Investigating forms of plasticity in three echinoid species from the tropical north Pacific

Abstract

Phenotypic plasticity allows marine invertebrate larvae to adjust morphology and development in response to environmental variability, but its expression across species and environmental gradients remains incompletely understood. We examined morphological plasticity in larvae of three tropical echinoids– Tripneustes gratilla, Echinometra mathaei, and Colobocentrotus atratus– reared under nine combinations of temperature (23, 26, 29 °C) and food concentration (200, 1000, 5500 algal cells mL⁻¹). Across seven experiments and six larval cohorts, we quantified post-oral arm length (POL), stomach length (SL), and median body length (MBL) to test for a morphological plastic response to these environmental variables. Tripneustes gratilla displayed consistent morphological plasticity across the three experiments, with low- and medium-food treatments producing the longest relative POL. Echinometra mathaei exhibited POL plasticity in both experiments, but in a nontraditional way that needs more observations to fully understand where this plasticity comes from. C. atratus showed a traditional plasticity trend (although not statistically significant) with the lowest food treatments producing the longest POL relative to body size in a single spawn. High-food treatments promoted longer SL across all species and experiments. For T. gratilla, we tracked time to metamorphic competence and beaker larval duration for and larval duration (PLD) in a single spawn in May of 2023. Developmental timing was plastic in T. gratilla: at 29 °C with high-food, larvae reached metamorphosis in ~25 days, while some medium-food larvae at 23 °C remained free-swimming after 366 days. Morphological plasticity was predominantly food-driven, with limited interaction effects from temperature. Larval duration was significantly affected by food availability, temperature, and their interaction in T. gratilla. These results show patterns of larval flexibility and suggest that morphology and larval duration is an environmentally responsive trait. Such plasticity may have important implications for dispersal, connectivity, and resilience under changing ocean conditions.