AFLP analyses of genomic DNA reveal no differentiation between two phenotypes of the vestimentiferan tubeworm Ridgeia piscesae

Ridgeia piscesae Jones, 1985, is the only vestimentiferan tubeworm species at the Juan de Fuca Ridge of the northeast Pacific, yet the range of sulphide microhabitats it populates and tube morphologies it expresses resemble those of other vestimentiferan tubeworm genera known at hydrothermal vent and deep-sea hydrocarbon seep sites throughout the world. Initially, two species of Ridgeia were formally described, and up to five were proposed based on distinct morphological phenotypes (Jones, 1985; Tunnicliffe & Fontaine, 1987; Tunnicliffe, 1991). Studies of variation in allozymes, in the mitochondrial cytochrome oxidase I gene, and in nuclear ribosomal genes, however, have detected no significant differentiation between different Ridgeia morphs (Southward et al., 1995; Southward et al., 1996; Black et al., 1998). The vent habitats on the Juan de Fuca Ridge are diverse, varying in vent fluid flow rate, chemical composition, temperature, and substratum types. The morphological forms of R. piscesae that inhabit these microhabitats differ considerably in length, diameter, and details of their body morphology, as well as in characteristics of their tube (Jones, 1985; Southward et al., 1995). The present study focuses on two of the most extreme phenotypes, which we refer to as long-skinny and short-fat. Long-skinny Ridgeia are found on basalt substrata in weak, diffuse flow environments of low sulphide levels, where temperatures generally range from ambient (2 °C) to only a few degrees above (Robigou et al., 1993; Urcuyo et al., 1998). Tubes of mature long-skinny individuals are relatively sturdy, rigid, and average one metre in length, tapering to a thin-walled posterior that is approximately 1 mm in diameter. Long-skinny Ridgeia resemble the cold seep vestimentiferan Lamellibrachia cf. lumeysi van der Land & Nørrevang, 1975, from the Gulf of Mexico, which has been shown to acquire sulphide through its roots at levels sufficient to sustain net chemoautotrophy (Freytag et al., 2001). Based on similar posterior tube permeability characteristics, it has been hypothesized that the longskinny morph of Ridgeia may also supplement its sulphide uptake in this manner, a useful adaptation for tubeworms that live in environments where sulphide is often undetectable around the animals’ plumes (Urcuyo et al., submitted). Short-fat individuals are found on sulphide edifices, in areas with visibly active venting fluid that is up to 30 °C and with sulphide concentrations around 200 μM (Robigou et al., 1993; Sarrazin et al., 1997). Short-fat tubes are very thin and relatively flimsy, averaging 15 to 20 cm in length, 1 cm in diameter, and displaying pronounced flanges along their length. This phenotype displays no roots and uses its plume to uptake sulphide. Two hypotheses can explain the morphological differences in R. piscesae in different microhabitats. The first hypothesis is that the various phenotypes are induced by environmental cues. In a single aggregation of Ridgeia in heterogeneous fluid flow at the Endeavour Segment of the Juan de Fuca Ridge, Urcuyo et al. (submitted) found two clusters of newly settled individuals. The tubeworms that had settled in an area of lower flow developed roots, while those in a higher vent flow area did not. Studies currently being conducted on the blood physiology of short-fat and long-skinny Ridgeia have detected morph-specific differences in the structure and function of haemoglobin molecules (J. Flores, PSU, pers. comm.). Initial studies have detected higher concentrations of the 400 kDa haemoglobin in the coelomic fluid of the short-fat morph as compared to the long-skinny morph. This suggests that short-fat Ridgeia can bind considerably greater concentrations of sulphide Cah. Biol. Mar. (2002) 43 : 363-366