An organ of equilibrium in deep-sea isopods revealed: the statocyst of Macrostylidae (Crustacea, Peracarida, Janiroidea)

Abstract

Isopoda (Crustacea, Peracarida) from the deep sea are relatively well studied but little is known about their lifestyles or the functional morphology and anatomy. The isopod family Macrostylidae, for example, is rather small in size, usually less than 1 cm in body length, and occurs mainly in the deep sea between 3000–6000 m. This family features a paired subepidermal structure on the posterior end of the pleotelson. It has been reported only in this family and was first mentioned by Hansen in 1916, who hypothesised that it represents a pair of statocysts. Nevertheless, neither the structure nor the function has been investigated until now. The shape of some related features, however, has already been used for species differentiation thus indicating that phylogenetically as well as systematically valuable information may be inherent in this feature. Here, the anatomy of this structure was studied based on four species of Macrostylidae from the North Pacific and Atlantic Oceans. It was digitally reconstructed from histological sections. The paired structure comprised two tergal invaginations, each with distinct muscular attachments and a modified seta that distally held a statocyst on the shaft. This resembles equilibrium organs reported from other organisms and thus the statocysts hypothesis seems reliable. Using energy-dispersive X-ray spectroscopy, the substance of the statolith could be determined as silicon dioxide. Based on these findings, the function of this organ and its potential phylogenetic and ecological implications are discussed.