In situ estimates of larval development and mortality in the New Zealand sea urchin Evechinus chloroticus (Echinodermata: Echinoidea)

Abstract

Synchronous spawning of the sea urchin Evechinus chloroticus Valenciennes in a closed fiord population (Doubtful Sound, New Zealand) provided a unique opportunity to calculate in situ rates of larval mortality and development. The resulting cohort of larvae was sampled 7 d after this spawning, and subsequently every 2 wk at 5 sites throughout the fiord. Instantaneous mortality rates for E. chloroticus larvae were calculated using 3 different models. Mortality (M) was found to be constant and estimated at M = 0.164, 0.173, and 0.085 d-1 for the 3 models; the most accurate estimate of mortality probably being M = 0.164 d-1. Larvae reach competency in Doubtful Sound between 18 and 31 d, which is 1.05- to 1.82-fold slower than maximum growth rates recorded in laboratory cultures. Larvae spent most of their development time (44.9 %) as 4-armed plutei, and only a small proportion of these larvae (11.5 %) survived beyond this stage: We suggest that the slower in situ larval development rate and the accumulation of larvae in the 4-armed pluteus stage indicate that these larvae are food-limited in the plankton. Given the high daily mortality rate, these food-limited early pluteus larvae may be a critical life history stage for E. chloroticus populations in Doubtful Sound.