The red muscle of the thresher-shark myotome is internalized and supplied by a simple rete in connexion with lateral vessels. It is probable that the system enables the fish to be warm-bodied.

INTRODUCTION

Warm-bodied sharks were first described by Carey & Teal (1969); subsequent observations by Carey and his colleagues (Carey et al. 1971) and by Carey (1982) showed that five lamnid species were able to maintain body temperatures 5–10 °C above the ambient water. All of these possess more or less elaborate retia linking a red muscle mass lying internally in the myotomes with the systemic circulation. In cold-bodied sharks, the red muscle lies superficially, just under the skin, and retia are absent. A sixth lamnid species, the big-eye thresher shark (Alopias superciliosus) may be warm-bodied; it has a simple muscle retial system (Carey et al. 1971) which has not been described in detail. The chance capture of a common thresher shark (A. vulpinus) enabled this short description of the retial system in this species.