Spirorbis spirorbis or Spirorbis borealis is a small (3–4 mm) coiled sedentary marine polychaete worm in the Serpulidae family that lives attached to seaweeds and eel grass in shallow saltwater. It is commonly called the sinistral spiral tubeworm and is the type species of the genus Spirorbis.[1]
S. spirorbis is a cross-fertilising hermaphrodite that broods its young in a tube attached to the worm inside the shell. The larvae are released at an advanced stage of development and spend just a few hours as free-living organisms before attaching themselves to the nearest suitable surface, often the same seaweed as the parent.
Description
S. spirorbis secretes a very small, unridged, off-white, calcareous tube. This is about five millimetres in diameter and forms a flat, clockwise spiral coil as seen from above. The worm retreats into its tube when above water but under water can be seen to have green tentacles.[2]
The species has a smooth, white, sinistral (left-handed) coiled shell encasing an orange body about 3 mm in length. The tube has a peripheralflange where it attaches to the substrate.[citation needed]
The worm has a short abdominal region and a slightly broader thorax terminating in ten stiff tentacles, used to filter food from the water. One of the tentacles is slightly larger than the rest and shaped like a saucer, which is used as an operculum. This seals the opening of the shell and serves to protect the worm from predators and desiccation when out of water.[citation needed]
Distribution and habitat
Spirorbis spirorbis on Fucus, on the beach at Mwnt, Ceredigion
S. spirorbis is a hermaphrodite. The segments at the front of the abdomen are female while those at the back are male.[3] The male and female gonads mature at much the same time but the sperm is usually released first. Fertilisation is external to the body but inside the tube.[4] The larvae are free swimming member of the plankton for a short time.[5] They then settle out. In a study, where fronds of Fucus serratus already colonised by adults were available for settling, the larvae avoided the most densely populated areas and favoured the concave grooves on either side of the midribs. When prevented from settling out for eight hours, they were much less selective and settled on any Fucus surface regardless of the presence of adults.[6]
References
^Daudin F. M. (1800). Recueil de mémoires et de notes sur l'espèces indites ou peu connues de mollusques, de vers et de zoophytes. Paris pp. 50 + 4 pl:
^ abJohn Barrett and C M Young, Collins Pocket Guide to the Sea Shore (1958) p.76
^ abcRead, G.; Fauchald, K. (Ed.) (2024). World Polychaeta Database. Spirorbis borealis Daudin, 1800. Accessed through: World Register of Marine Species on 2024-12-06
