Genetic relationships of the hydrocoral Millepora alcicornis and its symbionts within and between locations across the Atlantic

Although the hydrocoral Millepora alcicornis is a prominent and ecologically relevant amphi-Atlantic reef builder, little attention has been given to its endosymbionts which are also involved in the survival and adaptation success of the species in different environments. In this study, we resolve the genetic relationships between M. alcicornis and its symbionts (Symbiodiniaceae) within both sides and across the Atlantic. The COI and 16S-rDNA regions were selected for the host tissues, and the 23S-rDNA and ITS regions were chosen for the symbionts. Phylogenetic networks consistently showed that host populations from the eastern Atlantic archipelagos (Canary and Cape Verde Islands) were more related to western Atlantic populations than they were between them. However, results for Symbiodiniaceae species varied according to the molecular marker used. Samples from Mexico were grouped as Symbiodinium sp. (formerly Symbiodinium clade A) by both markers. Specimens from Puerto Rico were grouped as either Symbiodinium sp. or Breviolum sp. (formerly Symbiodinium clade B), according to the molecular marker used. Most samples from the eastern Atlantic were identified as Breviolum sp. by both markers, except for one sample from the Canary Islands and two samples from the Cape Verde Islands, which were identified as Cladocopium sp. (formerly Symbiodinium clade C) using ITS-rDNA. These results suggest that these two genera of Symbiodiniaceae may cohabit the same M. alcicornis colony. Because hydrocorals from the Canary Islands were phylogenetically related to the western Atlantic, but symbionts were more related to those of the Cape Verde Islands, the origin of the coral and its symbionts is probably different. This may be explained either by “horizontal” transmission, i.e. acquisition from the environment, or by a change in the dominant symbiont composition within the host. The flexibility of this hydrocoral to select symbionts, depending on environmental conditions, can provide new insight to understand how this coral may face ongoing climate change.

     

Phenological and morphological variations in the Mediterranean orange coral Astroides calycularis between two distant localities

Studies of intraspecific variation in the presence of different environmental features are essential to improving the knowledge of species population dynamics and structure. Astroides calycularis is an azooxanthellate scleractinian coral commonly found in shallow rocky habitats of the southwestern Mediterranean Sea. This study compares the fertilization period of two coral populations located in distantly separated localities; one in an upwelling area off the southern coast of the Iberian Peninsula, and the other on the southwestern coasts of Italy. Colony morphology varied between localities, the former having massive-shaped morphology with densely crowded polyps, the latter having bush-shaped morphology and separated polyps. These differences are possibly due to the different hydrodynamic conditions of their respective habitats. Gonochorism and planula brooding as sexual patterns and conditions of the coral coincide at both sites, but a delay in the timing of fertilization and planulation of the southern Iberian Peninsula population was observed, probably linked to a shift in the time at which seawater reaches its maximum temperature at each site.