Biology subjects

Lasker, H. R.

Publications and source records attributed to Lasker, H. R..

5 recordsLinked to original sources

bioRxiv2026

Species identity rather than colony size partitions shape variation in six Caribbean octocorals

Caribbean reefs are increasingly dominated by arborescent octocorals as scleractinian cover declines. The physical structure of these octocoral forests is established by the manner in which octocoral species generate and modify colony form as they grow. We tested whether colony shape exhibited species-specific architecture and whether it changed allometrically with size. Two-dimensional projections of colony form were modeled as a series of stacked bands, whose areas were measured from scaled in situ images of colonies of six sympatric Caribbean octocorals from St John, US Virgin Islands. Principal component analysis (PCA) of relative band areas resolved a single dominant axis, accounting for 67.9% of the variation, that separated top-heavy from bottom-heavy colony profiles. Species identity explained 42% of the total variation in relative form, whereas colony height only explained 2% and had no detectable effect on either PCA axis. The proportion of the colony outline occupied by branches increased with height, from a predicted 41% at 20 cm to 52% at 100 cm. Colonies therefore grew without a detectable redistribution of area along their vertical axis, filling a species-specific outline more densely rather than moving along a common allometric trajectory. The vertical geometry of an octocoral canopy may therefore depend more on which species are present than on the sizes its colonies have reached.

ecology↗

bioRxiv2025

Caribbean octocoral communities: finding the forest for the trees?

Octocorals have increased in abundance on many Caribbean coral reefs, and at some sites "octocoral forest" may be a better community descriptor than "coral reef." Implicit to the concept of a forest is that structural elements, trees, colonies, etc., alter the environment in ways that affect the structural elements themselves and the organisms that inhabit the forest. At what density do the structural elements create the emergent properties of a "forest?" Communities traditionally characterized as hardgrounds and coral reefs around Puerto Rico and St John, US Virgin Islands, varied in density of octocoral colonies from a few to >100 colonies m-2in surveys conducted in 2021 and 2022. Canopy cover was correlated with the density of colonies. Among the quadrats with the highest octocoral density there was no significant correlation between numbers of colonies and canopy cover. Frontal area, a measure related to the communitys effect on water flow, as well as the volume occupied by colonies followed patterns similar to canopy cover. Vertical profiles of flow velocity were measured from the substratum to 3 m above the bottom on a reef on St John where octocoral population density ranged from 0 to 16 colonies m-2. Profiles of orbital velocity exhibited perturbations which were more pronounced in locations with > 12 colonies m-2. Using the effect on flow as a criterion, 4 of the 8 surveyed sites would function as forests. Understanding the density at which emergent properties appear is critical to understanding the bio-physical interactions affecting the community.

ecology↗

bioRxiv2022

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

Caribbean octocorals have not suffered the decades long decline in abundance that has plagued reef-building scleractinian corals. Their success and the formation of octocoral forests has been attributed to their continuing recruitment to reef habitats. Assessing the processes controlling recruitment is essential to understanding the success of octocorals and predicting their future. Benthic grazers on coral reefs can facilitate the growth and recruitment of corals by reducing the abundance of competitive algal turfs and macroalgae or hinder corals through predation of coral tissue and recruits. We assessed the effects of grazing by fishes and the sea urchin Diadema antillarum and mesofaunal predation on octocoral recruitment in a series of manipulative experiments using varying grazer/predator exclusion and inclusion conditions in in situ and ex situ experiments. Exposure to fish and urchin grazing significantly reduced survival and recruitment of single-polyp octocorals, while turf-associated mesofauna did not significantly affect neither recruitment nor survival. We also found a positive relationship between octocoral recruitment and turf algae, a potential related response to the deleterious effect of grazing exposure. These data suggest that grazers and predators mediate the mortality bottleneck characteristic of recruitment. Thus, the declines in the abundance of grazing fishes and urchins throughout the Caribbean may have contributed to the increase in abundance of octocorals in the Caribbean, concurrent with the loss of scleractinians.

ecology↗

bioRxiv2020

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

O_LIAlgal 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. C_LIO_LIWe 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. C_LIO_LILaboratory 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. C_LIO_LISynthesis. 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. C_LI

ecology↗

bioRxiv2020

Embryogenesis, polyembryony, and settlement in the gorgonian Plexaura homomalla

Understanding the ontogeny and reproductive biology of reef-building organisms can shed light on patterns of population biology and community structure. This knowledge is particularly important for Caribbean octocorals, which seem to be more resilient to long-term environmental change than scleractinian corals and provide some of the same ecological services. We monitored the development of the black sea rod Plexaura homomalla, a common, widely distributed octocoral on shallow Caribbean reefs, from eggs to 3-polyp colonies over the course of 73 days. In aquaria on St John, U.S. Virgin Islands, gametes were released in spawning events three to six days after the July full moon. Cleavage started 3 hours after fertilization and was holoblastic, equal, and radial. Embryos were positively buoyant until becoming planulae. Planulae were competent after 4 days. Symbiodiniaceae began infecting polyps at around 8 days post fertilization. Development was typical for Caribbean octocorals, except for the occurrence of a novel form of asexual reproduction in octocorals: polyembryony. Fragmentation of embryos during development may represent a temporally varied tradeoff between number and size of propagules, in which large eggs have higher fertilization rates followed by polyembryony, which maximizes the number of surviving recruits by generating more, albeit smaller, larvae. Polyembryony may contribute to the success of some gorgonians on Caribbean reefs as other anthozoans are in decline.

developmental biology↗