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.

Grazers and predators mediate the post-settlement bottleneck in Caribbean octocoral forests

“…Plexaurids are the most abundant family of octocorals that spawned during the time of this study (Kahng, Benayahu, and Lasker 2011). The highest settlement pulse was coincident with times when there were many planulae in the water column similar to observations by Wells et al (2021).…”

“…Calcified worm tubes were present in similar abundances on all tiles, but they were only visible in the images used to determine abundance when they were not under other benthic cover groups such as turf algae. Opposite to the initial caging experiment but similar to findings of Wells et al (2021), turf algae reduced survival in the urchin inclusion and fish exclusion experiment (Fig. 4D).…”

“…At the conclusion of the experiment, invertebrates from one tile of each treatment were identified and counted to determine the effectiveness of each invertebrate removal treatment by soaking them in magnesium chloride for 30 minutes, shaking them, and then manually picking any remaining invertebrates off the tile. The undersides of the tiles, where octocorals primarily settle (Wells et al 2021; Martínez-Quintana and Lasker 2021), were photographed to categorize benthic cover after treatments were applied. Turf height was measured in five locations on the underside of the tiles.…”

“…Macroalgae have often been highlighted as a threat to juvenile recruitment of both scleractinians and octocorals (Wells et al 2021; Linares, Cebrian, and Coma 2012; Kuffner et al 2006; Hughes 1994; Bruno et al 2009; Roff and Mumby 2012). Proposed mechanisms for this negative interaction include allelopathic competition (Kuffner et al 2006; Rasher et al 2011; Bonaldo and Hay 2014), algae providing a reservoir for virulent disease (Nugues et al 2004; Casey et al 2014), microscale alterations of conditions in the benthic boundary layer (Carpenter and Williams 1993; Brown and Carpenter 2013), and physical interactions between polyps and algae (River and Edmunds 2001; Box and Mumby 2007).…”

“…While octocorals and scleractinians are both negatively affected by algal competition (Lirman 2001; Hughes et al 2007; Kuffner et al 2006; Lasker and Kim 1996; Wells et al 2021), they differ in their apparent response to grazers. Grazers frequently facilitate scleractinian growth, fecundity, and recruitment by removing algae (Lirman 2001; Stockton and Edmunds 2021; Nugues and Bak 2006; Foster, Box, and Mumby 2008; Idjadi, Haring, and Precht 2010).…”