Crustaceans as Symbionts: An Overview of their Diversity, Host Use and Life Styles.

The adoption of a symbiotic lifestyle is a major adaptation in the marine realm. Protection against environmental stress, escape from natural enemies, and nourishment are the major benefits obtained by symbiotic crustaceans from hosts. Hosts also represent a mating and nursery ground for symbiotic crustaceans. Costs and/or benefits experienced by hosts are diverse but may be subtle and challenging to measure. Costs suffered by hosts include physical injury, reduced feeding/growth rates, decreased fecundity and lifespan, and feminization, including castration, of male hosts. Some rhizocephalans are capable of altering host behaviors. The life cycles of symbiotic crustaceans vary widely. At one extreme, juveniles recruit directly into hosts from parental brooding chambers in crustaceans with direct/abbreviated development. Many symbiotic crustaceans with indirect development spend their larval life in the pelagic environment and establish themselves in/on their hosts during the first post-larval stage. At another extreme, the most complex life cycles occur in parasitic copepods and rhizocephalan cirripeds. Such life cycles involve one or two hosts and subtle or considerable changes in body morphology relative to that of their closest free-living relatives. The species richness of various symbiotic clades is higher than that of their closest free-living relatives. Whether the symbiotic lifestyle favors adaptive radiations in crustaceans is an outstanding and open question.