Nutritional ecology of sea urchin larvae: influence of endogenous and exogenous nutrition on echinopluteal growth and phenotypic plasticity in Tripneustes gratilla

Abstract

1. Marine invertebrates use egg nutrients to develop the functional feeding larva and then enter a facultative feeding period (FFP) when development can proceed without food because larvae are supported by maternal reserves. Facultative feeding reduces starvation risk and so is important for larval success. It may also influence egg size evolution because FFP length correlates with egg size. 2. We quantified energetic lipid (triglyceride, TG) utilisation in unfed and fed larvae as an index of larval nutritive condition in the tropical echinoid Tripneustes gratilla during the FFP to determine if fed larvae would use the buffer provided by endogenous provisions to improve their condition or accelerate development. We predicted that: (i) the condition of unfed and fed larvae should diverge before egg TG is exhausted and/or (ii) that the size of fed larvae should outstrip that of unfed larvae. 3. Temperate echinoplutei exhibit phenotypic plasticity, increasing the length of their food capture apparatus (arms) in nutrient-poor conditions. We examined the generality of this phenomenon in a morphometric analysis of growth in the tropical larva of T. gratilla. We hypothesized that plastic arm growth in starved larvae would occur before the FFP ends as a bet hedging strategy to prepare for hard times ahead when lack of reserves may render this response impossible. 4. We found that fed larvae diverged in condition (higher TG) but not in size compared with starved larvae before egg energetic lipids were exhausted. In addition, unfed larvae showed plastic arm growth before the end of the FFP. 5. Lack of divergence in growth of unfed and fed larvae suggests there may be an imperative to maintain a nutrient storage buffer against starvation due to unpredictable future planktonic food supply. The FFP of T. gratilla exceeds 8 days, considerably longer than that of other echinoids with similarly sized eggs, emphasizing that egg quality may be more important than egg size as a predictor of the FFP in fecundity-time models of egg evolution.