The Verdigris River in Kansas and Oklahoma, USA once held a diverse native unionid mussel fauna although a number of these populations have declined in richness and abundance. There is recent evidence that populations of some species of unionid mussels are increasing in parts of the Verdigris River in Kansas, but no current data exist for Oklahoma. In addition, zebra mussels (Dreissena polymorpha Pallas, 1771) have been introduced into a major impoundment on the Verdigris River, Oologah Lake, and may further threaten mussel populations downstream from this reservoir. This study updates the distribution and abundance of native mussels in the Oklahoma portion of the Verdigris River upstream and downstream of Oologah Lake, and documents the current distribution of zebra mussels in this region. Thirty-one sites were sampled and a significant increase in species richness and abundance of native mussels was observed as compared to a 1997 study. Two species of special interest, Cyprogenia aberti (Conrad, 1850) and Quadrula cylindrica (Say, 1817), were found. Zebra mussels were not found at sites upstream of Oologah Lake but were present at every downstream site. In September 2006, zebra mussel byssal threads were observed on shells of a number of native mussels downstream from Oologah Lake, but unionid richness and abundance were not significantly different between sites above and below the reservoir.

Distribution and abundance of the molluscan fauna was studied in the intertidal and subtidal soft-bottoms of the Ensenada de San Simón (NW Spain). Depth, grain size, and total organic matter were the most important factors in determining distribution patterns of molluscs in this inlet. Three major malacological assemblages have been determined in the Ensenada de San Simón, two of them subdivided in two facies. In the intertidal area of the inlet, one facies (A1) was located in areas associated with seagrass meadows of Zostera spp. and was dominated by Hydrobia ulvae (Pennant, 1777) whereas the second facies (A2) had a high dominance of H. ulvae, Cerastoderma edule (Linnaeus, 1758), and Tapes decussatus (Linnaeus, 1758). An impoverished facies of this community was present in reduced, muddy bottoms (Group C). In the subtidal bottoms, one group (B1) was located in the central part of the inlet with H. ulvae, Rissoa labiosa (Montagu, 1803), Turboella radiata (Philippi, 1836), Parvicardium exiguum (Gmelin in Linnaeus, 1791), Loripes lacteus (Linnaeus, 1758), and Abra nitida (Müller, 1789) as characteristic species. A second facies (B2) was found in outer areas of the inlet, characterized by Thyasira flexuosa (Montagu, 1803), Mysella bidentata (Montagu, 1803), Abra alba (Wood, 1802), and Nucula nitidosa Winckworth, 1930.

Echinochiton dufoei Pojeta et al., 2003 is now known from seven specimens. The new material shows the anterior end and allows for a full reconstruction of the animal. The hollow spines are circumsomal; they were flexible and perhaps moveable in rotary anterior-posterior directions. Possible functions for the hollow spines are discussed. The relationships of E. dufoei to other chitons and to other molluscs and mollusc-like organisms are presented.

A glutaraldehyde fixative developed for preserving the radula superior epithelium of the adult chiton Acanthopleura hirtosa (Blainville, 1825), was used in conjunction with conventional and microwave-assisted sample processing to produce high quality tissue preservation for light and electron microscopy. In addition, high-pressure freezing (HPF) and cryo-substitution were used to fix the radula tissue of juvenile specimens. Microwave-assisted fixation was preferred to conventional bench-top techniques due to the superior preservation of fine cell structure together with reduced processing times and chemical exposure. Although restricted to very small (<200 μm) samples, the quality of juvenile radulae processed by HPF was excellent. The improvements in tissue preservation using microwave and cryo-preservation techniques are therefore critical for obtaining accurate ultrastructural information on the radula in marine molluscs. In particular, these findings highlight additional processing options available for the study of cellular structures in biomineralizing tissues.

Traditional shell characters are insufficient to differentiate taxa within the polyplacophoran order Lepidopleurida. Additional morphological character sets from soft anatomy (e.g., gamete morphology, gill arrangement, and locations of gonopores and nephidiopores) have previously been described from only a small number of taxa. This study reports for the first time, positions of the gonopores and nephridiopores for 17 species in the Lepidopleurina. The position of both types of pores on the longitudinal body axis varies within a generalized range of the posterior third of the body; however, the separation between the pores as a proportion of the specimen's foot length varies from 3.7% to 17% in different species. Positions of pores relative to the serial gills are also variable within species, and future studies may require a new descriptive basis in order to resolve positional homology. The order Lepidopleurida occupies a critical position with respect to understanding larger-scale patterns in polyplacophoran (and molluscan) evolution.

The aesthete canals of fourteen chiton species were cast with epoxy, allowing detailed examination and comparison of the entire canal system that infiltrates their valves (shell plates). Some species in this study have been classified without question in the family Mopaliidae (Mopalia ciliata (Sowerby, 1840), Mopalia lignosa (Gould, 1846), Mopalia spectabilis Cowan and Cowan, 1977, Mopalia swanii Carpenter, 1864, Katharina tunicata (Wood, 1815)), while other species have been placed in that family by some workers but not others (Dendrochiton flectens (Carpenter, 1864), Dendrochiton lirulatus (Berry, 1963), Tonicella insignis (Reeve, 1847), Tonicella lineata (Wood, 1815), Tonicella lokii Clark, 1999, Tonicella marmorea (Fabricius, 1780), Nuttallochiton mirandus (Thiele, 1906), Plaxiphora aurata (Spalowski, 1795)), and one has never been placed in the Mopaliidae (Tonicia chilensis (Frembly, 1827)). The results provide additional evidence that there is high diversity in aesthete canal morphology but also some striking resemblances interpreted here as homologies, reaffirming that aesthete canal characters have considerable potential for phylogenetic analyses and for supporting classification ranks ranging from suborder to species. In this case, the results are broadly consistent with traditional classifications of mopaliids, but Tonicella and Dendrochiton (taxa not always thought not to be mopaliids) share many aesthete canal synapomorphies with undisputed mopaliids, whereas Plaxiphora (typically thought to be a mopaliid) has an aesthete canal system more similar to non-mopaliid members of the Acanthochitonina. These differences are in line with results of recent phylogenetic analyses of the Mopaliidae.

The hairy chiton Mopalia kennerleyi Carpenter, 1864 is distinguished from its congener Mopalia ciliata (Sowerby, 1840), and the identity of Chiton wosnessenskii von Middendorff, 1847 is clarified. Mopalia kennerleyi and M. ciliata are distinguished by setae structure, valve sculpture, and radular teeth. Their characteristics are illustrated and discussed, and their distributions defined.

Recent deep-sea trawling in the Monterey Sea Canyon, California has brought to light two previously unknown bathyal chitons. The new species, members of the family Ischnochitonidae, are placed in the genus Tripoplax Berry, 1919, here in raised to full generic rank on the basis of morphological and ecological characteristics. Tripoplax calypso spec. nov. and Tripoplax cowani spec. nov. are described, illustrated, and compared to similar species from the region.

The chiton fossil record is richer than previously reported in the literature. A newly compiled database comprised of Cambrian to Pleistocene fossil chitons totals 2594 occurrences of 900 species. Of the 900, 430 are named species known only as fossils, 123 are extant species that also have a fossil record, and 247 are indeterminate taxa. Most of the database (61%) consists of fossil chiton occurrences reported from localities other than type localities. A preliminary analysis of the data using the collector curve method suggests that the chiton fossil record has not been adequately sampled by geographic regions or geologic time. The fossil record of chitons is incomplete, sporadic, and geographically limited because the sampling record has been incomplete, sporadic, and geographically limited. The current database comprises enough information to discern diversity patterns throughout geologic time, but whether the patterns are real or artifacts of sampling inadequacy remains to be investigated.

Studies of gamete structure and fertilization biology have revealed much about the phylogeny of molluscs. Recent studies of fertilization in chitons support the view that the basal order of chitons, the Lepidopleurida, fertilize eggs, as most molluscs do, by fusing the entire sperm with the egg and transferring chromatin, mitochondria, and centrioles into the egg cytoplasm. However, current evidence suggests that all members of the order Chitonida inject only chromatin into the egg. These chitons, which include the controversial family Callochitonidae, share a series of synapomorphic characters based on their fertilization biology that makes them unique. Current evidence suggests that Callochitonidae are basal to this order and sister taxa to the remaining Chitonida, which have been divided into two suborders, the Chitonina and Acanthochitonina. New evidence indicates that Chitonina have at least two different mechanisms for penetrating the egg. One group of species has pores in the egg hull (e.g., Chaetopleura apiculata (Say in Conrad, 1834) and Stenosemus albus (Linnaeus, 1767), whereas a second group has a continuous dense layer on the surface of the egg hull that is digested by the sperm (e.g., Rhyssoplax tulipa (Quoy and Gaimard, 1835) and Stenoplax conspicua (Pilsbry, 1892)). However, the genus Ischnochiton Gray, 1847 appears to be polyphyletic, as several species have distinctive characters that typify other genera or families. In particular, this genus needs to be re-evaluated using modern morphological and molecular methods. All of the Chitonina have spiny-hulled eggs with narrow bases and are quite different from the second suborder, Acanthochitonina, which is characterized by large-hull cupules with wide bases. Within Acanthochitonina, some species have open-hull cupules, whereas most have closed ones. Open-cupule species lack micropores in the hull for sperm entry, whereas several closed-cupule species exhibit micropores between hull cupules. These features of the egg are accompanied by alterations in sperm structure, such as position of the mitochondria and structure of the basal body, acrosome, and flagellum. Knowledge of the gamete structure of individual species and their fertilization biology, as demonstrated here, provides a different series of characters that can help avoid mistakes that are inherent during early development of new methods, such as molecular analyses. New details of fertilization biology have made it possible to revise preliminary analyses and provide an updated phylogeny of chitons, which differs in some important respects from other recent publications.

Three new species of chitons are described from hydrothermal vent sites and methane seep sites around Japan: Deshayesiella sirenkoi n. sp. from the hydrothermal vent sites on the seamounts in the northern Mariana Islands area, Placiphorella okutanii n. sp. from Hachijo Depression in the Izu-Ogasawara (Bonin) Islands area where no active vent/seep area has been discovered, but the possibility of hydrothermal activity has been suggested, and Placiphorella isaotakii n. sp. from methane seep sites on the Kuroshima Knoll off Yaeyama Islands. Deshayesiella sirenkoi n. sp. as well as two previously known hydrothermal vent species, Leptochiton tenuidontus Saito and Okutani, 1990 and Thermochiton undocostatus Saito and Okutani, 1990, are vent/seep associated species, whereas the two Placiphorella may be guest species. Additional distributional records are given for the two known species.