Rhabdopleura mirabilis

Rhabdopleura mirabilis (M. Sars in G.O. Sars, 1872)

(Fig. 5A)

Type locality. Skraaven (= Skrova), Lofoten Islands, Norway, at 100‒300 fm (c. 183‒549 m) on mud.

Key features. Inception of ringed erect tubes is direct, with erect annulated zooid tubes produced directly from the creeping tube, not as blind branches. The creeping tube seldom bifurcates and is thickly covered by sediment particles, with erect tubes “always more or less bent in some part … sometimes like an S” (Sars 1872, p. 26). Zigzag sutures were neither mentioned nor illustrated, presumably because of obscuring particles covering the creeping tube, or they may be lacking. The largest colony fragment obtained by Sars was 4 cm long with erect tubes 6‒7 mm long and 200‒250 μm diameter. The living animal (arms, tentacles) was described as being covered by “intensely dark violet spots of colouring matter, which also occur on the buccal shield, and especially on its anterior freely projecting extremity, where they are very close together, forming a large, roundish dark spot” (ibid., p. 34). Pigmentation thus differs from that in R. normani, which is black and orange-brown, conferring a “spotted leopard-like appearance” (Lankester 1884, p. 630).

Comment. Sars was well aware of Allman’s species, concluding that it was “decidedly different” (Sars 1872, p. 27), lacking adherent particles and being “more strongly branched,” with shorter tubes and that “there appears some difference between the two species with regard to the manner of the division of the stem into chambers”; i.e. he noted the distinction between indirect-lateral and direct-frontal colony growth forms (as did Norman 1921, p. 99‒100), while not using that terminology.

Significantly, he also noted that, since the colony of R. mirabilis is “very small and entirely colourless, it is rather difficult to discover. Its presence is, however, easily detected by stirring the washed mud in a fine sieve with a feather or other instrument, when irregular fibres will be noticed therein. These fibres, covered by particles of mud, Rhizopod-shells, and fragments of mussel shells, will prove to be the creeping stem, whereupon, by closer investigation, there will be discovered the small, transparent, perpendicularly projecting cells.” The muddy substratum is remarkable for an organism that is normally thought to require a firm surface for its creeping stem. Coupled with the differences in colony growth form and zooid pigmentation, our conclusion is that R. mirabili s cannot possibly be synonymous with R. normani.

George Ossian Sars found the species in 1866. He later formally described it (Sars 1872), stating that his father, Michael Sars, had noted it in his catalogue of deep-sea animals, compiled in 1868, under the denomination of ‘ Halophilus mirabilis’.” This was in manuscript form as “Fortsatte Bemaerkninger om det dyriske Livs Udbredning i Havets Dybder” [= Continued Remarks on the Distribution of Animal Life in the Depths of the Sea]; it was not formally published and the combination Halophilus mirabilis was a nomen nudum. Notwithstanding, G.O. Sars (1872, 1874) deliberately attributed authorship to his father. Accordingly, we consider formal authorship of R. mirabilis to be M. Sars in G.O. Sars, 1872. The account by G.O. Sars (1872) is repeated verbatim in G.O. Sars (1874).