Genetic variation within Symbiodinium clade B from the coral genus Madracis in the Caribbean (Netherlands Antilles)

The internal transcribed spacer (ITS) region was sequenced in symbiotic dinoflagellates (zooxanthellae) from five morphospecies in the genus Madracis. The phylogeny of the symbionts is congruent with a companion phylogeny of the coral host. Comparison with known clade B symbiont ITS types reveals that M. mirabilis contains the B13 symbiont and that the other morphospecies contain the B7 symbiont. Madracis formosa also contains a previously undescribed type. The B7 and B13 symbionts appear to be highly specific to morphospecies in the genus Madracis. The host specificity between the B13 symbionts and its coral host may be the result of co-evolution of the coral-symbiont association and suggests that the brooding species, M. mirabilis, is reproductively isolated. Microhabitat differentiation associated with light utilization independent of depth is discussed.