The strange social structure of tiny fish called emerald coral gobies may be explained by family loyalty, new research shows.

Coral goby groups contain a single breeding male and female and — as «sequential hermaphrodites» — the subordinate gobies can take over either role if one of the breeders dies.

The puzzle for biologists is why breeders tolerate the smaller non-breeders sharing their space and competing for food.

One explanation is «kin selection» (favouring related individuals).

Reef fish are often assumed to disperse at random after hatching, meaning groups of adults should not be closely related, but the new study finds «positive relatedness» among gobies living close together.

«Groups of emerald coral gobies have a fixed structure in which the breeders are biggest,» said lead author Dr Theresa Rueger, of the University of Exeter and Boston University.

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