The energy consumption of echinoderms is low in comparison with that of other invertebrates. We demonstrated 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" invertebrates. Low energy consumption in echinoderms has been attributed to their high skeletal content and to catch connective tissues (CCTs) that maintain body posture by altering their mechanical properties with little energy expenditure. 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 occupied 53.5% of wet body weight, whereas body-wall muscles, including those of tube feet, occupied 5.1%. VO2 of the dermis in the stiff state (2.45 microl x g(-1) x 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 posture-maintenance activities contributed to the low energy consumption of holothurians.

The catch apparatus (CA) is the collagenous ligament at the spinal joint of sea urchins. It maintains spine posture by stiffening and allows spine movement by softening. A CA preparation, which was isolated from ossicles, was used to test the hypothesis that frictional forces between collagen fibers and ossicles are the source of stiffness changes. Isolated preparations of the CA changed in stiffness, thus falsifying the hypothesis. Another hypothesis proposes that muscle fibers, which represent a relatively small component of the CA, cause stiffening of the CA by contraction. Chemicals that evoked contraction in spine muscles did not always stiffen the CA: the CA of Heterocentrotus mammillatus softened in response to artificial seawater with potassium concentration elevated to 100 mM. This provided evidence against the muscle-based hypothesis. The present results suggest that the stiffness changes of the CA are based on changes in the mechanical properties of the extracellular components of the connective tissue and are therefore related to the connective tissue catch that is widespread in other echinoderms.