Aquatic Sowbug

Asellus aquaticus (Linnaeus)

Oniscus aquaticus Linnaeus, 1758, p. 637.—Otho Fabricius, 1780, p. 251.

Asellus aquaticus (L.).—Geoffrey St. Hilaire, 1764, p. 672.—Sars, 1899, p. 97.—Racovitza, 1919, pp. 31–41, figs. 1–6.—Richardson, 1905, pp. 428–431, fig. 486.—Stephensen, 1917, pp. 239–240.—Van Name, 1936, pp. 458–459, fig. 287.—Birstein, 1951, pp. 57–60, figs. 18–26.—Williams, 1962b, pp. 78–80.

Asellus vulgaris Latreille, 1803, p. 359.—Not Gould, 1841, p. 337.

Asellus grönlandicus? Krøyer, 1838, p. 318—Not Packard, 1867, p. 296.—Hansen, 1888, p. 190.

A complete synonymy for Asellus aquaticus would be extremely long and complex and is not needed here, as this species is probably restricted to the Palaearctic and more complete synonymies have been given elsewhere (e.g., Bovallius, 1886; Gruner, 1965). The synonymies and references given above cover the original description and generic transference, the most pertinent references to A. aquaticus in the Palaearctic, and all references to the synonyms of the species in North America.

Asellus aquaticus was first indicated as present in North America by Otho Fabricius (1780) who recorded it from Greenland. A further Greenland record was given by Krøyer (1838) who recorded what he said was undoubtedly the same species as Fabricius, but who tentatively gave it separate specific status (as A. grönlandicus), a separation of which Hansen (1888), who referred to it as “A. grønlandicus,” had great doubts. Richardson (1905) gave yet another record of A. aquaticus in Greenland based upon material sent her by the Museum of Comparative Zoology at Harvard and labeled “Asellus grönlandicus.”

Stephensen (1917) summarized the old records of A. aquaticus in Greenland, and on the basis of his summary and a personal communication from Dr. Ulrich Risen, who had examined many hundreds of freshwater localities in Greenland, I came to the conclusion (Williams, 1962b) that A. aquaticus is probably absent from Greenland. It is now necessary to add, however, that a collection of material in the Museum of Comparative Zoology at Harvard and clearly labeled “M C Z Greenland Asellus aquaticus (Linnaeus)” on examination proved to contain material which is probably A. aquaticus. The material consisted of two specimens, both of which had dried out and were in very bad condition. Gentle warming in alcohol helped to soften them, but unfortunately one was still completely unrecognizable at the species level. The other, however, though impossible to identify definitively to species, could be seen to prossess many features characteristic of A. aquaticus. The most important features are indicated in Figures 28A–C. From these it can be seen that the specimen (a male) possessed a prominent basal spur on the endopod of the second pleopod (a feature possessed by no other epigean species recorded from North America), its first pleopod can reasonably be reconstructed to a shape similar to that of the first pleopod of A. aquaticus, and the propodus of its first peraeopod lacked a triangular projection near the midpoint of its palm. This material would seem to be that examined by Richardson in 1905, and my reexamination of it, therefore, gives support to her identification as A. aquaticus.

The reexamination although solving partly one of the problems associated with records of A. aquaticus in North America, namely reliable identification, does not really clarify the present status of the species. The conclusion remains, it seems reasonable to state, that A. aquaticus is not present in Greenland, a conclusion with which Dr. Røen (personal communication, 7 June 1967), who has examined many further Greenland localities since his original communication to me (see Røen, 1962), still agrees. The specimens belonging to the Museum of Comparative Zoology may, as suggested by Røen (in Williams, 1962b, p. 80) for older records, have come originally from Denmark in ships’ water tanks, have been introduced temporarily to suitable waters in Greenland near ship bases there, and then have been collected as a “native” species.

It should be added that A. aquaticus has not been recorded for North America outside Greenland, and no further material has been encountered during the present study of North American collections. Packard’s (1867) record of A. aquaticus from Labrador related in fact to a terrestial isopod (Johansen, 1926).

The name Asellus vulgaris advanced by Latreille in 1803 for European material has been consistently synonymized with A. aquaticus (see, for example, Birstein, 1951; Gruner, 1965), and there is little to gainsay this. Gould recorded a taxon he referred to as “A. vulgaris? Latr.” from Massachusetts in 1841, but as he says only that is was common, was larger than the two species described by Say [1818, A. communis and A. lineatus (=Lirceus lineatus)], and that he could find no differences from “the foreign species,” we are left in considerable doubt as to identity of his specimens. In view of the almost certain absence of A. aquaticus from North America apart from Greenland, it is most unlikely that Gould’s specimens were A. aquaticus. Through the courtesy of Dr. H. W. Levi and Miss A. B. Bliss, I have examined all available material of Asellus in the Museum of Comparative Zoology, the principal institution for the deposition of zoological material in Massachusetts, and did not encounter any material that could have been seen by Gould. Gould’s species has been synonymized with A. communis by Richardson (1905) and Van Name (1936), but neither author advanced reasons for this synonymy. The identity of the species recorded by Gould must remain unknown.