen Well the chronozones Preboreal/Boreal, Atlantic, Subboreal and Subatlantic can be identified by 14C dating. The environmental changes within the seven regions through the Late Quaternary are evaluated by depicting the molluscan communities encountered in the seven Late Quaternary stages together with remarks on studies of the neighbouring areas. By following the marine communities through the Late Quaternary in the light of the classical bottom communities sensu C.G.J. Petersen, it is demonstrated how facies have changed both through time and space within the Danish marine realm. The well-established, more temperate Eemian marine fauna was closely associated with shallow-water environments. The inferred climatic changes reflect an interglacial–glacial cycle. However, the climatically induced changes during the Holocene in the marine environment were small and overshadowed by the facies changes. Out of the 341 species recorded in this study, 140 occur in the Eemian, 36 in the Early/Middle Weichselian and 41 in the Late Weichselian. The Holocene fauna is represented by 183 species of shell-bearing molluscs, of which the first recorded occurrence of 148 species has been radiocarbon-dated.

Shell gravel deposits on the Åland Islands, southwestern Finland, with special reference to the molluscan assemblages

Twenty‐six shell gravel sites at elevations between 3 and 35 m a.s.l. were found on the Åland Islands. The faunal composition was determined on 14 of these locations and Mytilus edulis and Macoma balthica were found everywhere. Another common species was Hydrobia ulvae, while H. ventrosa, Littorina saxatilis and Cerastoderma glaucum were more sporadic. This species assemblage implies that the salinity of the area was about 10C when the animals lived there. The deposits on the N‐NE hilltops generally have southerly to westerly aspects, while those of the south‐central hilltops are easterly. On the basis of a 14C‐dated stratified deposit it is suggested that each deposit was formed during a relatively short period of time. As a hilltop emerged from the sea, glacial debris was flushed down. Shell‐bearing molluscs colonized the bare rock surface and were flushed down and formed the shell gravel deposit as the shoreline displacement continued. At a later stage, as the more concave part of the hilltop was subject to wave action, erosion was weaker and the shell gravel deposit was covered by beach sand. Since the hills may be of different elevation, it is concluded that trying to date ancient shorelines from shell gravel deposits is not justified, as each deposit originates from the hill with which it is associated.

A comparison of living and dead molluscs on coral reef associated hard substrata in the northern Red Sea — implications for the fossil record

Fidelity of death assemblages to live shelly faunas is one of the major palaeontological questions, but quantitative data are scarce and most case studies on this topic have been performed in non-reef sediments. Therefore we studied different types of subtidal reef-associated hard substrata (reef flats, reef slopes, coral carpets, coral patches, rock grounds), each with different coral associations, in order to determine the agreement of assemblages of living and dead shell-bearing molluscs. A total area of 340.5 m 2 was investigated and 2846 individuals were counted at 68 sample localities ranging from shallow subtidal to 40 m water depth. Most taxa found dead in the study area were also found live and vice versa; differences in this respect can be related to quantitatively unimportant taxa. However, strong differences exist in the proportion of living and dead fauna, dominant taxa, and molluscan distribution patterns. The ratio of live to dead molluscs is high. Living molluscs are strongly dominated by taxa with distinct relations to corals, mainly Pedum, Coralliophila and Tridacna, and the encrusting gastropod Dendropoma. Five distinct groups of living molluscs can be differentiated and related to specific hard substrata, which are characterized by distinct molluscan life habits. In contrast, the death assemblages are always strongly dominated by encrusting bivalves, mainly Chamoidea and Spondylidae, and cerithiid gastropods in varying dominances. Correspondingly there is no significant correlation of the total abundance of living and dead molluscs and their overall similarity is only 6%. Similarity between living and dead faunas is above 50% at only 12 of the 68 sample locations, and at 17 sample locations significant correlations of living and dead molluscs were recognized. These correlations are mainly based on Chamoidea, which dominate both the living and dead fauna, on rock grounds. Therefore rock grounds are the only bottom type with consistent correlations and similarities of living and dead molluscs. The observed bias is due to the close relationship of molluscan life habits and post-mortem history of shells. Molluscs that live permanently attached to or within living corals (mostly bivalves and encrusting Dendropoma) can easily be overgrown after death by the large amounts of living substrata available. Rapid transport of dead shells into surrounding sediments or into crevices within corals is typical of gastropods that feed on corals. Molluscs that colonize dead surfaces preferentially accumulate on rock grounds. The ecologic information that can be derived from the shells depends on the different post-mortem histories of the faunas.

A comparative study of morphometry in shell-bearing molluscs

A comparative study of morphometry in shell-bearing molluscs

A Fossil Skull Fragment of a Walrus from the Mouth of the River Scheldt (Netherlands)

An occipito-parietal skull fragment of a male specimen of Odobenus antverpiensis (Rutten, 1907), from the mouth of the River Scheldt (Netherlands) is described and figured. The animal lived probably about the transition Scaldisian-Merxemian (Pliocene-Pleistocene). It lived in coastal marine surroundings rich in shell-bearing molluscs and is not necessarily an indication of cool conditions.

Notes on Shell-bearing Molluscs from Tagonoura, Suruga Bay

Notes on Shell-bearing Molluscs from Tagonoura, Suruga Bay

Notes on Shell-bearing Molluscs in Arari Bay and Heta Bay, the West Coast of the Izu Peninsula

Notes on Shell-bearing Molluscs in Arari Bay and Heta Bay, the West Coast of the Izu Peninsula