Widening the host range of the ectosymbiotic scale-worm Asterophilia culcitae (Annelida: Polynoidae) to three echinoderm classes, with data on its body color variation

The Polynoidae, commonly known as “scale-worms” due to the scale-like elytra on the dorsal surface, contains many species living in symbioses with other invertebrates. Most of these symbionts are host-specific, but some have a wide range of hosts. The genus Asterophilia includes two species living in shallow subtropical to tropical waters in the Pacific Ocean as ectosymbionts of asteroids and, more rarely, crinoids. Here, we recorded Asterophilia culcitae from asteroid hosts (Culcita novaeguineae, Linckia laevigata, L. guildingi, and Leiaster leachi) and, for the first time, from holothurian hosts [Stichopus chloronotus, Holothuria atra, H. (Stauropora) pervicax, and Bohadschia argus] along warm Japanese Pacific coasts. The cytochrome c oxidase subunit I sequences of the specimens from holothurians did not differ significantly from those on asteroid hosts, proving that A. culcitae has a wide host range across three different echinoderm classes: asteroids, holothurians, and crinoids. The general body color of A. culcitae was constantly reddish (female) or whitish (male), regardless of the host body color, although a previous study suggested that it differs in accordance with the host body color. However, we found that one individual from a holothurian host showed a different color pattern: A. culcitae typically shows three whitish or yellowish elytral mounds that have been suggested to mimic the tube foot of the asteroid hosts, whereas one individual from S. chloronotus had reddish brown translucent mounds, which we suggest might be cryptic on its holothurian host.