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.

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Over the last century, considerable research has been dedicated to investigating life history traits of coral planulae. However, despite the universality of coral propagules and their deduced ecological properties, there is no clear understanding of some basic biological issues such as ecological and evolutionary forces that determine sexual reproduction and larval behavior in hermatypic corals. To further evaluate ecological trajectories in sexual reproduction of the hermatypic coral Stylophora pistillata from Eilat, Red Sea, we collected 22 900 planulae from 313 S. pistillata colonies in situ during March to July (2004July ( to 2006. Variations in reproductive effort were observed between years, but during all reproductive seasons, the larvae tended to settle ex situ in aggregates of kin (61.71% to 82.4%; 2005 values). During a 6 mo survey, the spat grew at a rate of 3.4 to 5.4 polyps mo -1 . Peaks of planula shedding were documented for April to June, suggesting a shift in reproductive seasonality compared to the preceding 2 to 3 decades. Ex situ settlement of collected larvae revealed, for the first time, settlement polymorphism of early versus late larvae throughout the reproductive season; the March settlement rate was the highest (43.7%) and June was the lowest (26.2%). A reverse trend was documented for settlement rates on the first post-planulation night, with only 2.2% settlement in March versus 46.2% in July. Early season larvae of S. pistillata are therefore capable of dispersing farther than late season propagules. These results are diametrically opposed to the 'desperate larva hypothesis,' revealing different ecological traits of early versus late season larvae.