Interspecific and Intraspecific competition of Lytechinus variegatus (Variegated Sea Uchin) and Tripneustes ventricosus (West Indian Sea Egg)

Breeanne Thomas and Kaitlyn O’Toole

Lytechinus variegatus (Variegated Sea Urchin) and Tripneustes ventricosus (West Indian Sea Egg) are two species of echinoderm grazers found coexisting in Caribbean seagrass beds. According to theory, these species are able to co-exist due to intraspecific competition being higher than interspecific competition. Our experiment tested consumption by the two urchin species, both intraspecifically and interspecifically to see which type of competition would be highest. We collected urchins of both species and similar sizes from the bay. We kept track of their mass and consumption of turf algae over 48 hours in a darkened tank with running water. As the number of species increased, we observed a higher proportion of algae consumed on average for both the West Indian Sea Egg and Variegated Urchin. In trials containing both species, observed consumption was more than the expected. We found that competition had less of an effect on consumption when both species were present, interspecifically, than when a single species was present, intraspecifically. Therefore, competition between different species is less then competition among the same species.

Weighing of West Indian Sea Egg.