Blennies on offshore petroleum platforms in the Gulf of Mexico: Factors influencing assemblage structure
Assemblages closer to the equilibrium end of an equilibrium/non-equilibrium continuum are stable (in time and space), and usually regulated by biotic interactions. Assemblages closer to the non-equilibrium end are unstable (in time and space) and usually regulated by abiotic factors. Using blennies (Family Blenniidae; Hypsoblennius invemar, Scartella cristata, and Parablennius marmoreus ) found on offshore petroleum platforms in the northern Gulf of Mexico, I asked the following questions: Where is this assemblage of blennies located on an equilibrium/non-equilibrium continuum? Are interspecific interactions (such as competition and predation) important in maintaining the assemblage structure of these blennies? Initially, I recorded the pattern of abundance and distribution of four species of blennies on 13 offshore platforms. Using Moritita's Index of Similarity, I found that blenniid assemblages on different platforms were similar. Also, platforms surveyed two years apart and before and after Tropical Storm Danny had similar blenniid assemblages. Therefore, this assemblage of blennies appears closer to the equilibrium end of an equilibrium/non-equilibrium continuum. I investigated the effect of predation on the vertical zonation of blennies by identifying predators through documentation of the predator avoidance behavior of blennies. Predators were not distributed in a pattern which could restrict some species of blennies to certain depths or limit the total number of blennies at deeper depths. Interspecific competition was an important factor affecting blenniid assemblages because large Scartella cristata displaced small Hypsoblennius invemar from large barnacle cavities at 1 m. At 5 m where S. cristata was absent, H. invemar used large barnacle cavities. In aquarium tests with blennies captured between 10 and 15 m, Parablennius marmoreus was dominant over similar sized H. invemar . However, H. invemar was able to defend a cavity when given prior residence overnight. Almost half of the cavities at these depths were vacant, which provided H. invemar access to cavities. The effect of prior resident advantage changed the interspecific dominance relationship of these blennies and allowed H. invemar to spawn between 10 and 15 m.