Host growth and survivorship varies with endosymbiotic algal partner in developing cnidarians

Juvenile cnidarians acquire diverse assemblages of dinoflagellate symbionts within the family Symbiodiniaceae from the environment, and the dominant symbionts often differ from those in conspecific adult hosts. Such non-homologous symbionts may confer benefits to their juvenile hosts that are not available from homologous symbionts (those found in adult symbioses). Benefits gained by harboring several heterologous symbiont types remain poorly understood. We investigated host growth and survivorship with several symbiont partners that naturally occur within juvenile Briareurn asbestinum (Alcyonacea). We infected newly settled aposymbiotic juveniles with symbiont types within cultured Breviolurn (formerly lade B) and Symbiodinium (formerly lade A) that naturally occur within juveniles of this host. The symbiont partner community significantly affected overall growth and mortality rates of juveniles during this 11 mo laboratory study. Heterologous symbionts found within juveniles may confer survival benefits, but they vary in the relative contributions to host fitness. Symbiont types within Breviolum provided the greatest growth and survivorship benefits to B. asbestinum hosts, and early acquisition of these symbionts increased survivorship as those juveniles initially harboring a more mixed population exhibited higher mortality rates. These findings demonstrate that symbiont types that naturally occur in developing octocoral hosts have significantly different effects on host growth and survival, and some types in particular play an important role in octocoral symbioses.