Text Box:     Number 52 (February 2009)
Text Box: The Malacologist

A rapidly evolving secretome forms the shell

The recent sequencing of invertebrate genomes has allowed the investigation of the evolution and the process of biomineralization in gastropod molluscs. Over 25% of the genes expressed in the mantle of Haliotis encode secreted proteins, and almost 85% of the secretome encodes novel proteins. The expression of secretome genes is restricted to discrete mantle zones, each responsible for building one of the structural layers. Patterned expression of a subset of genes along the mantle is indicative of roles in shell ornamentation and pigmentation. Two novel genes are described that are involved in mother of pearl deposition. The unexpected complexity and evolvability of the secretome and the modular design of the molluscan mantle enables diversification of shell strength and design, and contributes to the variety of adaptive architectures and colours.

Jackson, Daniel J et al. BMC Biol. 4;40. 2006.


A Novel Extrapallial Fluid Protein in Pearl Oyster

Mollusc shell nacre is the result of a precisely controlled biomineralization process. A novel 38-kDa extrapallial fluid (EPF) protein, named amorphous calcium carbonate-binding protein (ACCBP), is actively involved in the process. In vitro, ACCBP can inhibit the growth of calcite and induce the formation of amorphous calcium carbonate. When ACCBP functions were restrained in vivo, the nacre lamellae grew in a screw-dislocation pattern, and low crystallinity CaCO3 precipitated from the EPF. ACCBP can also recognize different crystal phases and crystal faces. It can therefore modify the morphology of nacre lamellae by inhibiting the growth of undesired aragonite crystal faces and meanwhile maintain the stability of CaCO3 -supersaturated body fluid by ceasing the nucleation and growth of calcite. The results suggest that a "safeguard mechanism" of undesired crystal growth is necessary for shell microstructure formation.

Ma, Zhuojun et al. J. Biol. Chem. 282(32), 23253-23263. Aug 2007.


Advances in mollusc sclerochronology and sclerochemistry

A special issue of Geo-Marine Letters (28(5-6). Oct 2008.) compiles papers on marine, estuarine and freshwater mollusc shells as recorders of environmental and climatic conditions.


Granular chitin in nudibranch skin

Chitin in nudibranchs occurs as intracellular granules that fill the epidermal cells of the skin and the epithelial cells of the† stomach. In response to nematocysts fired by cnidarian prey, the epidermal cells of eolids release masses of chitin granules, which then form aggregates with the nematocyst tubules, insulating the animal. Granular chitin, while protecting the animal, does not interfere with the suppleness and flexibility of the skin, in contrast† to the stiffness of chitin armour. The specialized epidermis enables nudibranchs to live with and feed on Cnidaria.

Martin, Rainer et al. Biol. Bull. 213 (3), 307-315. Dec 2007.


Sea slug has acquired algal photosynthesis gene

Elysia chlorotica ingests plastids of the alga Vaucheria litorea. The plastids are sequestered in the digestive epithelium where they continue to photosynthesize for months. The ingested chloroplasts contain only enough DNA to encode about 10% of the proteins needed to function.† It is likely that the sea slug acquires the essential plastid proteins by horizontal gene transfer. A nuclear gene of oxygenic photosynthesis, psbO, is expressed in the animal and has integrated into the germ line.

Mary Rumpho et al. 2008. Proc. Nat. Acad. Sci. 10.1073/pnas.0804968105. ALSO at www.newscientist.com/article/dn16124-solarpowered-sea-slug-harnesses-stolen


Kleptoplasts help saccoglossans survive food shortages

The importance of photosynthetic products derived from kleptoplasts in several sacoglossan species has been investigated by analyzing the effect of kleptoplasts on the survival rates of† Elysia timida. The development of chlorophyll concentration, total length and survival rates of starved specimens in the light and in the dark was evaluated. Although chlorophyll concentrations were similar in both cases, after 28 days specimens kept in the dark showed a greater size decrease and a lower survival than those kept in the light. Evidently, kleptoplasts provide extra energy at the primary metabolism level to compensate for a shortage in food.

Casalduero, F.G. and Muniain, C. J. Exp. Mar. Biol. Ecol. 357(2) 181-187. 31 Mar 2008.


Giant African snail as a snail predator

Individuals of Achatina fulica (Bowdich, 1822) were observed preying on veronicellid slugs at two sites on Oahu, Hawaii, so it may pose a greater threat to terrestrial mollusc conservation than previously imagined.

Meyer, WM III et al. Am. Malacol. Bull. 24(1), 117-119. 21 Mar 2008.


New Pest Semi-Slug on Hawaii is a potential vector for human angiostrongyliasis

The semi-slug Parmarion cf. martensi Simroth, 1893, was first discovered on Oahu, Hawaii, in 1996 and on the island of Hawaii† in 2004. It is abundant in eastern Hawaii Island, displacing Veronicella cubensis (Pfeiffer, 1840) in some areas. It is now established in commercial papaya plantations, and reported as a pest in home gardens.† It also climbs on structures where it deposits its faeces and has the potential to transmit disease. It has been found to carry large numbers of infective third-stage larvae of the nematode Angiostrongylus cantonensis, the cause of human angio-strongyliasis and human eosinophilic meningo-encephalitis. A newly developed polymerase chain reaction test found 77.5% of P. cf. martensi were infected with A. cantonensis, compared with 24.3% of V. cubensis from the same areas.

Hollingsworth, RG et al. Pac. Sci. 61(4), 457-467. Oct 2007.


Palearctic freshwater mussel† diversity and the Comparatory Method as a species concept

The current taxonomy of freshwater Unionoida in the Palearctic is confused by two competing species concepts: the Biological Species Concept (BSC) and the Comparatory Method (CM). The CM uses the frontal contour of the shell as the primary/sole character to delimit bivalve species. In the literature, 45 Biological species in 16 genera are recognized in the Palearctic vs. 156 Comparatory species in 34 genera. I argue that CM recognizes species with no evolutionary or biological basis and the traditional, Biological species better represent actual species diversity.

Graf, DL Proc. Acad. Nat. Sci. Phila. 156(1), 71-88. Sep 2007.

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Identification of 'extinct' freshwater mussel species using DNA barcoding

70% of the North American freshwater mollusc species are extinct, endangered, or at risk.† Impoundments and other human impacts on the Coosa River of Alabama, Georgia and Tennessee of the southeastern USA alone are believed to have caused 50 mollusk species extinctions. DNA barcoding and molecular phylogenetic analyses confirm the re-discovery of four morphospecies in the genus Pleurobema (Unionidae) previously thought to be extinct from the upper Coosa basin. A fifth 'extinct' form was found in an adjoining basin. Prompt conservation efforts may preserve some of these taxa and their ecosystem.

Campbell D. et al. Mol. Ecol. Resour. 8(4), 711-724. Jul 2008.

Dreissena bugensis reaches western US

The small striped quagga mussel (Dreissena† bugensis) was discovered in Lake† Mead in January 2007, the first recorded occurrence in the western United States. The mussels have already created havoc in the Great Lakes and Mississippi Basin due to blocked water intake pipes and other physical and biological impacts, and have impacted native Unionid mussels. D. bugensis was introduced from the Ukraine to the Great Lakes in

ballast water where it has begun to replace the zebra mussel D. polymorpha, and is able to colonise much greater depths. Both D. bugensis and D. polymorpha can be striped or unicoloured, but the shell of D. bugensis is more rounded in profile and has a convex ventral edge,† while D. polymorpha has a pronounced carina and a flat ventral edge.

Roefer, PA† et al. SW. Hydrol. 6(6), 20-21. Nov-Dec 2007.


Were the small earliest molluscs just babies?

Millimetre-scale, exquisitely preserved molluscs are important constituents of many small shelly fossil assemblages and the focus of many studies of Cambrian molluscs, although centimetre-sized molluscs occur as early as the earliest Cambrian. A large, limpet-like mollusc from the Lower Cambrian of Spain preserves an apical shell indistinguishable from the millimetre-scale helcionellids that have come to epitomize the ancestral 'conchiferaní, providing evidence that at least some millimetre helcionellids are juvenile or larval shells, and suggesting that the generally small size of Cambrian molluscs may be a taphonomic artifact.

Mus, M. et al. Geology 36(2), 175-178. Feb 2008.


Global diversity of gastropods (Gastropoda; Mollusca) in freshwater

The world's gastropod fauna from continental waters comprises† >64,000 valid described species and >33-38 independent lineages of Recent Neritimorpha, Caenogastropoda and Heterobranchia† (including the Pulmonata). The caenogastropod component dominates in species richness and diversity of biology and has produced several highly speciose endemic radiations. Ancient oligotrophic lakes (e.g., Baikal, Ohrid, Tanganyika) are key hotspots of gastropod diversity; also noteworthy are lower river basins (e.g. Congo, Mekong, Mobile Bay), but small streams, springs and groundwater systems have† produced the most speciose associations. The freshwater gastropod fauna faces unprecedented threats from habitat loss and degradation and introduced fishes and other pests. Unsustainable use of ground water, landscape modification and stock pose the most significant threats to the large diversity of narrow range endemics in springs and ground water. The status of most taxa is unknown, and the considerable magnitude of extinction and high levels of threat indicated by the IUCN Red List is certainly a significant underestimate.

Strong, Ellen E; Gargominy, Olivier; Ponder, Winston F; Bouchet,† Philippe. Hydrobiologia 595(1), 149-166. Jan† 2008.


Endocrine-related reproductive effects in molluscs

Most studies of endocrine disruption have concentrated on vertebrate species, but invertebrates, especially molluscs, are now gaining more attention. One of the best-documented examples of endocrine disruption is imposex in some gastropod species. Molluscs, especially bivalves, are also good bioindicators. Recent results suggest that molluscs can be adversely affected by compounds that alter their reproduction and that vertebrate-type sex-steroids could be involved in these effects. Nevertheless, the endocrine system of molluscs appears to be dissimilar in many aspects to those of vertebrates and sex-steroids might not have the same importance in all mollusc species.

Ketata, Imen et al. Comp. Biochem. Physiol. C: 147(3), 261-270. Apr 2008.


The impact of trematodes on endocrine disruption in Potamopyrgus antipodarum

Potamopyrgus antipodarum is an invasive freshwater mollusc in Europe introduced from Australasia in the late 19th century and now commonly distributed throughout Europe. It has been increasingly utilised for ecotoxicological work, particularly the impact of endocrine disrupting chemicals. However, molluscs are commonly infected with trematode parasites, which can disrupt the endocrine system, and distort pollution studies. This study assessed the extent of trematode parasitism in European populations of P. antipodarum, summarizes the combined effects of parasitism and pollution on molluscan physiology and ecology and evaluates the implications for ecotoxicological studies.

Morley, NJ. Aquat. Sci. 70(2), 107-114. 2008.


Mollusc grazing limits lichen growth

When foliose epiphytic lichens were transplanted to two calcareous broadleaved deciduous forests, one poor in lichens, one rich, preventing the access of molluscs significantly reduced the loss of juvenile lichens, particularly in the naturally lichen-poor forest. Molluscs also severely grazed mature† thalli in the lichen-poor forest. Reducing the lichensí natural concentration of depsidones by rinsing with acetone increased subsequent grazing significantly, showing their role in herbivore defence. The results suggest that mollusc grazing is† important in shaping the epiphytic vegetation in calcareous deciduous forests.

Asplund, Johan and Gauslaa, Yngvar. Oecologia 155(1) 93-99. Feb 2008.


Impact of Crepidula fornicata on benthic community respiration

The American slipper limpet has been an†† invasive species in European bays and estuaries since the 1950s and can reach densities up to several thousands of individuals m -2. A comparison of dissolved oxygen and† dissolved inorganic carbon at a site with high (>1000 ind. M-2) and low (<200 ind. m-2) densities in the Bay of Brest (Brittany, France) showed community respiration (CR) was 1.5- to 3-fold higher at the station with high densities. CR was mainly controlled by C. fornicata biomass, temperature, and chlorophyll a concentration in the water column. C. fornicata is thus considered a source† of carbon influencing partial pressure of CO2 in seawater and favouring CO2 effluxes to the atmosphere.

Martin, Sophie et al. Mar. Ecol. Prog. Ser. 347, 51-60. Oct 2007.††


Cultivating Manila clam affects sand-mason Lanice

Cultivation of Manila clam Ruditapes philippinarum in the Chausey Archipelago, France, is highly mechanized: every step uses tractor-driven machinery. The Manila clam concessions are concentrated on Lanice conchilega (Pallas, 1766) bioherms, which are known to increase alpha-diversity and to locally modify sediment dynamics. L. conchilega populations are significantly affected within the concessions where their total abundances drastically decrease, their spatial patterns are modified and the associated benthic assemblages are significantly altered.

Toupoint, N. et al.† Mar. Pollut. Bull. 56(8), 1429-1438. Aug 2008.


Host-parasite interactions in oligotrophic stream ecosystems

Conservation strategies for the freshwater pearl mussel (Margaritifera margaritifera), and its vertebrate host fish are often poorly integrated. The genetic differentiation and diversity is compared with productivity and ecological habitat features of both species in nine streams from the drainage systems of the Danube, Elbe, Weser, Tuuloma, Kemijoki and Aulne. Genetic differentiation was more pronounced in pearl mussel than in brown trout. Genetic diversity of host and parasite was negatively correlated. The most oligotrophic, postglacially colonized areas represented genetic diversity hotspots with high conservation priority for pearl mussels, whereas their host fish displayed low diversity in these areas. These results suggest that genetic information from species with different life-history strategies, such as invertebrates and fish, should be considered simultaneously.

Geist, J and Kuehn, R. Mol. Ecol. 17(4) 997-1008. Feb 2008.

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