Abstract
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.
Highlights
Chitons (Mollusca, Polyplacophora) have a flattened, ovoid body with a muscular foot protected by eight dorsal shell valves
Twenty animals of each species were selected at random for ablation treatment. They were relaxed in a 7% MgCl2 solution for twenty minutes before ablations were performed on the Schwabe organ in Leptochiton asellus and the corresponding epithelial tissue either side of the mouth in Lepidochitona cinerea using a fine 125 mm extended-tip high temperature cautery pen (FIAB, Florence)
From the evidence presented here, we conclude that the Schwabe organ is paedomorphic, an elaboration of the larval eye which persists to adulthood in Leptochiton asellus
Summary
Chitons (Mollusca, Polyplacophora) have a flattened, ovoid body with a muscular foot protected by eight dorsal shell valves. A previously unknown sensory organ, the Schwabe organ, has recently been discovered and described throughout one of the two taxonomic orders of chitons [3] This is as a stripe of brownish pigmentation, visible to the naked eye, which extends posteriorly and laterally from either side of the mouth towards the top of the foot (Fig 1A and 1B). Clear ventral migration of the larval eye has only been reported in Leptochiton asellus [10]; Naef’s drawings of Lepidopleurus cajetanus larvae are ambiguous in this respect [11] This ventral migration in Lepidopleurida results in the larval eye attaining a position reminiscent of the Schwabe organ in adults (Fig 1D). We combine these techniques in order to further explore the possibility of a link between the Schwabe organ and the larval eye
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
