ON THE POSSIBILITY OF KIN GROUPS IN CORAL REEF FISHES 



Douglas Y. Shapiro 



Department of Marine Sciences, University of Puerto Rico 



Mayaguez, Puerto Rico 00708 



ABSTRACT 



This paper concerns the widely-held assumption that members of schools 

 of coral reef fishes are unlikely to be related because of pelagic dispersion 

 of their eggs or larvae. The most direct way for adult schools to contain 

 kin would be for siblings to remain together throughout the pelagic period 

 and then to settle together as juveniles. For pelagic sibling groups to 

 endure, mixing and dispersion by turbulent diffusion must be counter- 

 balanced by biological processes for the formation, maintenance or selective 

 survival of discrete egg or larval aggregations. I argue that there are 

 sufficient hints of the existence of such processes to warrant examination 

 of the possibility of kin groups in coral reef fishes. For example, demersal 

 spawners may reduce the period of passive dispersion almost to zero by 

 laying benthic eggs that release developmentally advanced larvae with the 

 sensory and motor ability to aggregate actively soon after hatching. Other 

 likely places to look for kin groups are among mouthbrooders , livebearers 

 and pelagic spawners like lionfishes or species like Anthias whose young 

 juveniles occupy large, sedentary groups. 



With rare exception, eggs and/or larvae of coral reef fishes undergo a pelagic 

 phase during which many are thought to travel large distances in open ocean. It is 

 generally assumed that pelagic eggs and larvae are thoroughly mixed, so that re- 

 cruits at any particular site on a reef represent a random sample of the reproduc- 

 tive products entering the local gene pool. If mixing occurs, kin are unlikely to 

 settle together or find one another after metamorphosis. Hence, it is widely 

 assumed that kinship plays no role in the evolution of coral reef fish behavior, 

 schools or social groups (e.g., Thresher , 1977). 



However, at the beginning of, during, and at the end of the pelagic phase, eggs 

 and larvae are clumped. Eggs are initially released in the sea by pelagic spawners 

 or laid on the substrate by demersal spawners in a small, tightly-clumped mass. 

 Thus, at the outset sibling eggs are closely aggregated. Little is known about 

 patterns of pelagic dispersion for coral reef fishes, but among commercial 

 temperate water fishes pelagic eggs and larvae are often found in patches (Hunter, 

 1980). Among these species patchiness is sufficiently prevalent that Hewitt (1981) 

 concluded that patchiness is an important characteristic of larval existence with 

 strongly positive survival value. At the end of the pelagic phase, coral reef fish 

 larvae metamorphose and settle on the reef. Larvae of at least 23 species in 

 several families settle in aggregates (Luckhurst & Luckhurst, 1977; Tal bot,£t aj . , 

 1978), often in bursts of recruitment over short periods of time (Victor, 1983). 

 Thus, one finds patches in all pelagic stages. The most direct way for adult 

 social groups, colonies or schools to contain kin would be for siblings to remain 

 together throughout the pelagic period and then to settle together as juveniles. 

 The aim of this paper is to outline some of the factors determining whether or not 

 sibling aggregations will persist through the pelagic phase. 



To place dimensions on the problem, I will first treat eggs as 'though they were 

 a passive, inert substance, such as dye, that is dispersed by diffusion. Assume 

 that 2,000-40,000 neutral or positively buoyant eggs are released in the water col- 

 umn at a depth of 10 m in a single spawn as a sphere 10-20 cm in diameter. If (1) 

 the eggs are non-adherent, (2) horizontal turbulent diffusion is radially symmetric, 



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