Mesopontia, Sak & Huys & Karaytuğ, 2008

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).