BackgroundAggregated settlement of kin larvae in sessile marine invertebrates may result in a complex array of compatible and incompatible allogeneic responses within each assemblage. Each such aggregate can, therefore, be considered as a distinct self-organizing biological entity representing adaptations that have evolved to maximize the potential benefits of gregarious settlement. However, only sparse information exists on the selective forces and ecological consequences of allogeneic coalescence.ResultsWe studied the consequences of aggregated settlement of kin larvae of Stylophora pistillata (a Red Sea stony coral), under controlled laboratory settings. When spat came into contact, they either fused, establishing a chimera, or rejected one another. A one-year study on growth and survivorship of 544 settled S. pistillata genotypes revealed six types of biological entities: (1) Single genotypes (SG); (2) Bi-chimeras (BC); (3) Bi-rejecting genotypes (BR); (4) Tri-chimera entities (TC); (5) Three-rejecting genotypes (TR); and (6) Multi-partner entities (MP; consisting of 7.5 ± 2.6 partners). Analysis of allorecognition responses revealed an array of effector mechanisms: real tissue fusions, transitory fusions and six other histoincompatible reactions (borderline formation, sutures, overgrowth, bleaching, rejection, and partner death), disclosing unalike onsets of ontogeny and complex modes of appearance within each aggregate. Evaluations at the entity level revealed that MP entities were the largest, especially in the first two months (compared with SG: 571% in the first month and 162% in the seventh month). However, at the genotype level, the SG entities were the largest and the colonies with the highest-cost-per-genotype were the TR and the MP colonies. The cost was calculated as reduced average genotype size, from 27% and 12% in the first month to 67% and 64% in the seventh month, respectively. In general, MP exhibited the highest survivorship rate (85%, after one year) and SG the lowest (54%).ConclusionIn view of the above, we suggest that the driving force behind gregarious kin settlements in Stylophora pistillata stems from gained benefits associated with the immediate and long-term increase in total size of the MP entity, whereas survivorship rates did not draw a parallel link. Furthermore, the biological organization of MP entity exhibits, simultaneously, an intricate network of rejecting and fusible interactions in a single allogeneic intimate arena, where proposed benefits surpass costs incurred by discord among founders. Above results and documentations on gregarious settlement in other marine taxa bring us to suggest that the 'group level' of kin aggregates may serve as a ubiquitous legitimate selection entity in the evolution of a sessile mode of life in marine organisms.
Coral populations have precipitously declined on Caribbean reefs while algal abundance has increased, leading to enhanced competitive damage to corals, which likely is mediated by the potent allelochemicals produced by both macroalgae and benthic cyanobacteria. Allelochemicals may affect the composition and abundance of coral-associated microorganisms that control host responses and adaptations to environmental change, including susceptibility to bacterial diseases. Here, we demonstrate that extracts of six Caribbean macroalgae and two benthic cyanobacteria have both inhibitory and stimulatory effects on bacterial taxa cultured from the surfaces of Caribbean corals, macroalgae, and corals exposed to macroalgal extracts. The growth of 54 bacterial isolates was monitored in the presence of lipophilic and hydrophilic crude extracts derived from Caribbean macroalgae and cyanobacteria using 96-well plate bioassays. All 54 bacterial cultures were identified by ribotyping. Lipophilic extracts from two species of Dictyota brown algae inhibited[50% of the reef coral bacteria assayed, and hydrophilic compounds from Dictyota menstrualis particularly inhibited Vibrio bacteria, a genus associated with several coral diseases. In contrast, both lipo-and hydrophilic extracts from 2 species of Lyngbya cyanobacteria strongly stimulated bacterial growth. The brown alga Lobophora variegata produced hydrophilic compounds with broad-spectrum antibacterial effects, which inhibited 93% of the bacterial cultures. Furthermore, bacteria cultured from different locations (corals vs. macroalgae vs. coral surfaces exposed to macroalgal extracts) responded differently to algal extracts. These results reveal that extracts from macroalgae and cyanobacteria have species-specific effects on the composition of coral-microbial assemblages, which in turn may increase coral host susceptibility to disease and result in coral mortality.
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