Scrupocellaria reptans

Scrupocellaria reptans: a widespread or complex species?

Scrupocellaria reptans has been reported from different localities in the Northeast Atlantic, North Sea and Mediterranean (Zabala & Maluquer 1988; Hayward & Ryland 1998), despite the morphological differences (e.g. shape of rhizoids and frontal scuta) among specimens from different areas.

Hincks (1880, p. 52, pl. 7, figs 1–7) described two kinds of rhizoids (smooth and hooked) among British specimens of Scrupocellaria reptans. The “toothed” rhizoids (Hincks 1880, pl. 7, fig 6) are similar to those figured by Ellis (1755, 1756a, 1756b, 1767), as well as those observed in Linnaeus’s specimens (Figure 11) and other specimens from European waters (Figs 2, 16, 17). The hooked rhizoids were also reported in specimens from Cornwall, U.K. (Couch 1844, pl. 23, fig. 3). The second type of rhizoid was characterized by Hincks (1880) as “ …simple, and giving off at the extremity a number of anastomosing fibrils forming a netted disk ” and figured by him in Plate 7, fig. 5. These rhizoids, of simple tubes, had been described earlier by Johnston (1847, p. 337, pl.

LVIII, figs 3, 4), with the distal end of each tube branching into two or three small knob-like processes. In both Johnston’s and Hincks’s specimens from the British coast (NHMUK 1842.12.19.2; NHMUK 1899.5.1.3; NHMUK 1899.5.1.359), the rhizoids are smooth with a branched distal adherent end. Hincks (1880) suggested that these two forms of rhizoid are an ecological adaptation in this species. Colonies with smooth rhizoids that form circular reticulate disks distally are attached to firm surfaces like rocks or algae, while hooked rhizoids are found deeper in soft substrata like sponges. Hooks were observed by Peach (1878), who also described smooth rhizoids in colonies found on Flustra foliacea.

Peach (1878) mentioned Busk’s Scrupocellaria macandrei from Spain and S. ferox from Bass Strait, which were characterized by rhizoids with hooks. Waters (1909, 1913) also used hook shape to distinguish some Scrupocellaria taxa. On the other hand, Prenant & Bobin (1966) cited the presence of both hooked and smooth rhizoids in some species (viz. Scrupocellaria reptans, S. diadema, S. maderensis, S. delilii). We analysed more than 50 specimens deposited at NHMUK that had been identified as S. reptans, but found only a few lots with hooked rhizoids. These specimens have hooks with a consistent shape and position along the rhizoid tubes. We found the same type of hooked rhizoids in Linnaeus’s specimens (Figs 7, 11), in a specimen found in herbarium material identified by Alfred Norman (NHMUK 1915.4.2.13), in four specimens from the west coast of Britain (NHMUK 1849.2.12.51, NHMUK 1963.3.6.35, NHMUK 1994.3.4.5–6 and NHMUK 1995.9.25.26) and in two colonies from the Thanet coast, southern England (NHMUK 1884.12.12.9). Careful examination of the Thanet colonies revealed that two different morphotypes occur together, but they are distinguished by the shape of their rhizoids and frontal scuta. The presence of these two phenotypes on the same shell suggested that they may not be conspecific. At the same time, hooked rhizoids have not been found in several colonies from western Britain, or in colonies from the east coast of Britain and North European waters, which suggests a more restricted distribution of the morphotype with hooked rhizoids.

Examination of Scrupocellaria at NHMUK showed that the occurrence and shape of hooked rhizoids are uniform in colonies of the same species, indicating a species-specific character rather than an environmental adaptation. In addition, we observed at least three kinds of rhizoidal surfaces in the Candidae ―hooked (Figs 2–3), smooth (Fig. 4) and ringed (Fig. 5), morphologically distinct in different species.

By light microscopy, the morphology of zooids of the two phenotypes previously identified as S. reptans (with and without hooks) appears similar, but detailed study using SEM shows differences between them (Figs 2, 12–17, 24, 26 —morphotype with hooked rhizoids, = Scrupocellaria reptans sensu stricto; and Figs 4, 18–23, 25, 27 — morphotype with smooth rhizoids, herein recognized as a distinct species). In later ontogeny both species are distinguished by scutum shape (Figs 14, 21). Despite having the same proportions of opesia and scutum length, S. reptans sensu stricto has a slender, less-branched scutum compared to specimens with smooth rhizoids. In Linnaeus’s specimens, the scutum is branched twice (sometimes shortly branched at the distal tip), with a large gap between each slender branch (Fig. 9). In early ontogeny, however, the frontal scuta are quite similar in both species (Figs 26–27) but become highly branched in colonies without hooks (Figs 20–21). Variation in the shape of frontal avicularia makes it difficult to compare young colonies of both morphotypes; both have frontal avicularia with the same position and orientation, but in early ontogeny the rostrum of the avicularium is slender and taller in S. reptans (Figs 24–27). In Linnaeus’s type material, the basal vibracular chamber is often absent, while in the morphotype with smooth rhizoids it is usually present.