Abstract
Summary This study provides a qualitative and quantitative description of larval development of Tetrapygus niger to metamorphosis. After fertilization, the length of the planktonic interval (developmental time) for T. niger at room temperature (14–18°C) ranged from 94 to 120 days. Competent larvae were induced to metamorphose by transferring them into culture vessels containing bacterial films and diatoms. About 40% of the larvae reached the juvenile stage, with complete metamorphosis from the feeding larval stage to the feeding adult stage taking 4–6 days. Although the morphology of T. niger larvae is atypical compared with most regular extant echinoid species, it is similar to another arbacioid species, Arbacia punctulata. This suggests that larval morphology needs to be included in studies aimed at establishing phylogenetic relationships. The large larval size (>3.0 mm close to metamorphosis) and uniform pattern of ciliation observed in T. niger larvae have also been observed in non-feeding larvae. However, as T. niger larvae feed, we believe these characteristics reflect a functional solution to the swimming and feeding requirements of a long planktonic life. The maximum size (>4.5 mm) achieved by T. niger larvae in this in vitro study represents the largest recorded for echinoid larvae.
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