Low Oxygen Consumption and High Body Content of Catch Connective Tissue Contribute to Low Metabolic Rate of Sea Cucumbers

The energy consumption of echinoderms is low in comparison with that of other invertebrates. We demon- strated this by measuring the oxygen consumption rate per unit of body weight (VO2) of the sea cucumber Actinopyga mauritiana: VO2 was 1/8 that of the "standard" inverte- brates. Low energy consumption in echinoderms has been attributed to their high skeletal content and to catch con- nective tissues (CCTs) that maintain body posture by alter- ing their mechanical properties with little energy expendi- ture. The former is not applicable to holothurians, and the latter has not been proven experimentally. We postulated that the large content of dermal connective tissue, which maintaines posture economically, contributes to the low energy consumption in holothurians. Body-wall dermis oc- cupied 53.5% of wet body weight, whereas body-wall mus- cles, including those of tube feet, occupied 5.1%. VO2 of the dermis in the stiff state (2.45 l g 1 h 1 ) was 1/10 that of the longitudinal body-wall muscle in contraction. The mechanical tests revealed that the stress at an imposed strain of 2% strain was 7 times greater in CCT than in muscles. These results showed that CCT could maintain posture more economically than muscles could. We concluded that the high content of connective tissue with energy-saving pos- ture-maintenance activities contributed to the low energy consumption of holothurians.