Males of basketstar, Astrochlamys bruneus, live grabbed to females, which are larger, so that it is difficult to separate them without breaking any of their leathery arms. The Amplexus reminds of some toads when several males embrace a female forming a kind of a "sexual ball".
In many specimens the male is situated in such a way that his mouth is covered by the female's dorsal disc and it will be hard for him to have a coffee break, otherwise he might lose the sexual favors of the female. In this situation the female might not only give the male a piggyback (phoresy) and, being more corpulent, defend him from potential predators, but also the male might eat from female's table (commensalism) or directly rob her food (kleptoparasitism).
Another interesting aspect is the one mentioned by Hendler (1991), could we see in this dimorphism a case of proterandic hermaphroditism? That means, the same individual is male first and thereafter, being larger, it becomes a female. Of course it would not be a case of real sexual dimorphism, as the different sizes would be determined by age and, therefore, for the sexual stage in which the individual was found.
Hendler, G. (1991) Echinodermata: Ophiuroidea. In: Giese, A.C., Pearse, J.S. & Pearse, V.B. (Eds.), Reproduction of marine invertebrates: echinoderms and lophophorates. The Boxwood Press, Pacific Grove, CA, pp. 355–511.
©Rafael Martín-Ledo 2010