Juvenile niches select between two distinct development trajectories and symbiosis modes in vent shrimps

Most animal species have a singular developmental pathway and adult ecology, but developmental plasticity is well-known in some like honeybees where castes display profoundly different morphology and ecology. An intriguing case is the Atlantic deep-sea hydrothermal vent shrimp Rimicaris hybisae/chacei that share dominant COI haplotypes but develops into either the symbiont-reliant hybisae with a hypertrophied head chamber (in the Mid-Cayman Spreading Centre) or the mixotrophic chacei with a narrow head chamber (on the Mid-Atlantic Ridge). Here, we use X-ray micro-computed tomography and fluorescent in situ hybridization to show that key anatomical shifts in both occur between the juvenile-subadult transition, when those developing into hybisae have fully established symbiosis but not chacei. On the Mid-Atlantic Ridge the diet of R. chacei has been hypothetically linked to competition with the obligatorily symbiotic congener R. exoculata, and we find anatomical evidences that R. exoculata is indeed better adapted for symbiosis - suggesting the chacei morph could be an adaptation to prevent competitive exclusion. Our results suggest that the two distinct development trajectories are likely selected according to the diet available to the juveniles, determined by whether energetically sufficient symbiont colonisation has occurred before reaching subadult stage.