This week's highlight taxon is the worm genus Hydroides. Hydroides unites about eighty species of the family Serpulidae, the tubeworms, an easily-found component of many a beach all over the world. Serpulids (previously commented on here) are a distinctive group of annelid worms that secrete themselves a tubular shell of calcium carbonate in which they live permanently attached to a rock or some other substrate (not uncommonly, that "other substrate" will be another tubeworm, leading to the production of tangled masses of worm tubes). Many authors have divided serpulids between two families, Serpulidae proper and Spirorbidae, but phylogenetic studies place spirorbids as a derived subgroup of Serpulidae rather than their sister group (Kupriyanova et al., 2006; Lehrke et al., 2006). Hydroides is not a spirorbid, so it remains in Serpulidae whatever the preferred arrangement.
With more than eighty species, Hydroides is a reasonably large assemblage, and it is distributed worldwide. Among the more distinctive features of the genus is the division of the spinose operculum into two tiers, a lower (rather daisy-like, in my opinion) ring called the funnel and an upper ring of spines called the verticil. The features and arrangement of the verticil spines are often the main distinguishing characters between species, but this can be complicated somewhat by changes in spine morphology over the course of growth and regeneration (ten Hove & Ben-Eliahu, 2005). It is also notable that at least one Caribbean species, Hydroides spongicola, has been recorded as showing a tendency towards reduction of the operculum; this species lives in close association with the highly toxic touch-me-not sponge (Neofibularia nolitangere) and presumably the host sponge's irritating spicules offer all the protection the tubeworm needs (Lehrke et al., 2006).
The first species of Hydroides to be described was H. norvegica (now Hydroides norvegicus to match the gender of the genus) in 1768 by Johan Ernst Gunnerus, Bishop of Trondhjem*. An English translation of Gunnerus' original description was published by Moen (2006); as shown in the reproduction above from Moen (2006) of the first few lines, the original is not among the easiest of reads. Interestingly enough, Gunnerus bestowed a different name on the animal ("Hydroides norvegica") from the tube that it lived in ("Serpula norvegica") - I suspect that this may represent a different philosophy from the present about how to deal with animals versus their products, as opposed to any doubt about whether the one had produced the other. In a letter to Linnaeus (or von Linné as he'd become by then), Gunnerus also described his doubt about just what type of animal it was he'd described - a hydrozoan, or possibly a sea cucumber? It seems that its sessile lifestyle had quite put him off the idea of it being a worm**.
*Among other animals described by Gunnerus are the basking shark and Lineus longissimus, the world's longest ribbon worm and possibly the longest of all living animals.
**Remember, Gunnerus and Linnaeus were both working in a largely non-evolutionary paradigm. As such, their classifications were not intended to reflect an organism's "affinities" in the sense that we'd understand them, but rather to reflect how their overall features compared to other animals.
These days, Hydroides species are among the most intensely studied of all marine annelids. The motives for this interest are primarily economical - not surprisingly, Hydroides have often earned the ire of humans through their penchant for attaching themselves to the humans' nice clean boats and jetties. A number of Hydroides species have been transported and established outside their native ranges by human agency - probably in ballast water for the most part, though H. ezoensis was transported from Japan to France on the shells of oysters imported to stock oyster farms (Thorp et al., 1987). With this economic focus, it is not surprising that the majority of studies appear to have been on factors affecting larval development and settlement. One interesting point is that Hydroides larvae are far more likely to settle somewhere already inhabited by tubeworms than on a completely fresh surface (Scheltema et al., 1981), a common pattern among sessile organisms that do not reproduce by budding. After all, there's safety in numbers.
Hove, H. A. ten, & M. N. Ben-Eliahu. 2005. On the identity of Hydroides priscus Pillai 1971 – taxonomic confusion due to ontogeny in some serpulid genera (Annelida: Polychaeta: Serpulidae). Senckenbergiana Biologica 85 (2): 127-145.
Kupriyanova, E. K., T. A. Macdonald & G. W. Rouse. 2006. Phylogenetic relationships within Serpulidae (Sabellida, Annelida) inferred from molecular and morphological data. Zoologica Scripta 35: 421-439.
Lehrke, J., H. A. ten Hove, T. A. Macdonald, T. Bartolomaeus & C. Bleidorn. 2006. Phylogenetic relationships of Serpulidae (Annelida: Polychaeta) based on 18S rDNA sequence data, and implications for opercular evolution. Organisms Diversity & Evolution 7 (3): 195-206.
Moen, T. L. 2006. A translation of Bishop Gunnerus’ description of the species Hydroides norvegicus with comments on his Serpula triqvetra. Scientia Marina 70S3: 112-123.
Scheltema, R. S., I. P. Williams, M. A. Shaw & C. Loudon. 1981. Gregarious settlement by the larvae of Hydroides dianthus (Polychaeta: Serpulidae). Marine Ecology Progress Series 5: 69-74.
Thorp, C. H., S. Pyne & S. A. West. 1987. Hydroides ezoensis Okuda, a fouling serpulid new to British coastal waters. Journal of Natural History 21 (4): 863-877.