A new species of Hyalopomatus (Serpulidae: Polychaeta) which lacks an operculum: is this an adaptation to low oxygen?

Abstract

Hyalopomatus cancerum n.sp., epizoic on spider-crabs (Encephaloides) in a low-oxygen area of the Arabian Sea, differs from other species of the genus in lacking opercula. Larger serpulids in Indian Ocean ‘Galathea’ samples from great depths, tentatively referred to Protis simplex Ehlers, mostly bear vesicles on tips of pinnulate radioles. These are too small to occlude the tube mouth, so perhaps they and the larger vascularised vesicles of Apomatus may help in respiration. The BIOFAR Survey off the Faroe Islands showed that Protula (always non-operculate) and Apomatus are the main serpulid genera in the deeper channels. Protis, Protula and Apomatus,which are amongst the best-known of deep-sea serpulids, are like early postlarval stages of Serpula and Hydroides in lacking opercula, or in having thin-walled opercular vesicles on pinnulate stalks. They thus support the view that hypomorphy is somewhat characteristic of abyssal taxa.

Considering non-operculate serpulids of shallow seas, many species of Spiraserpula lack opercula, but secrete sharp ridges and spines on the inner walls of their tubes, which must deter or trap predatory tube-invaders; Floriprotis may be protected by coral, whilst Salmacina, Paraprotula, Microprotula and Paraprotis dendrova Uchida incubate their embryos, so have special respiratory needs.