Methodology from DOI: 10.1111/ivb.12201

Reefs were censused between Cabritte Horn (18°18.4560 N, 63°43.6620 W) and White Point (18°18. 8610 N, 64°43.9090 W) on the south shore of St. John, and surveys were completed at multiple sites that were haphazardly selected as the field schedule permitted. The same vicinities were censused in 2 years, but replicate areas of reef were not relocated. Fringing reefs along the south shore of St. John are richly populated by octocorals and scleractinians that have been studied since 1987 (Rogers & Miller 2006; Edmunds 2013; Edmunds & Lasker 2016), and in this study, they were evaluated for interactions between colonies of Millepora spp. and octocorals. Surveys were completed in two summers (July and August of 2014 and 2016) at 9–14 m depth, and they focused on arborescent octocorals at sites close to those that have been surveyed for decades (Edmunds 2013).

Objective 1: Abundance of octocoral encrustation

To calculate the percentage of arborescent octocorals encrusted by colonies of Millepora spp., reefs were censused using band transects (2014, 10 9 2 m) and quadrats (2015, 1 9 1 m), that were randomly placed along a constant isobath at each site, but with depths varying among sites. Arborescent octocorals were counted by genus when their holdfasts were vis- ible within the band transects and quadrats, and colonies were inspected for encrustations of Millepora spp. Colonies of octocorals were scored as encrusted if any portion of their surface was covered by colonies of Millepora spp., and octocorals were identified to genus where this was possible; fully encrusted colonies often were impossible to identify and were scored as “unknown” octocorals. For all octocorals (i.e., pooled among taxa) and for each genus (where possible), densities of encrusted and Millepora-free colonies were calculated using data pooled among sites. The abundance of encrusted octocorals was expressed as a percentage of all colo- nies censused each year.

Objective 2: Initiation of octocoral encrustations

The likelihood that octocorals became encrusted through pursuit by Millepora spp. was evaluated from evidence that Millepora spp. colonies were orienting their growth toward nearby octocorals (sensu Wahle 1980). This possibility was determined by searching for examples of this growth orientation and, further, by measuring the distance from octocorals encrusted by Millepora spp. to other Millepora spp. colonies (called originating colonies), from which pursuit leading to encrustation could have been staged. The reef adjacent to encrusted octocorals was searched for originating colonies, and in 2014, these surveys were completed up to 1.5 m from the holdfasts of encrusted octocorals. In 2015, this distance was increased to 2.0 m to provide a more exhaustive census for possible originating colonies.

Our methods to evaluate the origins of colonies of Millepora spp. on octocorals were limited by the inability to census colonies over time in order to observe the progression of pursuit (by Millepora) or its outcome (overgrowth of octocorals). As an alternative to repeated censuses, we focused our 2015 measurements of distances between hydrocoral and octocoral colonies to include only those octocoral colonies encrusted by just a few centimeters of hydrocoral growth. Because these encrustations were at least as common in St. John as they were in Jamaica in the 1970s, when pursuit by Millepora was first observed (Wahle 1980) (described below), it was reasonable to expect that among large numbers of encrusted octocorals, we would observe interactions of varying ages including pursuit in its earliest stages. These encrustations likely had a recent origin and, therefore, signs of initiation by pursuit were more likely to be evident. Partially encrusted colonies were identified to genus where this was possible, and the distance to the nearest colony of Millepora spp. within 2 m of the holdfast was recorded.