Data describing interactions between colonies on St. John, Virgin Islands in 2014 (NCEI Accession 0278869)

Acquisition Description:
Methodology from Gambrel, B. and Lasker, H.R., 2016

Octocoral colonies at East Cabritte and Europa Bay were surveyed to assess the spatial distribution and,in particular, the incidence of colonies in close proximity to each other. At each site, five 10 m × 1 m parallel belt transects were set up perpendicular to shore at10 m intervals starting at an arbitrarily selected point. Erythropodium caribaeorum and the encrusting form of Briareum asbestinum were not included in the surveys because they are not branching and, therefore,do not compete for space in the canopy. Each octocoral ≥5 cm in height was identified to species level in the field when possible; otherwise a small, 3 cm long sample was collected from colonies ≥15 cm tall for sclerite examination under a microscope. Images of the colony and a close-up image of the collected branch were also obtained. Identifications were based on Bayer (1961) and Sánchez (2009).

Colonies were divided into 2 classes, those in close proximity to a neighbor, cases in which a colony’s branches or base were within 5 cm of another octocoral colony, and those more distantly spaced. When the branches of 2 colonies are <5 cm apart, branch movement driven by currents and wave action often leads to contacts between the branches. Large colonies whose bases are within 5 cm of each other almost always have branches within 5 cm of each other, and small colonies with bases <5 cm apart at the time of settlement will almost inevitably come into contact with each other as they grow in the canopy. We recorded all instances of interactions. We did not distinguish between interspecific and intraspecific interactions since our goal was to first determine the overall incidence of competition among branching octocorals at the 2 sites.

At East Cabritte, which had a greater density of octocorals than Europa Bay, 4 randomly selected 1×1 m2 quadrats on each transect were surveyed while all 10 quadrats on each transect were surveyed atEuropa Bay. The height of each colony was measured to the nearest centimeter. Each colony was assessed for proximity to adjacent colonies and for effects of proximity, tissue damage to branches in close proximity to an adjacent colony and/or an asymmetric colony form. Only the octocorals with an asymmetric colony form attributable to the presence of another octocoral were scored as asymmetric in this study.Colonies that were asymmetric as a result of growing adjacent to a physical obstruction were not included as our focus was on the incidence of competition among octocorals. Examples of the different effects are shown in Fig. 1.