Several invertebrate systems have been developed to study various aspects of the eye and eye disease including Drosophila, Planaria, Platynereis, and most recently, the cubozoan jellyfish Tripedalia; however, molluscs, the second largest metazoan phylum, so far have been underrepresented in eye research. This is surprising as mollusc systems offer opportunities to study visual processes that may be altered by disease, vision physiology, development of the visual system, behavior, and evolution. Malacologists have labored for over a century as morphologists, systematists, physiologists, and ecologists in order to understand the structural and functional diversity in molluscs at all levels of biological organization. Yet, malacologists have had little opportunity to interact with researchers whose interests are restricted to the biology and development of eyes as model systems as they tend not to publish in the same journals or attend the same meetings. In an effort to highlight the advantages of molluscan eyes as a model system and encourage greater collaboration among researchers, I provide an overview of molluscan eye research from these two perspectives: eye researchers whose interests involve the development, physiology, and disease of the eye and malacologists who study the complete organism in its natural environment. I discuss the developmental and genetic information available for molluscan eyes and the need to place this work in an evolutionary perspective. Finally, I discuss how synergy between these two groups will advance eye research, broaden research in both fields, and aid in developing new molluscan models for eye research.

Light sensitive rhabdoms in the octopus retina increase in cross-sectional area in the dark and shrink in the light. Growth in the dark is due to the formation of microvilli in an avillar region of the photoreceptor cell membrane and lengthening of rhabdomere microvilli already present. Diminution in the light is the result of the disassembly and shortening of the same microvilli. Each microvillus contains an actin filament core that must be assembled or disassembled in the dark or hght, respectively. To understand the regulation of the construction and breakdown of rhabdomere microvilli in the light and dark, we used centrifugation to separate the rhabdom membranes followed by Western blotting and Rho pull-down assays to investigate the role of Rho GTPases in this process. Western blotting showed a difference in the distribution of Rho in rhabdom membrane and supernatant fractions. In the hght, Rho was mostly present in the supernatant but in the dark it was found in the fraction enriched with rhabdom membranes. Complementing these results, pull-down assays showed that Rho is activated in the dark but in the light, Rho is mostly inactive. We believe that in the dark, activated Rho binds to the rhabdom membrane and initiates signaling pathways, leading to growth of rhabdomere microvilli. In the light, Rho is present in the soluble fraction, is inactivated, and is likely bound to a Rho GDI. Receptors involved in the activation of Rho in the dark are undetermined and may involve rhodopsin or another membrane protein.

The unique, double-retina, concave mirror eyes of scallops are abundant along the valve mantle margins. Scallops have the most acute vision among the bivalve molluscs, but little is known about how eyes vary between scallop species. We examined eye morphology by immunofluorescent labeling and confocal microscopy and calculated optical resolution and sensitivity for the swimming scallops Amusium balloti (Bernardi, 1861), Placopecten magellenicus (Gmelin, 1791), Argopecten irradians (Lamarck, 1819), Chlamys hastata (Sowerby, 1842), and Chlamys rubida (Hinds, 1845) and the sessile scallops Crassadoma gigantea (Gray, 1825) and Spondylus americanus (Hermann, 1781). We found that eye morphology varied considerably between scallop species. The eyes of A balloti and P. magellenicus had relatively large lenses and small gaps between the retinas and mirror, making them appear similar to those described previously for Pecten maximus (Linnaeus, 1758). In contrast, the other five species we examined had eyes with relatively small lenses and large gaps between the retinas and mirror. We also found evidence that swimming scallops may have better vision than non-swimmers. Swimming species had proximal retinas with inter-receptor angles between 1.0 ± 0.1 (A. balloti) and 2.7 ± 0.3° (C. rubida), while sessile species had proximal retinas with inter-receptor angles between 3.2 ± 0.2 (C. gigantea) and 4.5 ± 0.3° (S. americanus). Distal retina inter-receptor angles ranged from 1.7 ± 0.1 (A balloti) to 2.8 ± 0.1° (C. rubida) for swimming species and from 3.0 ± 0.1 (C. gigantea) to 3.6 ± 0.2° (S. americanus) for sessile species, but did not appear to correlate as strongly with swimming ability as proximal retina inter-receptor angles did. Finally, we found that optical sensitivity differed between species, measuring from 3 ± 1 (A. balloti) to 21 ± 10 µm² · sr (C. hastata) for proximal retinas and from 2 ± 1 (C. gigantea) to 8 ± 5 µm² · sr (C. hastata) for distal retinas. These differences, however, did not appear to correlate with ecological factors such as a scallop species' swimming ability, preferred substrate type, or range of habitat depth. In light of these and previous findings, we hypothesize that scallop distal retinas may perform tasks of similar importance to all species, such as predator detection, and that proximal retinas may perform tasks more important to swimming species, such as those associated with the visual detection of preferred habitats.

Two types of multi-cellular eyes have been identified in the Bivalvia. Paired cephalic eyes occurring internally above the anterior end of the ctenidia are seen only in representatives of the Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea. These eyes, comprising a pit of photo-sensory cells and a simple lens, are thought to represent the earliest method of photoreception. Many shallow-water marine, estuarine, and freshwater bivalves also possess simple photoreceptive cells in the mantle that enable them to respond to shadows. In some other marine, shallow-water taxa, however, a second type of more complex photoreceptors has evolved. These comprise ectopic pallial eyes that can be divided into three broad categories, in terms of their locations on the (i) outer mantle fold in representatives of the Arcoidea, Limopsoidea, Pterioidea, and Anomioidea, (ii) middle fold in the Pectinoidea and Limoidea, and (iii) inner fold in the Cardioidea, Tridacnoidea, and Laternulidae (Anomalodesmata). Eyes do not occur in deep-sea bivalve taxa. Where ectopic pallial eyes occur, they measure amounts of light and integrate intensities from different directions, thereby supplying information to the individual possessing them about the distribution of light in its immediate environment. This does not mean, however, despite broad, phylogenetically related advances in pallial eye complexity, that any bivalve can perceive an image. A revised picture of the independent evolution of ectopic pallial eyes in the Bivalvia is provided. In bivalves, pallial fold duplication has resulted in improvements to the peripheral visual senses, albeit at different times in different phylogenies and on different components of the mantle margin. This has been achieved, it is herein argued, through: (i) selective gene-induced ectopism; (ii) pigment cup evagination in Category 1 eyes; (iii) invagination in Categories 2 and 3; and (iv) natural selection. The invaginated distal retina in representatives of the Pectinidae and Laternulidae provides the potential for image formation and the detection of movement. In the absence of optic lobes capable of synthesizing such information, however, these complex eyes must await matching cerebral sophistication.

This review showcases one group of gastropod's ability to perceive light through the eyes. The central question is simple: what are the visual performances and tasks of cephalic eyes in gastropods? That topic in itself is rather broad and is here applied to pulmonate gastropods, coming from terrestrial and aquatic biomes as well as different habitats and microhabitats, exhibiting different life-styles and light-tolerances. Therefore, the main objectives have been to analyze (1) anatomical and ultrastructural eye characteristics, (2) optical systems, (3) image-forming capabilities and possible functional consequences of eye size and design, (4) interactions between gastropods and their environment mediated by the visual information obtained through the eye, and (5) the specific visual tasks that the eyes serve. During the course of this study, a range of variations (= adaptations) in both optical and retinal design parameters, including eye size, aperture size, quality of optical image, retinal shape, sampling density, and optical sensitivity were discovered. All species of pulmonate gastropods studied have paired simple camera-type eyes that operate with advanced fixed focal-length optics. However, in terrestrial snails and slugs as well as freshwater limpets, the optics cannot produce a focused image on their shallow retinas. This seems to indicate that eyes in these species are not designed to receive a focused image and are likely to measure only the average light intensity or quality over large angles rather than resolve fine image details. The aquatic snails examined are able to focus a sharp image on the photoreceptive layer of the retina due to the deepenings of the latter (at least in a localized region). Although there is a significant correlation between specialization of the eye (e.g., quality of optical image, sensitivity, and resolution) for a particular visual task in a specific habitat that the species encounters, there is no correlation between cellular composition of the retina and light/dark preferences. Their high optical sensitivity allows terrestrial snails to perform the necessary visual tasks in both bright and dim light, whereas the eye in aquatic species functions preferentially under bright light conditions. In conclusion, pulmonate gastropods use their eyes primarily for the following two kinds of visual tasks: (1) discriminating objects and possible enemies in their environment and (2) monitoring the environmental brightness level to orient towards dark places. The first type of visual task is characteristic of the aquatic snails and is served by image-forming eyes; the second is typical of terrestrial snails and slugs and is best served by a blurred image. Attention is given to visual ecological adaptations, specific visual needs, and the evolutionary history of gastropods.

The genus Buccinanops (d'Orbigny, 1841) (Caenogastropoda, Nassarriidae) is endemic to the SW Atlantic Ocean, and the name implies no eyes, due to the lack of visible eyes in adults. We recognize for the first time the occurrence of eyes during several developmental stages within Buccinanops. Eye spots in Buccinanops cochlidium (Dillwyn, 1817) were observed during intracapsular development and in hatchlings and juveniles. Eyes were histologically confirmed in embryonic cephalic tentacles; they were comprised of sensory cells, supportive cells, a lens, and an optic nerve cord. The ontogenetic history of the eyes of B. cochlidium is discussed.

Diverse crystallins (abundant water-soluble proteins) are responsible for the optical properties of transparent cellular eye lenses and are multifunctional proteins that have been recruited from stress proteins and enzymes by enhanced lens expression. The major (S-crystallins) and minor (Ω-crystallin) cephalopod crystallins were recruited from glutathione S-transferase (GST) and aldehyde dehydrogenase (ALDH), respectively. S-crystallins underwent multiple gene duplications while Ω-crystallin appears to be encoded in a single-copy gene. Except for one S-crystallin (considered a “molecular fossil”), S-crystallins lack enzyme activity due to mutation and insertion of a variable central peptide by exon shuffling. The Ω-crystallin is the sole crystallin in scallops. Scallop Ω-crystallin does not bind the co-factor NAD /NADH, lacks enzyme activity, and is a tetramer but migrates as a dimer by gel filtration, suggesting structural adaptations for crystallin function. Similar transcription factors (Pax6 among others) appear to drive high lens expression of crystallin genes in molluscs and other species consistent with convergent recruitment of the non-homologous crystallin genes.

The general expression of the transcription factor gene Pax-6 homologues and the overall presence of the photopigment opsin have cast doubt on the polyphyletic evolution of photoreceptors. Herein it is proposed that the evolutionary pathway of photoreceptors reflects two different, successive processes: (i) the (monophyletic?) differentiation of photoreception as such, mediated by a specific transcription factor gene (such as Pax-6 or sine oculis) and (ii) the genetic information of that induction factor (normative unit for photoreception). The latter stimulates the (polyphyletic) differentiation of the photoreceptors themselves through its multiply convergent co-options with variable network-modifications (intercalation of different genes). The expression of transcription factor genes does not per se imply homology of the differentiated photoreceptors (but at most some pattern of homoiology). The differentiation of both receptor types, ciliary versus rhabdomeric, in one and the same cell during development (of veliger larvae; Blumer 1996) shows them to be interchangeable structures (mere morphs). Apparently dependent of functional requirements, the structural type of the receptive organelle has no direct bearing upon the homology identification of the photoreceptors. This obviates the need to propose separate (ciliary and rhabdomeric) precursor cells in metazoans. A possible primitive “dermal” receptive cell which was polyphyletically adapted for metazoan photoreceptors is discussed. The rich morphological diversity of photoreceptors (including the larval organs) in Mollusca appears to represent in-group differentiations. Their polyphyletic lines are surveyed and the fine structure of eyes of three pteriomorph Bivalvia species—Lima lima (Linnaeus, 1758) (with a subdivided retina), Chlamys varia (Linnaeus, 1758), and Pseudamussium peslutre (Linnaeus, 1771) (with incomplete proximal retina)—is reported.

Bivalve molluscs are known for shadow responses involving closure or retraction of the siphon and valve adduction. In representative genera [Spisula solidissima (Dillwyn, 1918), Mercenaria mercenaria (Linnaeus, 1758), Lima scabra (Born, 1778)], the pallial nerves contain photosensitive fibers that exhibit physiological shadow responses. These photoreceptors are inhibited by light but trigger an excitatory burst of spikes to a shadow, the off-response. Equivalent responses are seen in bivalve eyes, e.g., in optic fibers from the siphon tentacle eyes of Cardium edule (Linnaeus, 1758) and the mantle eyes of scallops (Pectinidae) and file clams (Limidae). In scallops, they form a distal retinal layer of ciliary receptors, distinct from a proximal microvillar layer that is excited by light. In off-receptors (ciliary), light inhibition is the result of a hyperpolarizing receptor potential with spikes generated on the rebound depolarization at dimming. In contrast, the proximal on-receptors are excited by light with spikes generated by depolarizing receptor potentials. The inhibitory effect is first-order, i.e., a direct response to light, as is excitation for the proximal receptors. With separate retinas and the absence of synaptic contact, these are the primary receptors. Aside from Pecten Müller, 1776 and Lima Bruguiére, 1797, the only other bivalve eyes in which receptor potentials have been investigated are those of the giant clam Tridacna maxima (Röding, 1798). Here there are two types of hyperpolarizing, light-inhibited primary receptors, one of which generates spikes at light offset, the other non-spiking. The inhibitory response is universal in bivalve photoreception, unique among the eyes of invertebrates, but similar in polarity to chordate photoreceptors although the ionic mechanisms are different. Receptor physiology is discussed relative to image formation in bivalve eyes.

Snail shells persist in the environment after death, but we know little about the rate at which shells decompose. Assumptions about the rate of shell decomposition are relevant to conservation biologists who find empty shells or biologists using empty shells to make inferences about assemblages of living individuals. I put shells in 1.6 mm mesh litter bags (excluding macro-grazers) in Delaware and northern Michigan, U.S.A. and monitored shell mass annually for 7 years. Decomposition rates differed among species, but I found no difference in rates at two sites with different habitats. Surprisingly, loss of periostracum had no effect on shell decomposition rate. At the locations and habitats studied, decomposition rate of snails averaged 6.4% per year, excluding shells that broke during the experiment (shell half life = 11.5 years), or 10.2%, including shell breakage (half life = 7.5 years). Half lives would likely be shorter if macro-grazers had access to shells. These results caution us to draw conclusions carefully when including empty shells in inferences about assemblages of living individuals.

A survey for bivalves was conducted at 25 sampling stations on the Mexican Central Pacific shelf off Jalisco and Colima, during the summer of 1988. The bivalves were sampled with a Van Veen grab at 16 stations with medium sand, sandy silt, and silty clay substrata at depths between 18 and 112 m. A total of 5,196 individuals belonging to 59 genera and 95 species of bivalves were found. A systematic list is provided with the relative abundance and density (individuals/m²) for each species and information on depth, type of substratum, bottom water temperature, and oxygen concentration for each station. The twelve most common species (>100 individuals/station) in descending order of abundance were: Nuculana laeviradius (Pilsbry and Lowe, 1932), Crassinella pacifica (C. B. Adams, 1852), Corbula nasuta G. B. Sowerby I, 1833, Anadara adamsi Olsson, 1961, Parvilucina approximata (Dall, 1901), Nucula declivis Hinds, 1843, Corbula ira Dall, 1908, Radiolucina cancellaris (Philippi, 1846), Cyclopecten pernomus (Hertlein, 1935), Nuculana lobula (Dall, 1908), Parvilucina mazatlanica (Carpenter, 1857), and Gouldia californica Dall, 1917. The bathymetric patterns in the abundance and species composition of the bivalve community and their relationship to environmental parameters are discussed. The structure of the assemblages differed with depth, with peak abundances and species richness (1) between 24 and 40 m with medium sand and sandy silt substrata and (2) at intermediate depths between 71 and 74 m, with sandy silt and silty clay substrata. The species characterizing shallow, intermediate, and deep zones were the most abundant or those exclusive of each zone. Diversity, dominance, and evenness decreased at the deeper stations. The distinctive species composition of these zones may be the result of variation in depth, oxygen concentration, and substratum.

A gravid female of the cranchiid squid genus Bathothauma Chun, 1906 was collected from 1027 m depth in excellent condition, other than having a ruptured ovary and tentacles reduced to stubs. The 2 mm diameter eggs, 18 spermatangia embedded in the skin of her mantle, head, and eye, and the very pronounced nidamental glands indicate full sexual maturity. The eggs are over three times larger than those previously reported in the genus.

We surveyed Indiana collections of freshwater gastropods from 220 museum collection lots and found 39 species inhabiting Indiana historically. Collection dates of museum material ranged from 1900 to 2006, with a median date of 1986. We collected 17,593 gastropods at 123 sites, including 86 sites where museum material was previously collected. Our surveys were combined with recent literature surveys and indicate a total of 36 species are currently present in Indiana. The Indiana fauna is composed of three species that are apparently secure globally, and 36 species that are widespread, abundant, and globally secure, including two exotics. However, three species are locally extinct and many others are locally imperiled or vulnerable. The majority of freshwater gastropod taxa in Indiana are of local conservation concern. The causes of local gastropod extinctions are unknown but likely include agricultural impacts, hydrologic alterations from reservoirs, and pollution. We recommend thorough inventory, recognition, and protection of the aquatic gastropods in Indiana.

The feeding behavior and diet of the federally threatened land snail Triodopsis platysayoides (Brooks, 1933) are reported. The species is atypical among eastern North American land snails in that it remains active and feeding during hot, dry summer months while other land snail species occurring in the region may become motionless or are compelled to estivate. Triodopsis platysayoides has also coevolved with a rare mammal, the Alleghany wood-rat, Neotoma magister. Clearly, where the wood-rat and T. platysayoides coexist, wood-rats furnish a nearly constant food supply to the snail, including wood-rat excrement and a host of wood-rat harvested provisions carried into the snail's location. Triodopsis platysayoides includes as part of its diet fungi, lichens, flower blossoms of the tulip tree Liriodendron tulipifera, deceased gray cave crickets Euhadenoecus fragilis, gray cave cricket excrement, yellow birch Betula allegheniensis, and sweet birch Betula lenta leaves. Senescent leaves of the birch may form a significant pool of foliar calcium available to the snail in an otherwise acidic environment. Triodopsis platysayoides was witnessed feeding on the vacant shells of Xolotrema denotatum (Fèrussac, 1821), Mesomphix cupreus (Rafinesque, 1831), and its own kind, presumably for the calcium carbonate content, a critical mineral in regulation of bodily functions and shell building. Peak activity for the species occurred after nightfall whereas peak feeding occurred when temperatures were between 18 and 23 °C and relative humidity was between 70% and 85%.

As part of a complete biotic survey for Little Mahoning Creek watershed, Western Pennsylvania Conservancy (WPC) used timed searches to survey for freshwater mussels along the entire mainstem of Little Mahoning Creek (56.8 km). The survey captured all historic sites sampled as early as 1905. The 15 sites revealed a high diversity of unionids (10 species) and several rare and endangered taxa, including Pleurobema sintoxia (Rafinesque, 1820), Villosa iris (I. Lea, 1829), and Alasmidonta marginata (Say, 1818). Present survey results indicate an established unionid community with several taxa increasing their distributions compared to historical surveys. Upstream dispersal was halted at the Savan Dam at river kilometer 36.69. No live or dead unionids were found at six sites upstream of the dam. Bray-Curtis similarity indices were calculated between present survey and historical survey accounts. Species composition in the present survey compares favorably (86.14%) with collections by Ortmann (1909) and Bogan and Davis (1992). Several locations have relatively high unionid diversity, despite being located in an area with anthropogenic perturbations.

Bayou Bartholomew is a low-gradient river system that drains much of southeastern Arkansas and northeastern Louisiana, U.S.A. As one of the few southeastern streams remaining un-impounded, the Arkansas reach of the bayou harbors a rich freshwater molluscan fauna. Collecting efforts have historically focused on documenting freshwater mussel and fish diversity, and there was no prior survey focusing on freshwater gastropods. This survey of the drainage yielded 13 gastropod species representing three orders and seven genera. Pulmonates were most abundant in low-order reaches of the drainage, while gill-breathing snails dominated higher-order reaches. Co-occurrence analyses indicated that pulmonates occurred significantly more often with other pulmonates than they did with gill-breathers; this trend was also observed in gill-breathers. Both stream order and predominant substrate influenced species richness and abundance. Our findings were consistent with other published studies on freshwater snail distribution but may be confounded by drought conditions experienced during the survey.