Behavioural strategy of the ectosymbiotic crab (Sestrostomasp.) during ecdysis of the crab and its upogebiid shrimp host

Yuto Shiozaki,Gyo Itani

doi:10.1017/s0025315420000594

Abstract

AbstractMoulting is essential for crustacean growth, but is one of the causes of mortality, because a crustacean cannot move during and just after its ecdysis. In the cases of ectosymbiotic crabs, escape from the host's hostile response may also be a problem during its own ecdysis. In this study,Sestrostomasp. (Varunidae), an ectosymbiotic crab which clings to the ventral abdomen of upogebiid shrimps with legs that can walk, was studied to clarify how the crab moults and maintains association with the host. Five cases of crab ecdysis were observed, where the crab moulted with its legs clinging to the host abdomen, without detaching from the host body. Time required for moulting was 14–21 min. Shedding of the old exoskeleton (active phase) took only 40–59 s.Sestrostomasp. detached from the host abdomen and waited in the burrow tube during shrimp ecdysis. The crab then reattached at the same location on the host when shrimp moulting was complete. Our results suggest thatSestrostomasp. are able to maintain a symbiotic relationship with the same shrimp host after its own ecdysis as well after ecdysis of its host.

Highlights

Moulting is an essential behaviour for growth in individuals which are covered with their exoskeleton, such as crustaceans
The old exoskeleton is shed in the active phase
The sequence of moulting behaviour in the crab was as follows (Figure 2, Supplementary video S1): (1) the thoraco–abdominal membrane was ruptured; (2) the exuvia was pushed up diagonally forward and new body extended behind; (3) after having shed the exoskeleton, the crab still clung to the exuvium, which was still clinging to the ventral abdomen; (4) the crab moved onto the host shrimp abdomen, and clung there, after detaching its exuvium

Summary

Introduction

Moulting is an essential behaviour for growth in individuals which are covered with their exoskeleton, such as crustaceans. Body expansion occurs by water absorption during moulting (Philippen et al, 2000). Moulting is composed of two phases: a passive, followed by an active phase (Philippen et al, 2000). The thoraco–abdominal membrane is ruptured by body expansion. The old exoskeleton is shed in the active phase. Especially in the active phase, crustaceans cannot move (Waddy et al, 1995). In the case of Homarus americanus, the lobster loses mobility during the active phase for 10–20 min (Waddy et al, 1995). In the blue swimming crab, Portunus pelagicus, the time required for carapace hardening is between 9 and 17 h (Azra et al, 2019)

Methods

Results

Conclusion