Early Life-History Dynamics of Caribbean Octocorals: The Critical Role of Larval Supply and Partial Mortality

Abstract

Recruitment is a key demographic process for maintenance of local populations and recovery following disturbance. For marine invertebrates, distribution and abundances of recruits are impacted by spatiotemporal variation in larval supply, settlement rates and post-settlement survival. However, for colonial and modular organisms, differences in survival and growth between settlers and colonial recruits may also affect recruitment patterns. In the Caribbean, shifts in the benthic community structure favoring octocoral’s have been detected, and recruitment has been suggested as key for octocoral’s resilience. Hence, we studied octocoral recruitment dynamics, and evaluated the role of pre-settlement, settlement and post-settlement processes in recruit’s densities. We performed the study at two sites with different octocoral densities, on the south coast of St. John, United States Virgin Islands, and distinguished between processes occurring to recently settled polyps and to colonial recruits. At both sites, we monitored P. homomalla settlers on settlement tiles for 3 months, and colonial recruits of two of the most abundant genera (Eunicea and Pseudoplexaura) for 3 years. In addition, we assessed whether recruits morphological traits affected recruitment and divided recruits of the genus Eunicea based on the presence of large calyces. The major contributor to both, single-polyps and colonial recruit densities was larval supply. Single-polyp densities were not limited by the availability of space, settlement cues, or early post-settlement survival. Height was the only predictor of survival and growth of colonial recruits, with potential growth rates increasing with height. However, large recruits suffered partial mortality often, distorting the relationship between recruit age and size, and causing most recruits to remain in the recruit size class (≤5 cm) longer than a year. Octocorals have been resilient to the conditions that have driven the decline of scleractinian corals throughout the Caribbean, and recruitment has been key to that success. Our results are crucial to understand early life history dynamics of Caribbean octocorals, and highlights the need to standardize the definition of recruit among colonial and modular taxa to facilitate inter-specific comparisons, and to understand future changes in coral reef community assemblages.

Portfolio effects and functional redundancy contribute to the maintenance of octocoral forests on Caribbean reefs

“…6 ) is suggestive of response variation and portfolio effects. Changes in the < 20 cm size class over the course of the study reflect differential mortality to small colonies as well as recruitment, which also varied over the course of the study 74 . The capacity of large (i.e., tall) octocoral colonies to form a cohesive canopy is an important community-scale feature of octocoral forests 75 , 76 , and it is positively associated with octocoral recruitment on shallow (< 9 m depth) 77 and mesophotic 78 reefs.…”

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

“…Settlement tiles (n = 40) were initially deployed and conditioned in an octocoraldominated reef in Grootpan Bay, St. John, U.S. Virgin Islands (18.309° N, 64.719° W) on April 12, 2021 for 62 days. This site is also known as East Cabritte in other studies (Lasker et al 2020;Martínez-Quintana and Lasker 2021). The tile undersides had 22 3-mm deep vertical grooves providing refugia for algae and invertebrate recruits.…”

“…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.…”

“…Octocoral forests provide some of the same functions as scleractinian reefs (e.g., Privitera-Johnson, Lenz, and Edmunds 2015; Tsounis, Steele, and Edmunds 2020) and are resilient to climatic events like hurricanes (Lasker et al 2020) unlike Caribbean scleractinians (Gardner et al 2003). Octocoral success has been attributed to reduced direct competition with scleractinians, higher fecundity, and greater juvenile survivorship and recruitment (Ruzicka et al 2013; Lenz et al 2015; Bartlett et al 2018; Lasker et al 2020; Tsounis and Edmunds 2017; Martínez-Quintana and Lasker 2021). While these mechanisms are key to understanding shifting reef communities, the finer details of octocoral recruitment processes remain understudied compared to those of scleractinians.…”

Growing up is hard to do: a demographic model of survival and growth of Caribbean octocoral recruits

“…Most studies of recruitment among octocorals have sampled on scales greater than the time needed for many recruits to bud, and therefore have included small single branch colonies as recruits, i.e. , those of a few centimeters in height ( Lasker, 2013 ; Lasker & Porto-Hannes, 2015 ; Martínez-Quintana & Lasker, 2021 ; Yoshioka, 1996 ). Martínez-Quintana & Lasker (2021) distinguished primary polyps or single-polyp recruits from colonial recruits.…”