Chelantermedia gen. nov.

Type and only species. Chelantermedia composita sp. nov.

Diagnosis. Body elongated, length about 5 times longer than width of pereonite 2. Lateral margins of pereonites straight, Pereonites 6, 7 and pleotelson dorsally fused. Pereopod chelate, enlarged, carpus ventrally with 1 slender seta midway and 1 slender seta proximal to claw-seta.

Etymology. The first part of the name refers to the chela of pereopod I. Chela (Latin, a claw) and intermedia (Latin, in between) reflect the intermediate set of characters between Desmosomatidae and Nannoniscidae – as it occurs for example in the chela of pereopod I.

Discussion. The new genus shows a mixture of characters of Desmosomatidae Sars, 1897 and Nannoniscidae Hansen, 1916, two families that are poorly differentiated. The new species, C. composita, will be important in phylogenetic studies leading to better understanding of the relationships within the family Desmosomatidae. It possesses a combination of character states not compatible with any existing genus. No desmosomatid genus has pereonites 6 and 7 and the pleotelson fused as in some nannoniscid genera, for example Rapaniscus Siebenhaller and Hessler, 1981. All nannoniscid genera including species with fused pereonites 6 and 7 (Hebefustis Siebenhaller and Hessler, 1977, Rapaniscus, Regabellator Siebenhaller and Hessler, 1981, Nannoniscoides Hansen, 1916, Nannoniscus Sars, 1870, Saetoniscus Brandt, 2002) have an antennula with a bulbous distal article. The new genus does not possess such a specialisation, the flagellar articles of the antennula tapering towards the distal article. Pereopod I resembles most species of Prochelator Hessler, 1970, Chelator Hessler, 1970 or Oecidiobranchus Hessler, 1970. Prochelator is the only one of these three that includes species with biramous uropods but none has fused pereonites.