Class Polyplacophora Research Articles

Phylogenomic analyses shed light on the relationships of chiton superfamilies and shell-eye evolution.

Mollusca is the second-largest animal phylum with over 100,000 extant species representing eight classes. Across 1000 extant species in the class Polyplacophora, chitons have a relatively constrained morphology but with some notable deviations. Several genera possess "shell eyes", i.e., true eyes with a lens and retina that are embedded within the dorsal shells. The phylogeny of the major chiton clades is mostly well established, in a set of superfamily-level and higher level taxa supported by various approaches, including morphological studies, multiple gene markers, mitogenome-phylogeny, and phylotranscriptomic approaches. However, one critical lineage has remained unclear, namely Schizochiton which was controversially suggested as being the potential independent origin of chiton shell eyes. Here, with the draft genome sequencing of Schizochiton incisus (superfamily Schizochitonoidea) plus assemblies of transcriptome data from other polyplacophorans, we present phylogenetic reconstructions using both mitochondrial genomes and phylogenomic approaches with multiple methods. We found that phylogenetic trees from mitogenomic data are inconsistent, reflecting larger scale confounding factors in molluscan mitogenomes. However, a consistent and robust topology was generated with protein-coding genes using different models and methods. Our results support Schizochitonoidea as the sister group to other Chitonoidea in Chitonina, in agreement with the established classification. Combined with evidence from fossils, our phylogenetic results suggest that the earliest origin of shell eyes is in Schizochitonoidea, and that these structures were also gained secondarily in other genera in Chitonoidea. Our results have generated a holistic review of the internal relationship within Polyplacophora, and a better understanding of the evolution of Polyplacophora.

A landmark-based geometric morphometric approach to quantify deviations from bilateral symmetry in polyplacophorans

The class Polyplacophora (chitons) represents a dorsoventrally flattened mollusk group that has an oval-shaped body covered with eight overlapping sclerites providing bilateral symmetry. Chitons show abnormalities (teratologies) that are characterized by symmetry deviations between the right and left sides of their bodies. As these deviations do not result in damage that affects vital functions, chitons are able to reach adult stages. In this study, we quantify the asymmetric condition of the species Chiton articulatus using a landmark-based geometric morphometric approach to assess variation in shape and bilateral symmetry. A geometric configuration of 22 landmarks and 50 semi-landmarks was created to evaluate shape variation in abnormal and deformed specimens compared to normal Chiton articulatus specimens. Vectors of change in the body shape configurations of chitons indicate that the greatest change occurs in the anterior part of the body, with less change in the middle and posterior parts. This gives chitons a widened appearance and provides anatomic compensation to restore the bilateral symmetry of the body scleritome. The diverse abnormalities and deformities had little impact on shape variations and confirmed that the coalescence condition is an intermediate step between a normal condition and the abnormal conditions of hypomerism or hypermerism. The low levels of fluctuating asymmetry expressed in C. articulatus indicate that despite living in areas of high stress, such as the rocky intertidal coast, this species maintains stability in its development and shape. Our results can serve as a model for studying bilateral symmetry deviation in polyplacophorans.

Species Composition of Molluscs in the Novorossiysk Bay

It is shown that the distribution of species composition of malacofauna of the coastal zone is uneven. The species diversity of malacofauna of the coastal zone of the Novorossiysk Bay includes 17 species of mollusks: 11 species (65% of all species found) belong to the class Gastropoda, 5 species (29%) to the class Bivalvia, and 1 species (6%) to the class Polyplacophora. The basis of abundance throughout the studied coastal part is created by bivalve mollusks, predominantly of the genus Mytilaster, and gastropods of the genus Rissoa. The Black Sea is inhabited by representatives of 3 classes of mollusks: gastropods, bivalve mollusks and shell mollusks. Species composition of coastal mollusks by status consists of 3 groups: mass (5 species), common (4 species) and rare (8 species).

Tightening the girdle: phylotranscriptomics of Polyplacophora

ABSTRACT The phylogenetic relationships within the molluscan class Polyplacophora (chitons) have been studied using morphology, traditional Sanger markers and mitogenomics, but, to date, no analysis has been carried out using transcriptomic and genome-wide data. Here, we leverage the power of transcriptomes to investigate the chiton phylogeny to test current classification schemes and the position of Callochitonidae, a family whose phylogenetic position and taxonomic assignation are uncertain because of conflicting results from past studies. Using multiple data matrices with different taxon occupancy thresholds and inference methods, including both concatenated and coalescence-based approaches, we find a consistent resolution of the chiton phylogeny. Our results support a system with the orders Lepidopleurida, Chitonida and Callochitonida, with the latter two as sister groups. This resolution is compatible with recent mitogenomic results and rejects the position of Callochiton as a member of Chitonina.

Life-history features and oceanography drive phylogeographic patterns of the chiton Acanthochitona cf. rubrolineata (Lischke, 1873) in the northwestern Pacific.

Chitons are a group of marine mollusks (class Polyplacophora) characterized by having eight articulating shell plates on their dorsal body surface. They represent suitable materials for studying the spatiotemporal processes that underlie population differentiation and speciation in ocean environments. Here we performed population genetic analyses on the northwestern Pacific chiton Acanthochitona cf. rubrolineata (Lischke, 1873) using two mitochondrial gene fragments (COI and 16S) from 180 individuals sampled from 11 populations among the coastal waters of Korea, Japan, and China. The phylogenetic network uncovered a reticulated relationship with several sub-haplogroups for all A. cf. rubrolineata haplotypes. SAMOVA analyses suggested the best grouping occurred at three groups (ΦCT = 0.151, P < 0.0001), which geographically corresponds to hydrographic discontinuity among the coastal regions of Korea, Japan, and China. The assumed limited dispersal ability of A. cf. rubrolineata, coupled with northeasterly flowing, trifurcate warm currents, might have contributed to the genetic differentiation among the three groups. Meanwhile, a high level of within-group genetic homogeneity was detected, indicating extensive coastal currents might facilitate gene flow among the populations within each group. Bayesian skyline plots demonstrated significant population expansion after the Last Glacial Period (110-25 thousand years ago) for all studied populations except the Japan group. Together these results suggest that the present-day phylogeographic patterns of A. cf. rubrolineata are strongly affected by the interplay of historical and/or contemporary oceanography and species-specific life-history features.

Listado actualizado y claves para Polyplacophora (Mollusca) en Cuba

Actualmente existe un parcial desconocimiento sobre cuáles especies de quitones están presentes en Cuba y sobre su correcto arreglo taxonómico, además, se carece de claves dicotómicas para su identificación. Empleando el listado de los moluscos marinos cubanos elaborado por Espinosa (2006), se actualizó la clase Polyplacophora empleando los criterios taxonómicos de García-Ríos (2003), Espinosa et al. (2012) y de Kaas y Van Belle (1998). Esta actualización permitió registrar por primera vez para Cuba un orden, dos subórdenes, cuatro familias, siete subfamilias, 12 géneros y 27 especies de quitones vivientes y se proponen claves que permiten la identificación de los taxones.

Polyplacophora (Mollusca) from reef ecosystems and associations with macroalgae on the Coast of Alagoas, Northeastern Brazil

Brazilian marine molluscs, especially Gastropoda and Bivalvia, are relatively well studied. However, information on the class Polyplacophora is more scarce, particularly on reef-dwelling forms. This study aimed to qualitatively and quantitatively analyze aspects of polyplacophorans from reef ecosystems and their associations with macroalgae on the coast of Maceio (state of Alagoas, Brazilian Northeast). The study area included five coral reefs at Ipioca, Ponta do Prego, Ponta do Meirim, Riacho Doce and Ponta Verde, as well as two sandstone reefs, located in Guaxuma and Sereia. The samples were obtained by snorkelling along the intertidal and subtidal reef zones to a depth of up to five meters during low tides, between 2009 and 2011. In addition, the chitons associated with three macroalgae of the Ponta Verde coral reef were studied based on collections made over 12 years (from the summer of 1998 to the winter of 2009). Three replicates with an area of 25 cm2 were collected from each of the following species of macroalgal phytals: Amphiroa fragilissima (Rhodophyta), Caulerpa racemosa (Chlorophyta) and Dictyota cervicornis (Phaeophyta). A total of 715 individuals (110 juveniles and 605 adults) were identified, including Acanthochitona terezae Guerra, 1983, Ischnochiton striolatus (Gray, 1828) and Ischnoplax pectinata (Sowerby II, 1840). Acanthochitona terezae was found for the first time in the area. Ischnochiton striolatus was the most abundant species in the reef ecosystem and in association with macroalgae. The greatest number of individuals of all three polyplacophorans identified (adults and juveniles) was found on the phytal A. fragilissima.

Just undiscovered or invasive? The first record of a chiton (Mollusca: Polyplacophora) from the remote Ascension Island, southern Atlantic Ocean

During a recent expedition to Ascension Island in the southern Atlantic Ocean, a polyplacophoran of the genusAcanthochitonawas collected, corresponding to the first recorded species of this molluscan class from this remote island. A scanning electron microscope study reveals a close affinity toAcanthochitona subrubicunda, which was so far only known from the Cape Verde Archipelago. Due to the lack of comparable type material the new finding warrants a confirmed identification and is only tentatively attributed toAcanthochitona subrubicunda. It is hypothesized that the class Polyplacophora was either overlooked during earlier surveys or colonized the island in more recent time. If the latter scenario took place, a transport by ships is thinkable.

Grazing under experimental hypercapnia and elevated temperature does not affect the radula of a chiton (Mollusca, Polyplacophora, Lepidopleurida)

Chitons (class Polyplacophora) are benthic grazing molluscs with an eight-part aragonitic shell armature. The radula, a serial tooth ribbon that extends internally more than half the length of the body, is mineralised on the active feeding teeth with iron magnetite apparently as an adaptation to constant grazing on rocky substrates. As the anterior feeding teeth are eroded they are shed and replaced with a new row. The efficient mineralisation and function of the radula could hypothetically be affected by changing oceans in two ways: changes in seawater chemistry (pH and pCO2) may impact the biomineralisation pathway, potentially leading to a weaker or altered density of the feeding teeth; rising temperatures could increase activity levels in these ectothermic animals, and higher feeding rates could increase wear on the feeding teeth beyond the animals' ability to synthesise, mineralise, and replace radular rows. We therefore examined the effects of pH and temperature on growth and integrity in the radula of the chiton Leptochiton asellus. Our experiment implemented three temperature (∼10, 15, 20 °C) and two pCO2 treatments (∼400 μatm, pH 8.0; ∼2000 μatm, pH 7.5) for six treatment groups. Animals (n = 50) were acclimated to the treatment conditions for a period of 4 weeks. This is sufficient time for growth of ca. 7–9 new tooth rows or 20% turnover of the mineralised portion. There was no significant difference in the number of new (non-mineralised) teeth or total tooth row count in any treatment. Examination of the radulae via SEM revealed no differences in microwear or breakage on the feeding cusps correlating to treatment groups. The shell valves also showed no signs of dissolution. As a lineage, chitons have survived repeated shifts in Earth’s climate through geological time, and at least their radulae may be robust to future perturbations.

Species richness and community structure of class Polyplacophora at the intertidal rocky shore on the marine priority region no. 33, Mexico

Marine Priority Region No. 33 (MPR 33) lies in the State of Guerrero, where the National Commission for Knowledge and Use of Biodiversity has declared a lack of information on marine species, Class POLYPLACOPHORA among others. This program was carried out on the rocky intertidal zone of MPR 33 with the objectives to: 1) determine species richness, 2) community composition based on families represented, 3) estimate the density, 4) index of diversity, 5) analyze body proportions and 6) geographic distribution of species. Three surveys on five sites were conducted during 2009 and 2012, using one mt2 quadrant for systematic sampling procedure; collecting all POLYPLACOPHORA specimens found within the quadrants, then proceeding with their preservation. 441 organisms were analyzed where 17 species were identified, on nine genera within six families. Fifteen new species were recorded for the MPR 33. Family ISCHNOCHITONIDAE Dall, 1889 was the best represented in species richness and CHITONIDAE Rafinesque, 1815 in abundance. Density of 8.82 organisms/m2, the H′ index was 2.38 bits/individuals and J′ = 0.57. The species Chiton (Chiton) articulatus Sowerby, 1832 showed the highest density, greater size and was found in all five collection sites. Three species, (Chiton (C.) articulatus, Ischnochiton (Ischnochiton) muscarius (Reeve, 1847) and Chiton (Chiton) alboli-neatus Broderip & Sowerby, 1829), were considered wide. Intertidal species richness was associated with habitat stability and wave intensity; the more unstable sites had greater richness while most stable sites lower. Species inventory and understanding the communities’ interactions of the marine fauna of the State of Guerrero are important; therefore these efforts must be continued.

Functional morphology and molecular evolution of chiton ‘shell’ eyes

Event Abstract Back to Event Functional morphology and molecular evolution of chiton ‘shell’ eyes Daniel I. Speiser1*, Sonke Johnsen2 and Todd H. Oakley1 1 University of California Santa Barbara, Ecology, Evolution, and Marine Biology, United States 2 Duke University, Biology Department, United States Image-forming eyes have evolved multiple times in invertebrates and it is almost certain that they have done so through separate step-wise processes. Here, we ask: 1) what are the functional consequences associated with the separate steps of eye evolution and 2) what are the changes in genotype associated with the transitions between these steps? To address these two questions, we are working with chitons (Phylum Mollusca; Class Polyplacophora), chosen because they have unique sensory structures embedded in their shell plates (“aesthetes”) that vary from the ancestral condition of non-pigmented, light-sensitive cells (“photoreceptors”), to pigmented cell clusters (“eyespots”), to what are likely the most recently evolved lens-bearing eyes of any animal (“eyes”; see Figure 1). Chitons also display multiple light-influenced behaviors that are simple, stereotyped, and easily-manipulated. Thus, chitons may reveal how new light-interacting structures (photoreceptors, pigment cells, and lenses) drive new light-influenced behaviors (light-detection, directional photoreception, and spatial vision). We are also using transcriptome sequencing to compare gene expression between the aesthetes of chitons with non-pigmented photoreceptors, eyespots, and eyes. Through behavioral trials and morphological examination, we find that the chiton Acanthopleura granulata forms images using birefringent ‘shell’ lenses made of solid aragonite (visible in Figure 1). Chitons that lack lenses do not demonstrate spatial vision but, intriguingly, are more sensitive to small changes in illumination than chitons with eyes. Our behavioral trials also indicate that A. granulata’s eyes provide equal angular resolution in both air and water. We hypothesize that one of the two refractive indices of the birefringent chiton lens places a focused image on the retina in air, whereas the other does so in water. Through transcriptome sequencing, we find that the aesthetes of chitons -- including those from species with non-pigmented photoreceptors, eyespots, and eyes -- generally express a wide range of vision-related genes, including G-proteins, arrestins, and several types of opsin. Finally, using immunohistochemistry we find that the non-pigmented photoreceptors, but not the eyes, of A. granulata express an r-type opsin similar to those expressed by the cephalic eyes of many invertebrates. We hypothesize that A. granulata has multiple photo-systems that operate via different phototransduction pathways and mediate different light-influenced behaviors. Figure 1 Keywords: chitons, Spatial Vision, eye evolution, Comparative genomics, phylogenomics, Behavior, Animal Conference: International Conference on Invertebrate Vision, Fjälkinge, Sweden, 1 Aug - 8 Aug, 2013. Presentation Type: Oral presentation preferred Topic: Development and evolution Citation: Speiser DI, Johnsen S and Oakley TH (2019). Functional morphology and molecular evolution of chiton ‘shell’ eyes. Front. Physiol. Conference Abstract: International Conference on Invertebrate Vision. doi: 10.3389/conf.fphys.2013.25.00011 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 01 Mar 2013; Published Online: 09 Dec 2019. * Correspondence: Dr. Daniel I Speiser, University of California Santa Barbara, Ecology, Evolution, and Marine Biology, Santa Barbara, CA, 93106-6150, United States, dispeiser@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Daniel I Speiser Sonke Johnsen Todd H Oakley Google Daniel I Speiser Sonke Johnsen Todd H Oakley Google Scholar Daniel I Speiser Sonke Johnsen Todd H Oakley PubMed Daniel I Speiser Sonke Johnsen Todd H Oakley Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

‘Catch Mechanism’ found in Molluscan Animals other than Bivalves

Some muscles of bivalve molluscs (phylum Mollusca, class Bivalvia) such as the smooth part of the adductor and the byssus retractor are known as ‘catch muscles’ that can maintain high passive tension for long periods with little energy expenditure after their active contraction. The state of this high passive tension is called ‘catch’ and it has been hypothesized that some special ‘catch mechanism’ exists for the maintenance of the tension. We have developed an in vitro catch assay in which the catch state is reconstituted by using proteins purified from bivalve catch muscles. With this assay, we have revealed that the catch tension is due to binding of actin filaments to myosin filaments which can be directly visualized under a light microscope. We have also found that twitchin, a giant protein associated with myosin filaments structurally related to connectin/titin, and to less extent, vertebrate cardiac myosin binding protein C, is an essential key component for the catch mechanism. Twitchin was originally found in a nematode Caenorhabditis elegans, and was later found widely in many invertebrates. This fact raises a question whether animals having twitchin other than bivalves also have the catch mechanism. To answer this, we prepared synthetic thick filaments containing myosin and twitchin from organs of molluscan animal species other than bivalves. We used foot of a spiral shellfish Monodonta labio (class Gastropoda) and a chiton Liolophura japonica (class Polyplacophora), and inner wall of suckers of an octopus Octopus vulgaris (class Cephalopoda). We performed the in vitro catch assay and found that the catch mechanism, at least at the molecular level, exists also in these molluscan animals. The results suggest that the catch mechanism is distributed more widely in Animal Kingdom than we know.

Polyplacophora species richness, composition and distribution of its community associated with the intertidal rocky substrate in the marine priority region No. 32 in Guerrero, Mexico

The coast of Acapulco is located within the Marine Priority Region No. 32 (MPR No. 32) at the State of Guerrero, Mexico. The National Commission for the Knowledge and Use of Biodiversity has stated the lack of information on marine species that live there, among which are the Class Polyplacophora. This study aimed to: 1) determine species richness, 2) the community structure based on the representative families, 3) estimate the density, 4) locate species distribution, 5) determine their degree of occurrence (hierarchical position) within the community, 6) analyze the structure of length and width across the populations and 7) estimate the diversity index. The intertidal rocky substrate at seven beaches was sampled from 2009 to 2012. The sampling unit 1 m2, while the area sampled was 10 m2. It was analyzed 2.548 specimens of Polyplacophora, with 20 species identified. Tonicellidae and Ischnochitonidae were the Families better represented inspecies richness and Chitonidae in abundance. Lepidochitona flectens is a new record for the Mexican Tropical Pacific and Stenoplax mariposa for MPR. No. 32. The density was 25.48 specimens/m2. Eight dominant species were determined. Chiton albolineatus had the highest density. Ischnochiton muscarius, Chaetopleura unilineata, Chiton albolineatus and Chiton articulatus presented wide distribution. Chiton articulatus showed the largest size in length and width. H’ = 2.01 bits/ individual. Numerous dominant species were present with broad and regular distribution, probably due to Polyplacophora body design, which allows for better adaptation to the rigorous environmental conditions of the rocky intertidal.

First record of Acanthopleura vaillantii (Mollusca: Polyplacophora) from Iran - Chabahar Bay in the Oman Sea

Chitons are an ancient molluscan lineage with a generally conservative appearance and grazing habit (Ernisse, 2004). Most Chitons (Polyplacophora) live in the intertidal zone and shallow water, but some species dwell in rather deep water. Chitons are distributed in all oceans from the Polar regions through to the tropics. Despite the larger number of molluscan species recorded from Chabahar Bay (e.g. Ashja Ardalan, 1993; Samaei, 1994) and the Iranian coasts of the Persian Gulf (e.g. Kosuge, 1998; Hosseinzadeh et al., 2001), there is no record of Acanthopleura vaillantii de Rochebrune, 1882 in Chabahar Bay. This is the first time that this species of the class Polyplacophora is recorded in Chabahar Bay (Figure 1).

A new genus and two new species of multiplacophorans (Mollusca, Polyplacophora, Neoloricata), Mississippian (Chesterian), Indiana

Deltaplax new genus, Deltaplax burdicki new species, and Deltaplax dellangeloi new species (Mollusca, Polyplacophora, Neoloricata, Multiplacophora) from the Mississippian Lower Buffalo Wallow Group (Chesterian) of Indiana, USA are described. the new genus is established by one partially articulated and one associated specimen with marginal fringes of two types of large spines, bilaterally symmetrical head and tail valves, and fifteen medial valves arranged in three longitudinal columns similar to those described previously for other multiplacophorans. the two specimens represent separate species differentiated by morphologies of the auxiliary valves, one type of spine, and subtrapezoidal versus triangular tail valves. the tail valve of the articulated specimen also had sutural laminae that projected under the preceding intermediate valve. the presence of sutural laminae allows for placement of multiplacophorans in Subclass Neoloricata of Class Polyplacophora. the head, intermediate, and tail valves are mucronate with comarginal growth lines and ridged insertion plates that probably inserted into soft tissue comparable to the girdle of modern polyplacophorans. the new specimens also indicate one left-handed, one auxiliary, and one right-handed valve in multiplacophorans was equivalent to a single bilaterally-symmetrical intermediate valve of extant polyplacophorans. However, multiplacophoran head valves have plates that project from the lower layer at the lateral margins and articulate with the first intermediate valves that overlap the head and second intermediate valves. These features have not been observed in more typical neoloricates, fossil or modern. Pending systematic revision of the class, Multiplacophora thus is retained as a separate order distinguished by the unique shared characters.

Towards a phylogeny of chitons (Mollusca, Polyplacophora) based on combined analysis of five molecular loci

This study represents the first phylogenetic analysis of the molluscan class Polyplacophora using DNA sequence data. We employed DNA from a nuclear protein-coding gene (histone H3), two nuclear ribosomal genes (18S rRNA and the D3 expansion fragment of 28S rRNA), one mitochondrial protein-coding gene (cytochrome c oxidase subunit I), and one mitochondrial ribosomal gene (16S rRNA). A series of analyses were performed on independent and combined data sets. All these analyses were executed using direct optimization with parsimony as the optimality criterion, and analyses were repeated for nine combinations of parameters affecting indel and transversion/transition cost ratios. Maximum likelihood was also explored for the combined molecular data set, also using the direct optimization method, with a model equivalent to GTR + I + Γ that accommodates gaps. The results of all nine parameter sets for the combined parsimony analysis of all molecular data (as well as ribosomal data) and the maximum-likelihood analysis of all molecular data support monophyly of Polyplacophora. The resulting topologies mostly agree with a division of Polyplacophora into two major lineages: Lepidopleuridae and Chitonida (sensu Sirenko 1993). In our analyses the genus Callochiton is positioned as the sister group to Lepidopleuridae, and not as sister group to the remaining Chitonida (sensu BucklandNicks & Hodgson 2000), nor as the sister group to the remaining Chitonina (sensu Buckland-Nicks 1995). Chitonida (excluding Callochiton) is monophyletic, but conventional subgroupings of Chitonida are not supported. Acanthochitonina (sensu Sirenko 1993) is paraphyletic, or alternatively monophyletic, and is split into two clades, both with abanal gills only and cupules in the egg hull, but one has simple cupules whereas the other has more strongly hexagonal cupules. Sister to the Acanthochitonina clades is Chitonina, including taxa with adanal gills and a spiny egg hull. Schizochiton, the only genus with adanal gills that has an egg hull with cupules, is the sister-taxon to one of the Acanthochitonina clades plus Chitonina, or alternatively basal to Chitonina. Support values for either position are low, leaving this relationship unsettled. Our results refute several aspects of conventional classifications of chitons that are based primarily on shell characters, reinforcing the idea that chiton classification should be revised using additional characters.

Gastropod arginine kinases from Cellana grata and Aplysia kurodai. Isolation and cDNA-derived amino acid sequences

Arginine kinase (AK) was isolated from the radular muscle of the gastropod molluscs Cellana grata (subclass Prosobranchia) and Aplysia kurodai (subclass Opisthobranchia), respectively, by ammonium sulfate fractionation, Sephadex G-75 gel filtration and DEAE-ion exchange chromatography. The denatured relative molecular mass values were estimated to be 40 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isolated enzyme from Aplysia gave a K m value of 0.6 mM for arginine and a V max value of 13 μmole Pi min −1 mg protein −1 for the forward reaction. These values are comparable to other molluscan AKs. The cDNAs encoding Cellana and Aplysia AKs were amplified by polymerase chain reaction, and the nucleotide sequences of 1608 and 1239 bp, respectively, were determined. The open reading frame for Cellana AK is 1044 nucleotides in length and encodes a protein with 347 amino acid residues, and that for A. kurodai is 1077 nucleotides and 354 residues. The cDNA-derived amino acid sequences were validated by chemical sequencing of internal lysyl endopeptidase peptides. The amino acid sequences of Cellana and Aplysia AKs showed the highest percent identity (66–73%) with those of the abalone Nordotis and turbanshell Battilus belonging to the same class Gastropoda. These AK sequences still have a strong homology (63–71%) with that of the chiton Liolophura (class Polyplacophora), which is believed to be one of the most primitive molluscs. On the other hand, these AK sequences are less homologous (55–57%) with that of the clam Pseudocardium (class Bivalvia), suggesting that the biological position of the class Polyplacophora should be reconsidered.

Molluscan Phylogeny: The Paleontological Viewpoint

Stasek (1) theorized that the extant mollusks are the progeny of three separate lineages that separated before the phylum was well established. He wrote that no known intermediate forms, fossil or living, bridge the "enormous gaps between any two of the three lineages," and therefore treated each as a separate subphylum. These subphyla are (i) the subphylum Aculifera Hatscheck 1891, containing only the class Aplacophora, derived from the most primitive ancestors of the Mollusca; (ii) the subphylum Placophora von Jhering 1876, containing only the class Polyplacophora, and emphasizing the pseudometamerism of its more advanced premollusk ancestor; and (iii) the subphylum Conchifera Gegenbaur 1878, containing the Monoplacophora and the other classes derived from it. We point out that the Polyplacophora may be derived from the Monoplacophora instead of a more primitive ancestral stock. We also suggest that the Conchifera can be separated into two major lineages, each worthy of the rank of subphylum. The fossil record indicates that the Monoplacophora gave rise to the Gastropoda, Cephalopoda, Rostroconchia, and possibly Polyplacophora, and that the Pelecypoda and Scaphopoda are derived from the Rostroconchia. These last three classes thus form a lineage that diverged from the Monoplacophora in the Early Cambrian. They emphasized a shell form that in all groups is primitively open at both ends, allowing the gut to remain relatively straight, with an anterior mouth and posterior anus. They became burrowing (infaunal) deposit or filter feeders. We coin the term Diasoma (through-body) for the subphylum containing these three classes (Rostroconchia, Pelecypoda, and Scaphopoda). The remaining three classes (Monoplacophora, Gastropoda, and Cephalopoda) emphasize a conical univalved shell, usually twisted into a spiral. The relatively small single aperture forces the anus to lie close to the mouth, and the gut is bent into a "U." Most are surface-dwelling (epifaunal) grazers or carnivores. We coin the name Cyrtosoma (hunchback-body) for the subphylum containing these three classes. Strictly speaking, the cyrtosomes are the ancestors of the diasomes but, in fact, both subphyla appeared and began to diversify within a few million years in the Early Cambrian. Note added in proof: After proofs were corrected we were informed that the new genus Opikella (40) is preoccupied by (Opikella = Oepikella) Thorslund 1940, an Ordovican ostracod. We rename the mollusk genus Oepikila.

Classification of the Class Polyplacophora, with a List of Japanese Chitons

Classification of the Class Polyplacophora, with a List of Japanese Chitons