Brooding snake star

Amphipholis squamata is a very small brittle star, with a central disc of only a few millimetres, that has been reported from every ocean except the Southern Ocean ^[1]. It is a hermaphrodite that keeps its embryos inside the body and releases crawling young, so it has no drifting larval stage ^[2]. Genetic work shows that the name covers a complex of lineages, at least 4 of them reproductively separate species in the French Mediterranean alone ^[3].

Adults are tiny: the disc measures roughly 1 to 3 mm across, and a general upper limit of 5 mm is given for the species ^[1],[2]. In a Scottish population the estimated size at maturity was a disc diameter of about 1.5 mm, and most of the reproductive output came from individuals of 2.00 to 2.99 mm ^[4].

The colour is highly variable. When 680 animals from one Mediterranean locality were photographed, 9 classes (one of them undetermined) could be defined from the colours of the body and arms ^[1]. Colour does not identify a lineage: the colour forms do not line up neatly with the mitochondrial clades, and no structural feature has been found that separates geographic areas or colour varieties ^[1],[3]. The distinct lineages inside the complex therefore cannot be told apart by eye ^[3].

The species is regarded as nearly cosmopolitan. It occurs in all oceans except the Southern Ocean and has been found from just below the surface down to 1300 m ^[1]. It is accepted in the World Register of Marine Species, which treats it as a marine and brackish animal ^[5].

On the shore it lives hidden: under stones, in rock cracks and among the fronds of algae ^[1]. In the Mediterranean it was sampled from patches of coralline red algae ^[2] and from blocks of the calcareous alga Corallina officinalis, collected by snorkelling in 0 to 1 m of water ^[1]. Densities vary strongly between patches; single quadrats held anywhere from 0 to more than 100 animals, and a Sicilian lagoon has been reported to hold up to 2000 individuals per litre of algae ^[2].

The animal is a simultaneous hermaphrodite, with male and female gonads that both end in the brood pouches (bursae) ^[2]. Embryos develop inside these pouches and crawl-away juveniles are released, with no free larva ^[1],[2]. In the Firth of Forth, brooding went on all year, with the most embryos and newly released young in summer; the young left the parent at a disc diameter of about 0.8 to 1.2 mm ^[4].

The mating system is unusual. In 190 animals from one Mediterranean site, not one heterozygote was found with 3 microsatellite markers, and in 91 brooding adults all of the 369 offspring carried the genotype of their parent ^[2]. The authors conclude that self-fertilisation is extremely frequent, while not excluding rare episodes of sexual mixing ^[2]. Such selfing could help a single founder start a new population, which may help explain the wide range of a species that does not disperse as larvae ^[2].

Feeding was studied in the Firth of Forth, where animals had full stomachs through the year and fed most in summer, when the water was warmest. Nearly all of them (99.4%) had taken in fine particles, and a wider variety of plant and animal items was also found in the gut ^[4]. The same study found that about 3% of the animals carried an ectoparasitic copepod, Cancerilla tubulata ^[4].

Dispersal over long distances is probably passive. The species has been seen in mats of floating algae, and work on New Zealand coasts supports rafting on macroalgae as an occasional means of travel ^[1],[6]. Even so, the genetic structure around New Zealand indicates restricted regional dispersal with sporadic long-distance events, and 4 divergent lineages were found there that can occur together in the same place ^[6]. Several lineages also occur in syntopy on the French Mediterranean coast, without a clear ecological explanation so far ^[3],[1].