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Background The radula, a chitinous membrane with embedded teeth, is one important molluscan autapomorphy. In some taxa (Polyplacophora and Patellogastropoda) one tooth type (the dominant lateral tooth) was studied intensively in the last decades with regard to its mechanical properties, chemical and structural composition, and the relationship between these parameters. As the dominant lateral tooth is probably one of the best studied biological materials, it is surprising, that data on elements and mechanical properties of the other tooth types, present on a chiton radula, is lacking. Results We provide data on the elemental distribution and mechanical properties (hardness and elasticity, i.e. Young’s modulus) of all teeth from the Polyplacophora Lepidochitona cinerea (Linnaeus, 1767) [Chitonidae: Ischnochitonidae]. The ontogeny of elements, studied by energy-dispersive X-ray spectroscopy, and of the mechanical properties, determined by nanoindentation, was analysed in every individual tooth type. Additionally, we performed breaking stress experiments with teeth under dry and wet condition, highlighting the high influence of the water content on the mechanical behaviour of the radula. We thereby could determine the forces and stresses, teeth can resist, which were previously not studied in representatives of Polyplacophora. Overall, we were able to relate the mineral (iron, calcium) content with the mechanical parameters (hardness and Young’s modulus) and the breaking force and stress in every tooth type. This led to a better understanding of the relationship between structure, material, and function in radular teeth. Further, we aimed at determining the role of calcium for the mechanical behaviour of the teeth: we decalcified radulae by ethylene diamine tetra acetic acid and performed afterwards elemental analyses, breaking stress experiments, and nanoindentation. Among other things, we detected that wet and decalcified radular teeth could resist highest forces, since teeth have a higher range of bending motion leading to a higher capability of teeth to gain mechanical support from the adjacent tooth row. This indicates, that the tooth material is the result of a compromise between failure reduction and the ability to transfer forces onto the ingesta. Conclusion We present novel data on the elemental composition, mechanical properties, and the mechanical behaviour of chiton teeth, which allows conclusions about tooth function. We could also relate the parameters mentioned, which contributes to our understanding on the origins of mechanical property gradients and the processes reducing structural failure in radular teeth. Additionally, we add more evidence, that the elemental composition of radular is probably species-specific and could be used as taxonomic character.

A revision of Black Sea chitons of the genus Lepidochitona has been conducted. The revision revealed new features of intrageneric similarity and differences of Black Sea species as well as with Mediterranean species. Species similarity of Black Sea and Mediterranean representatives of Lepidochitona cinerea is confirmed. A new species L. bondarevi sp. nov. is described. Previously its specimens were attributed erroneously to L. caprearum. The new species differs from L. caprearum in having a less broad head valve, shorter apophyses, a much longer postmucronal area on the tail valve, 7–8 longitudinal grooves around the upper half of dorsal girdle spicules and smooth dorsal girdle needles.

A new shallow-water species Lepidochitona cubensis sp. nov. collected in the Caribbean Sea off Cuba in sea grass Thalassia testudinum is described. This chiton is closest to the species L. bullocki with which it forms a separate group within the genus, differing from other species of the genus by the presence of small square grooves on tegmentum of valves.

This study describes and recognises, using histological and microscopical examinations on a morphometrical basis, several gonad traits through the early life stages of Chiton articulatus and C. albolineatus. Gonadal ontogenesis, gonad development stages, sexual differentiation, onset of the first sexual maturity, and growth sequences or \"early life stages\" were determined. In addition, allometry between lengths and body weight pooled for both sexes per each chiton were calculated using equation Y = aX(b) . A total of 125 chitons (4≤TL≤40 mm, in total length \"TL\") were used. All allometric relations showed a strong positive correlation (r), close to 1, with b-values above three, indicating an isometric growth. Gonadal ontogenesis and gonad development stages were categorised into three periods (\"Pw\" without gonad, \"Pe\" gonad emergence, and \"Pf\" gonadal sac formed) and four stages (\"S0\" gametocytogenesis, \"S1\" gametogenesis, \"S2\" mature, and \"S3\" spawning), respectively. Compound digital images were attained for each process. Periods and stages are overlapped among them and between species, with the following overall confidence intervals in TL: Pw 6.13-14.32 mm, Pe 10.32-16.93 mm, Pf 12.99-25.01 mm, S0 16.08-24.34 mm (females) and 19.51-26.60 mm (males), S1 27.15-35.63 mm (females) and 23.45-32.27 mm (males), S2 24.48-40.24 mm (females) and 25.45-32.87 mm (males). Sexual differentiation (in S0) of both chitons occurs first as a female then as a male; although, males reach the onset of the first sexual maturity earlier than females, thus for C. articulatus males at 17 mm and females at 32 mm, and for C. albolineatus males at 23.5 mm and females at 28 mm, all in TL. Four early life stages (i.e., subjuvenile, juvenile, subadult, and adult) are described and proposed to distinguish growth sequences. Our results may be useful to diverse disciplines, from developmental biology to fisheries management.

The discovery of a sensory organ, the Schwabe organ, was recently reported as a unifying feature of chitons in the order Lepidopleurida. It is a patch of pigmented tissue located on the roof of the pallial cavity, beneath the velum on either side of the mouth. The epithelium is densely innervated and contains two types of potential sensory cells. As the function of the Schwabe organ remains unknown, we have taken a cross-disciplinary approach, using anatomical, histological and behavioural techniques to understand it. In general, the pigmentation that characterises this sensory structure gradually fades after death; however, one particular concentrated pigment dot persists. This dot is positionally homologous to the larval eye in chiton trochophores, found in the same neuroanatomical location, and furthermore the metamorphic migration of the larval eye is ventral in species known to possess Schwabe organs. Here we report the presence of a discrete subsurface epithelial structure in the region of the Schwabe organ in Leptochiton asellus that histologically resembles the chiton larval eye. Behavioural experiments demonstrate that Leptochiton asellus with intact Schwabe organs actively avoid an upwelling light source, while Leptochiton asellus with surgically ablated Schwabe organs and a control species lacking the organ (members of the other extant order, Chitonida) do not (Kruskal-Wallis, H = 24.82, df = 3, p < 0.0001). We propose that the Schwabe organ represents the adult expression of the chiton larval eye, being retained and elaborated in adult lepidopleurans.

We describe the karyotype, location of nucleolus-organizing regions (NORs) and heterochromatin composition and distribution in Lepidochitona caprearum (Scacchi, 1836). The examined specimens had 2n=24 chromosomes; the elements of pairs 1-4 were metacentric, subtelocentric those of the fifth pair, telocentric the elements of other pairs. NOR-FISH, Ag-NOR- and CMA3 banding showed NORs localized on pericentromeric regions of a medium small sized, telocentric chromosome pair. After C-banding or digestions with restriction enzyme NOR associate heterochromatin only was cytologically evident, resulting CMA3 positive. The comparison with chromosome data of other chitons, other than to evidence a karyotypic similarity of Lepidochitona caprearum to species of suborder Acanthochitonina, allows us to infer that chromosome evolution in the suborder mainly occurred via reduction of the number of the chromosomes by centric fusions, which took place repeatedly and independently in the different lineages of Acanthochitonina.

BACKGROUND: Two types of excretory systems, protonephridia and metanephridial systems are common among bilaterians. The homology of protonephridia of lophotrochozoan taxa has been widely accepted. In contrast, the homology of metanephridial systems – including coelomic cavities as functional units – among taxa as well as the homology between the two excretory systems is a matter of ongoing discussion. This particularly concerns the molluscan kidneys, which are mostly regarded as being derived convergently to the metanephridia of e.g. annelids because of different ontogenetic origin. A reinvestigation of nephrogenesis in polyplacophorans, which carry many primitive traits within molluscs, could shed light on these questions. RESULTS: The metanephridial system of Lepidochitona corrugata develops rapidly in the early juvenile phase. It is formed from a coelomic anlage that soon achieves endothelial organization. The pericardium and heart are formed from the central portion of the anlage. The nephridial components are formed by outgrowth from lateral differentiations of the anlage. Simultaneously with formation of the heart, podocytes appear in the atrial wall of the pericardium. In addition, renopericardial ducts, kidneys and efferent nephroducts, all showing downstream ciliation towards the internal lumen, become differentiated (specimen length: 0.62 mm). Further development consists of elongation of the kidney and reinforcement of filtration and reabsorptive structures. CONCLUSIONS: During development and in fully formed condition the metanephridial system of Lepidochitona corrugata shares many detailed traits (cellular and overall organization) with the protonephridia of the same species. Accordingly, we suggest a serial homology of various cell types and between the two excretory systems and the organs as a whole. The formation of the metanephridial system varies significantly within Mollusca, thus the mode of formation cannot be used as a homology criterion. Because of similarities in overall organization, we conclude that the molluscan metanephridial system is homologous with that of the annelids not only at the cellular but also at the organ level.

Recent collecting provided fossil chitons (Mollusca, Polyplacophora) from upper Eocene to lower Oligocene deposits of Washington State, U.S.A. The study material consists of 140 valves from six localities in the Quimper, Makah, Lincoln Creek, Crescent and Gries Ranch formations. The material is mostly incomplete or very fragmented and/or worn so that a precise appreciation of diagnostic features has been in many cases difficult if not impossible. Fourteen species were identified, seven are described as new: Lepidopleurus propecajetanus n. sp., Leptochiton sp., Ischnochiton goederti n. sp., Ischnochiton? sp. A, Ischnochiton? sp. B, Stenoplax quimperensis n. sp., Stenoplax sp. A, Stenoplax sp. B, Lepidozona cowlitzensis n. sp., Lepidochitona lioplax (Berry, 1922), Lepidochitona washingtonensis n. sp., Lepidochitona squiresi n. sp., Lepidochitona sp. and Craspedochiton eernissei n. sp. The only species previously described from the study area is Lepidochitona lioplax and it is also the most common chiton in the study material (67% of the total). No species other than L. lioplax has been found in more than one locality. In four of these localities only a single species has been collected. The basal Lincoln Creek Formation at the Porter Creek site supplied the richest and most diverse chiton assemblage with seven species and 86% of the valves. The discovery and formal identification of such a diverse Paleogene fauna from the northeastern Pacific provides a comparative base for a better appreciation of Polyplacophora biogeography and evolution.

The number of the main macro-invertebrate species grazing on the microalgal film on rocky coasts was evaluated at six closely located stations on Mallorca (Balearic Islands). The main bioerosive species at the sites studied were Melaraphe neritoides (L., 1758), Patella rustica (L., 1758), Monodonta turbinata (Born, 1780) and Lepidochitona corrugata (Reeve, 1848). The transects considered shared the same general environmental conditions and species pool. The aim of this study was to compare the effects exerted by the vertical gradient with those exerted by other sets of environmental variables. These sets were, namely, (1) inter-seasonal differences, (2) inter-transect differences (comprising degree of wave-exposure and lithological differences) and (3) effects of the micro-morphology (i.e., crevices, basin pools and other small-scale structures). The main null hypothesis verified was that species composition remains constant after assessing the effect of the vertical gradient. This hypothesis is largely rejected here. The main factor modelling the species composition was the vertical gradient (accounting for 31.6% of variability) whereas the percentages of variability yielded by the other sets individually were smaller but significant (P<0.001). Inter-transect differences (including degree of wave exposure and lithology) accounted for 24.9% of the variability. Seasonal differences accounted for 6.3%, small-scale morphology for 4.5%, and wave-height for 1.6%. The main conclusions obtained from these results are that the most basal strip of the coastline undergoes the largest bioerosive rates. Similarly, the sites exposed to wave action will undergo a larger bioerosive impact than the sheltered sites (possibly because they are more damp). These biotic and abiotic effects taken together would result in a maximisation of erosion rates in areas that are porous, exposed and located near the sea.

The extracellular hull of chiton eggs is often elaborated into cupules or spines that may be open or closed to the external environment. Scanning electron microscopy was used to examine the location of fertilizing sperm in eggs that had been exposed to a dilute sperm suspension to create natural fertilization or to a sperm concentrate to induce polyspermic egg penetration. The effect of cupules on sinking rates was tested in cupulous (free-spawning) and non-cupulous (brooding) species, by timing descent of eggs over a fixed distance in a large container of seawater. Densities of eggs were compared on Percoll gradients and found to be similar. It was found that hull cupules focus the sperm to specific regions of the egg surface in both brooding and free-spawning species. Furthermore, protruding cupules act as parachute structures that can significantly reduce sinking rates.