The Sensory Tubercle On the Eyestalk of the Shrimp Crangon Crangon (Linnaeus, 1758) (Decapoda, Caridea)

Abstract

AbstractMany podophthalmous Crustacea present a more or less salient external structure near and in proximal position to the compound eyes. It is called sensory pore, eye papilla or eye tubercle, according to shape and author. Some SEM and TEM ultrastructural studies on several Caridea and Penaeidae have revealed a complex structure with at least two chemoreceptor-like organs: the main sensory pore (MSP) and the lateral sensory pore (LSP). MSP is a thin discoidal cuticular zone perforated by micropores. Beneath it there is a layer of sensory-like cells, each with two ciliary outer segments branching into a sub-cuticular cavity. The dcndrite parts of the sensory cells are surrounded by supporting cells. The tissue organisation reminds of that of a Vertebrate olfactive mucosa. The LSP consists of a cluster of bipolar sensory cells often ending in a pit similar to an invaginated chemoreceptive seta, found in Arthropoda. The present study deals with the common shrimp Crangon crangon (Linnaeus, 1758), which displays a highly salient conical papilla. This differs in shape from the sensory pores anatomically studied so far. Ultrastructural observations show an important LSP which ends at the tip of the cone rather than in a pit. The equivalent of the MSP is sunken into the base of the cone. This pattern is original and closer to that found in the Penaeidae than to that of the Caridea observed. Therefore, the MSP-LSP complex of Decapoda, in spite of a similar basic structure, presents very different external aspects in different species. It is also confirmed that this complex is very close to, but separate from, the internal organ of Bellonci.