Abstract
A population of the fissiparous sea star Stephanasterias albula (Stimpson) was studied at North Lubec, Maine, U.S.A. This population consisted of a dense (up to 40/m 2 localized group of roughly 5500 small ( R, the length of longest arm from disc center to arm tip, nearly always <18 mm) contagiously distributed individuals. Very small individuals ( R<3 mm) were totally absent suggesting that larval recruitment had not occurred recently. Fission is a seasonal phenomenon that takes place most frequently in the spring and summer. Incidence of fission appears to be correlated with seasonal changes in photoperiod. Rates of regeneration following fission are sufficient to allow individuals of S. albula to split once every 1 or 2 yr. Examination of 1-μm plastic sections from even the largest specimens revealed no trace of a differentiated reproductive system and of associated aboral haemal and coelomic rings, strong evidence that S. albula is obligately fissiparous. The ecological and developmental consequences of this finding are considered.
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