A study of the endogenous activity rhythms of the marine isopod Exosphaeroma truncatitelson

Abstract

The endogenous activity rhythms of the South African endemic isopod Exosphaeroma truncatitelson were explored. Isopods were collected on days coinciding with either a neap or spring tide. High- and low-energy beaches, which vary in the intensity of wave action, were selected as study sites. Isopods, in groups of 10–30, were placed in glass tanks with different experimental conditions: darkness + no sand, darkness with sand and natural ambient light (no sand). Activity of the isopods was measured for 50 h by recording the number of isopods swimming actively during a 30-s period. These isopods exhibited an endogenous activity rhythm which coincided with the tidal cycle even though removed from their natural habitat. Cosine curves were fitted to the observed activity data and statistical differences across variables were investigated. Peak activity was shown to occur shortly after high tide. Similar activity cycles were observed for both neap and spring tides and for different times of the year. Results also indicated that the activity cycles at both low- and high-energy beaches were similar. Two survival advantages of this rhythm are proposed: prevention of stranding on high shores and competitive avoidance with other local isopod species.

Keywords:

• activity rhythm
• endogenous
• Exosphaeroma truncatitelson
• isopod

Acknowledgements

Professor Charles Griffiths (Department of Biological Sciences, University of Cape Town) is thanked for his help in identifying the species of isopod. Dr Anabela Brandão and Dr Susan Johnston from the Department of Mathematics and Applied Mathematics, University of Cape Town, are thanked for suggestions regarding statistics suitable for this study. Thanks also to the various ESKOM EXPO judges, who provided suggestions for improving this project, and for providing funding to attend the international INTEL Science fair in Pittsburgh, USA, in 2012, where this project was presented.

Notes

1. S34°08′13.05′′, E18°19′42.99′′.

2. S34°09′30.85′′, E18°25′57.03′′.