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Animals have evolved numerous mechanisms to perceive and interact with the environment that can be translated into different sensory modalities. However, the genomic and phenotypic features that support sensory functions remain enigmatic for many invertebrates, such as bivalves, an ecologically and economically important taxonomic group. No repertoire of sensory genes has been characterized in bivalves, representing a significant knowledge gap in molluscan sensory biology. Here, we gather multiple lines of evidence to explore the specialized sensory function of bivalve tentacles in the common jingle shell, Anomia simplex. In addition to applying microscopy techniques, we performed transcriptome sequencing of dissected tentacles using phylogenetically-informed annotation to identify candidate receptors. Our results demonstrate the expression of candidate GPCRs, including one opsin type, five small-molecule receptors, and 11 chemosensory-related receptors, supporting the involvement of sensory neurons in the organ, likely in association with the ciliated receptor cells observed along the tentacle surface. In addition, we identified seven ionotropic receptors as putative chemosensory receptors and one member of the Piezo mechanosensitive ion channel, which might be involved in touch sensation by ciliated sensory receptors. Our results provide the first evidence of putative sensory genes expressed in a bivalve sensory organ, representing an important starting point to investigate chemosensation in this class. 

usp.br Ethanol-preserved specimens represent one of the most common resources for biological research. However, little is known of how ethanol preservation may change tissue morphology and impact the interpretation of trait quantification in structures, such as eyes. While scallop eyes are an interesting system for investigating eye evolution and visual adaptations, cross-species comparisons mainly depend on museum specimens. Therefore, to test whether ethanol-preserved specimens serve as accurate indicators of natural eye morphology, we investigated the effects of preservation on selected traits, such as eye and pupil diameter, in the scallop Argopecten irradians . We also compared ethanol-preserved eyes to paraformaldehyde (PFA)-fixed eyes to investigate possible impacts on retinal morphology. Our results demonstrate that eye size does not change with short-term preservation, whereas pupil size becomes significantly larger, likely due to the contraction of actin fibers during dehydration. When comparing measurements among eyes and treatments, eye size correlates to pupil size, but is not correlated to body size. We found that ethanol-preserved eyes provide close estimates of retinal traits, with similar photoreceptor spacing distance and number of photoreceptor cells, compared to samples fixed in PFA. These findings might also be applicable in the context of other mollusks, especially bivalves and gastropods, with delicate visual systems. Our study provides evidence that ethanol- preserved eyes exhibit tissue-specific differences that should be acknowledged in morphological studies. For example, pupil size should be investigated while accounting for post-preservation effects. Other traits, such as lens shape, are inconsistent and severely impacted by preservation. Finally, eye size and some photoreceptor cell measurements can be helpful to describe natural morphology. 

Many marine organisms have a biphasic life cycle that transitions between a swimming larva with a more sedentary adult form. At the end of the first phase, larvae must identify suitable sites to settle and undergo a dramatic morphological change. Environmental factors, including photic and chemical cues, appear to influence settlement, but the sensory receptors involved are largely unknown. We targeted the protein receptor, opsin, which belongs to large superfamily of transmembrane receptors that detects environmental stimuli, hormones, and neurotransmitters. While opsins are well-known for light-sensing, including vision, a growing number of studies have demonstrated light-independent functions. We therefore examined opsin expression in the Pteriomorphia, a large, diverse clade of marine bivalves, that includes commercially important species, such as oysters, mussels, and scallops. Methods Genomic annotations combined with phylogenetic analysis show great variation of opsin abundance among pteriomorphian bivalves, including surprisingly high genomic abundance in many species that are eyeless as adults, such as mussels. Therefore, we investigated the diversity of opsin expression from the perspective of larval development. We collected opsin gene expression in four families of Pteriomorphia, across three distinct larval stages, i.e., trochophore, veliger, and pediveliger, and compared those to adult tissues.Results We found larvae express all opsin types in these bivalves, but opsin expression patterns are largely species-specific across development. Few opsins are expressed in the adult mantle, but many are highly expressed in adult eyes. Intriguingly, opsin genes such as retinochrome, xenopsins, and Go-opsins have higher levels of expression in the later larval stages when substrates for settlement are being tested, such as the pediveliger. Conclusion Investigating opsin gene expression during larval development provides crucial insights into their intricate interactions with the surroundings, which may shed light on how opsin receptors of these organisms respond to various environmental cues that play a pivotal role in their settlement process.