Mesopontia, Sak & Huys & Karaytuğ, 2008

Sak, Serdar, Huys, Rony & Karaytuğ, Süphan, 2008, Disentangling the subgeneric division of Arenopontia Kunz, 1937: resurrection of Psammoleptastacus Pennak, 1942, re-examination of Neoleptastacus spinicaudatus Nicholls, 1945, and proposal of two new genera and a new generic classification (Copepoda, Harpacticoida, Arenopontiidae), Zoological Journal of the Linnean Society 152, pp. 409-458 : 443

GENUS MESOPONTIA GEN. NOV.

Lang (1965) noted that there is no close affinity between A. dillonbeachia and A. subterranea , but believed the former was ‘... most nearly related to the species A. acantha and A. secunda ...’, both of which currently belong to the acanthus lineage of Neoleptastacus . Arenopontia dillonbeachia shares with Arenopontia (as redefined herein) and Neoleptastacus the presence of an outer spine and an inner geniculate seta/claw on P1 enp-2, and the absence of sexual dimorphism on the P3 endopod; however, the morphology of the P5 indicates an intermediate position between both genera. In Arenopontia , the innermost element is setiform and defined at the base (as in Psammoleptastacus and Onychopontia gen. nov.); in A. dillonbeachia it is modified into a strong articulating spine, whereas in Neoleptastacus the inner corner of the P5 is modified into a spinous process. The pinnate nature of this process and the presence of a posterior surface suture in some species (e.g. N. spinicaudatus ; Fig. 15E) indicate the Neoleptastacus condition originated from the incorporation of a spinous inner element (as expressed in A. dillonbeachia ). The transitionary state of the P 5 in conjunction with the nonprehensile P1 exclude A. dillonbeachia from both Arenopontia and Neoleptastacus , and consequently it is here designated as the type of a new genus. A unique feature for the genus is the bicuspidate dorsomedial process on the caudal ramus.

Diagnosis: Arenopontiidae . Urosomites: without conspicuous surface ornamentation. Anal somite: without paired dorsolateral spinous processes. Anal operculum: not modified. Hyaline frills of abdominal somites with narrow rectangular lappets. Caudal ramus: without dorsolateral spur, but with bicuspidate process near medial margin. P1 exopod: threesegmented, short; exp-1 longest, with outer spine; exp-3 with two spines and two geniculate setae. P1 endopod: not prehensile, about as long as exopod; enp-2 with outer spine and inner geniculate seta. P2–P3 endopods: two-segmented; inner serrate seta of P2 enp-2 present. P3 endopod: with outer distal element. P4 endopod: with outer distal element well developed. Armature formula as follows:

Exopod Endopod P2 0.0.021 0.120

P3 0.0.021 0.020

P4 0.0.121 0.020

P3 endopod male: not sexually dimorphic, twosegmented. P5: with outer basal seta and four discrete elements in both sexes; innermost element a strong bipinnate spine. P 6 male: with two setae.

Type and only species: Arenopontia dillonbeachia Lang, 1965 = Mesopontia dillonbeachia ( Lang, 1965) comb. nov.

Etymology: The generic name is derived from the Greek mesos (MESOS meaning the middle), and the suffix pontia (PONTIA, meaning the sea), commonly used in the formation of interstitial copepod names, and refers to the morphology of the P5, which exhibits a transitionary state between the Arenopontia condition (innermost element setiform) and the Neoleptastacus condition (inner spinous process).