Abstract
Symbiodinium are dinoflagellate photosynthetic algae that associate with a diverse array of marine invertebrates, and these relationships are comprehensively documented for adult animal hosts. Conversely, comparatively little is known about the associations during larval development of animal hosts, although four different metazoan phyla (Porifera, Cnidaria, Acoelomorpha, and Mollusca) produce larvae associated with Symbiodinium. These phyla represent considerable diversities in larval forms, manner of symbiont acquisition, and requirements on the presence of symbionts for successful metamorphosis. Importantly, the different requirements are conveyed by specific symbiont types that are selected by the host animal larvae. Nevertheless, it remains to be determined whether these associations during larval stages already represent mutualistic interactions, as evident from the relationship of Symbiodinium with their adult animal hosts. For instance, molecular studies suggest that the host larval transcriptome is nearly unaltered after symbiont acquisition. Even so, a symbiosis-specific gene has been identified in Symbiodinium that is expressed in larval host stages, and similar genes are currently being described for host organisms. However, some reports suggest that the metabolic exchange between host larvae and Symbiodinium may not cover the energetic requirements of the host. Here, we review current studies to summarize what is known about the association between metazoan larvae and Symbiodinium. In particular, our aim was to gather in how far the mutualistic relationship present between adult animals hosts and Symbiodinium is already laid out at the time of symbiont acquisition by host larvae. We conclude that the mutualistic relationship between animal hosts and algal symbionts in many cases is not set up during larval development. Furthermore, symbiont identity may influence whether a mutualism can be established during host larval stages.
Highlights
Marine invertebrates typically present indirect development and their generally planktonic larvae act as the dispersive stage
Our review provides an analysis on the relationship between Symbiodinium and marine invertebrate larvae
Some species may digest symbionts while others actively engage in metabolite exchange and certain larval forms apparently select symbiont types while others do not
Summary
Marine invertebrates typically present indirect development and their generally planktonic larvae act as the dispersive stage. Especially scleractinian corals within the Cnidaria, engage in a mutualistic symbiosis with the photosynthetic dinoflagellates in the genus Symbiodinium (Stat et al, 2006; Venn et al, 2008). The majority of these corals are broadcast spawners and newly-formed planula larvae, while lecithotrophic, must acquire their symbionts horizontally (Harrison and Wallace, 1990; Baird et al, 2009; Harrison, 2011). The planktonic larvae acquire different free-living Symbiodinium strains from the environmental pool before selecting one or more specific strains at a later stage (Belda-Baillie et al, 1999; Weis et al, 2001; Coffroth and Santos, 2005)
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