Welcome to the jungle: Algal turf negatively affects recruitment of a Caribbean octocoral

Abstract

1. Algal cover has increased and scleractinian coral cover has steadily declined over the past 40 years on Caribbean coral reefs. In contrast, octocoral abundance has increased at those sites where octocoral abundances have been monitored. The effects of algal cover on recruitment may be a key component in these patterns, as upright octocoral recruits have the potential to escape competition with algae by growing above the ubiquitous algal turfs. However, the impacts of algal turf on octocorals have not been tested. 2. We used laboratory and field recruitment experiments to examine impacts of algal turf on settlement and then survival of newly-settled octocorals. Tiles were preconditioned on a Caribbean reef, allowing algae to settle and grow. Tiles were then partitioned into three treatments: lightly scrubbed (0% turf cover), left alone (19% turf cover), or kept for 15 days in a sea table without fish or large invertebrate herbivores (50% turf cover). Planulae of the common Caribbean octocoral Plexaura homomalla were allowed to settle and metamorphose on the tiles for six days. Tiles were then deployed onto a reef and survival of those recruits was monitored for seven weeks. Settlers that recruited to the tiles after deployment to the reef were also monitored. 3. Laboratory recruitment rate was significantly higher in lower turf cover treatments. Field survival was significantly reduced by increased turf cover; for every 1% increase in turf cover, polyps died 1.3% faster. In a model parameterized by the observed field survival, polyps exposed to 100% turf cover had a 2% survival rate over 51 days, while polyps exposed to no turf cover had a 32% survival rate over the same time. 4. Synthesis. We found that high densities of turf algae can significantly inhibit recruitment of octocorals. Octocoral survival rates were similar to those published for scleractinians, but field settlement rates were much higher, which likely contributes to the higher resilience of octocorals to disturbances. The factors that influence recruitment are critical in understanding the dynamics of octocorals on Caribbean reefs as continuing declines in scleractinian cover may lead to more octocoral-dominated communities in the Caribbean.

Spawning, embryogenesis, settlement, and post‐settlement development of the gorgonian Plexaura homomalla

“…Nine days later, the tiles were deployed onto an octocoral‐dominated reef (18.309°N, 64.719°W) and monitored 2, 4, and 10 weeks after fertilization. Settlement and survival rates on these tiles in the presence of varying levels of algal turf cover are reported in Wells et al (2020).…”

High ecological resilience of the sea fan Gorgonia ventalina during two severe hurricanes

“…Competition with macroalgae, for example, is detrimental for small scleractinians ( Box & Mumby, 2007 ), whereas arborescent octocorals can quickly extend above macroalgae from a small site of attachment ( Jackson, 1979 ; Borgstein, Beltrán & Prada, 2020 ). The positive demographic implications of rapid growth are augmented by recruitment, which is modest relative to scleractinians when octocoral recruits are considered as colonies ≤ 10 cm tall ( Lasker et al., 2020 ), but can be high when measured as polyps on settlement tiles evaluated over days-weeks ( Wells et al, in review ). Together, these features are likely to be important in supporting greater ecological resilience of octocorals relative to scleractinians ( Tsounis & Edmunds, 2017 ), and promoting octocoral persistence to form structurally dominant communities on contemporary reefs ( Lasker et al., 2020 ).…”