Colour adaptation mechanisms of the sea star symbiotic shrimp Zenopontonia soror during host shift experiments

Mimetic colouration is a widespread survival strategy that allows organisms to reduce predation by blending into their environment through adaptations in colour, texture, or shape. Symbiotic crustaceans are no exception and frequently exhibit host-specific morphological traits, including mimetic pigmentation. In some cases, however, ecological pressures force symbionts to switch hosts. When the new host exhibits a different chromotype (i.e., colour morphotype), the symbiont may adjust its colouration accordingly to maintain effective camouflage. This study investigates whether the symbiotic shrimp Zenopontonia soror can adapt its colouration to that of its host sea star, Culcita novaeguineae. An experimental host chromotype switch was conducted, and changes in shrimp colouration were monitored over a 12-day period using a standardized photography protocol. To explore potential pigment acquisition pathways, the diet of the symbiotic pair was examined through direct observation of the stomach contents using optical and scanning electron microscopy, complemented by stable isotope ratio analyses. The results demonstrate a clear chromatic adaptation of the symbiont to its new host following the host switch: yellow shrimp individuals progressively acquired the purple colouration of their new host within 12 days. Stable isotope data and stomach content analyses both indicate food sharing and direct feeding on host tegument by the symbiont. These results suggest that Z. soror could acquire pigments from its host by consuming its tissues and stealing some of its food intake, which would allow colour adaptation and question its symbiotic status as a commensal.