Acidostoma obesum, Bate & Westwood, 1861

Stoddart, H. E. & Lowry, J. K., 2012, Revision of the lysianassoid genera Acidostoma and Shackletonia (Crustacea: Amphipoda: Acidostomatidae fam. nov.), Zootaxa 3307 (1), pp. 1-34 : 2

The Acidostoma obesum problem

The genus Acidostoma in the North Atlantic Ocean has had a somewhat troubled taxonomic history. Bate & Westwood (1861) described the new species Anonyx obesus . They illustrated the whole animal, the first antenna, gnathopods, urosome and telson, but no mouthparts. Lilljeborg (1865a, b) illustrated the species, including the mouthparts, and proposed the new genus Acidostoma . Sars (1890) also illustrated the species and these two accounts formed the concept of this species for the next 90 years, based mainly on the cleft telson. Until Dahl's (1964) paper, North Atlantic and Mediterranean specimens with a cleft telson were called A. obesum and specimens with a notched telson were called A. laticorne Sars, 1879 , with the few exceptions of specimens with a cleft telson and a notch on the urosome which were called A. nodiferum Stephensen, 1923 . Dahl (1964) showed that some of the literature records of A. laticorne actually represented an undescribed species, also with a notched telson, which he described as A. neglectum .

Lincoln (1979) changed the concept of A. obesum by reporting that the telson was apically notched, not deeply cleft as reported by Bate & Westwood. Lincoln based his assessment of A. obesum on a specimen held in the Natural History Museum, London (registration number BMNH 1952:5:7:2) and labelled as Holotype. We have examined this specimen and are convinced that it is not the specimen which Bate & Westwood (1861: 98) used for their original description of the species. Our reasons are:

— the specimen labelled as 'Holotype' shows characteristics of a male; the specimen Bate & Westwood described shows characteristics of a female.

— the specimen labelled as 'Holotype' has an extremely well-developed callynophore on the first antenna; Bate &

Westwood did not indicate a callynophore either in their whole-animal drawing or in their enlargement of the first antenna.

—the specimen labelled as 'Holotype' has a dorsal notch on urosomite 1; Bate & Westwood's illustration of the urosome shows a straight dorsal margin on urosomite 1.

—the specimen labelled as 'Holotype' has an apically notched telson; Bate & Westwood illustrated and described a deeply cleft telson. Lincoln (in litt.) has suggested to us that Bate & Westwood had misinterpreted the telson by viewing it somewhat obliquely.

We find it difficult to accept that Bate & Westwood misinterpreted/misrepresented all four aspects cited here. The callynophore is such a gross structure in this genus it would have been impossible to ignore or overlook. We therefore cannot accept this specimen as the holotype of A. obesum and do not accept Lincoln's subsequent interpretation of the nomenclature of the species.

We are convinced that the specimen used by Bate & Westwood to describe A. obesum (i.e. the holotype) is lost and that there is no other extant material of this species from Bate’s collections. The specimen currently considered as the holotype is not the specimen described by Bate & Westwood. To stabilise the taxonomy of the genus we establish a neotype for Acidostoma obesum .

To accommodate the literature records of A. obesum with a cleft telson, [such as those of Lilljeborg (1865), Sars (1890), Chevreux & Fage (1925) and Dahl (1964)], Lincoln (1979) proposed the new species name A. sarsi . Lincoln realised that the shape of urosome 1 is a sexually dimorphic character, dorsally notched in the male and straight in the female. Consequently he recognised A. nodiferum as the male of his new species and included A. nodiferum as a junior synonym of A. sarsi . However, as noted by Vader (1984) the name A. nodiferum should have precedence over A. sarsi . In our opinion both of these names are junior synonyms of A. obesum .

An additional consequence of Lincoln’s concept change was that A. neglectum became a junior synonym of A. obesum . In our opinion A. neglectum is a valid species.

Because of the complex series of changes involving the names A. obesum , A. nodiferum , A. neglectum and A. sarsi , it is extremely difficult to determine which species is represented by many of the literature records of these names, especially where there are no illustrations or description. We have compiled synonymies for each species, but many records need to be checked and re-assessed.

It is unfortunate but necessary that the following changes be made:

— that a neotype be established for A. obesum ;

— that Bate & Westwood’s original description of A. obesum be accepted;

— that A. neglectum be reinstated as a valid species;

— that A. sarsi is a junior synonym of A. nodiferum and both names are junior synonyms of A. obesum .