Longevity, Life History, and Relative Body Wall Size in Sea Urchins

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Abstract
Annual survival rates in 38 populations of 17 sea urchins species in the Indo—West Pacific were related to relative size of the body wall and exposure to the surf. The urchin species are: Heterocentrotus trigonarius, H. mammillatus, Colobocentrotus atratus, Echinometra mathaei, E. oblonga, and Heliocidaris erythrogramma in the family Echinometridae; Prionocidaris baculosa in the Cidaridae; Stomopneustes variolaris in the Stomopneustidae; Echinothrix calamaris, E. diadema, Centrostephanus rodgersi, Diadema paucispinum, and D. setosum in the Diadematidae; Tripneustes gratilla, another Tripneustes sp. (possibly the same as T. gratilla), and Toxopneustes pileolus in the Toxopneustidae; and Salmacis belli in the Temnopleuridae. Populations were studied at Hawaii, Enewetak Atoll, Queensland, New South Wales and Western Australia, Sri Lanka, Seychelles, Kenya, Zanzibar, and Israel (Eilat). Live animals were dissected to determine the size of body components. The relative size (mass) of the body wall for a species is expressed by the constant α in the equation B = αTb, where B is wet mass of the body wall and T is total wet mass. A common β value of 0.976 ± 0.004 (SE) was used for all species, so α values can be used to compare relative body wall sizes among species. Values of α ranged from 0.966 for Heterocentrotus trigonarius (Enewetak Atoll) to 0.360 for Salmacis belli (Moreton Bay, Queensland, Australia). Parameters of the Richards growth function were determined from animals tagged with tetracy—cline. Tagged animals were collected after they had been in the field for 1 yr. Growth parameters were used with parameters from size—frequency distributions to estimate Z, the mortality coefficient. Annual survival probability p is e—z. Stepwise multiple regression was used to examine the relationship between annual survival probability (p) and two independent variables, α and E, where E is a subjective measure of exposure to surf (1 = most exposed). The function is p = 1.20α + 0.061E — 0.350. Survivorship increases with increased relative size of the body wall and with increased protection from the surf. The positive relationship between survival probability and relative body wall size supports the hypothesis that survival is related to allocation of resources to maintenance. The significance of longevity in urchins probably is that it is related to the predictability of survival of prereproductive individuals. The greater the unpredictability, the longer life must be. Long life requires a greater investment in maintenance mechanisms and hence, among other adaptations, a more massive body wall.