A revision of the genus Arenopontia Kunz, 1937 (Copepoda, Harpacticoida, Arenopontiidae), including the description of five new species Author Sak, Serdar Department of Biology, Faculty of Science and Literature, Balıkesir University, Balıkesir, Türkiye. Author Karaytuğ, Süphan Department of Biology, Faculty of Science, Mersin University, Mersin, Türkiye. Author Huys, Rony Department of Life Sciences, Natural History Museum, Cromwell Road, London SW 7 5 BD, UK. text Zootaxa 2024 2024-04-04 5433 1 1 50 http://dx.doi.org/10.11646/zootaxa.5433.1.1 journal article 10.11646/zootaxa.5433.1.1 1175-5326 10953646 06E5A735-A276-41D7-A9EE-B09642D953B6 Arenopontia nesaie Cottarelli, 1975 sensu Mitwally & Montagna (2001) Description. Mitwally & Montagna (2001) : 535–538; Figs 11–12 . Distribution. Egypt , Alexandria ; Bir Masoud, El Mamoura and El Shatby beaches. Body length. 305–427 μm ( ♀ ), 244–305 μm ( ♂ ). Wells (2007) noted that, assuming Mitwally & Montagna’s (2001) setation formula of P1–P4 is correct, their material cannot be assigned to Arenopontia . Sak et al. (2008) pointed out that the unorthodox armature pattern results from failure to distinguish between ornamentation elements (such as long spinules) and genuine setae/spines. Obvious observational errors include the 3-segmented condition of the mandibular palp and the reported presence of an outer seta on P1 enp-1 and P3–P4 enp-1, and of four elements on P2 exp-3. The distal segment of the P4 exopod also appears to be rotated in their Fig. 11G probably as a result of imperfect mounting. The presence of a prominent spinule [misinterpreted as a setation element by Mitwally & Montagna (2001) ] at the outer distal corner of P1 enp-1 places their material in Group II. Within this group, the Egyptian population shares the dorsal spur on the caudal ramus with A. nesaie and A. gunduzi sp. nov. , however, displays a P1 enp-1:exp ratio (1.85) that is intermediate between the respective values of these species. The elements on the female P5 are distinctly longer than in A. nesaie and the caudal rami appear shorter. No information was given on the number of setae on the male P6 but the variability illustrated for the male P5 indicates that there was more than one species in their samples. Consequently, A. nesaie sensu Mitwally & Montagna (2001) is considered a taxon of doubtful identity pending the re-examination of additional material. Species identification The nine valid species of Arenopontia can be divided in two groups based on the number of spatulate setae on the apical segment of the antennule, spinular ornamentation along the outer margin of P1 enp-1, and the number of elements on the male P6. Although the respective states of these three characters appear to be linked in each group, there is no congruence with other characters such as the number of elements on the fifth legs, the ornamentation of the anal operculum and the presence/absence of a dorsal spur on the caudal ramus. With the exception of A. riedli , which can readily be distinguished from its congeners by the more primitive armature on the endopods of P2–P3, accurate identification of Arenopontia species is notoriously difficult. A simple dichotomous identification key is difficult to construct, however species can reliably be identified by considering the differentiating characters summarised in Table 1 . Identifications made with the key below must be confirmed by reference to the original descriptions in the literature. 1. P2–P3 with two apical elements on enp-2............................................................ A. riedli . P2–P3 with one apical element on enp-2...................................................................2. 2. P1 enp-1 with single prominent spinule near outer distal corner of segment ( e.g. Fig. 15E )...........................3. Outer margin of P1 enp-1 with three sets of (typically two, occasionally three) spinules, more or less evenly distributed along length of segment ( e.g. Fig. 4A )..........................................................................5. 3. Caudal ramus with raised spinular row dorsally near inner margin; P5 of both sexes with five elements.................................................................................................... A. basibuyuki sp. nov. Caudal ramus with dorsal spur; P5 of both sexes with four elements.............................................4. 4. P1 enp-1 1.7 times as long as exopod; P4 enp-1 distincly shorter than exopod; P 5 ♀ innermost element at least as long as outer apical element.......................................................................... A. gunduzi sp. nov. P1 enp-1 twice as long as exopod; P4 enp-1 as long as exopod; P 5 ♀ innermost element distinctly shorter than outer apical element...................................................................................... A. nesaie . 5. Anal operculum spinulose, with coarse spinules; P 5 ♀ with five elements.......................... A. syltensis sp. nov. Anal operculum pinnate with fine spinules, or smooth; P 5 ♀ with four spinules....................................6. 6. P1 enp-1 1.3 times as long as exopod and 5.0 times as long as wide.............................. A. anatolica sp. nov. P1 enp-1 at least 1.5 times as long as exopod and at least 6.9 times as long as wide.................................7. 7. P1 enp-1 1.5 times as long as exopod and 6.3 times as long as wide................................... A. subterranea . P1 enp-1 1.7 times as long as exopod and 8.5 times as long as wide............................... A. adriatica sp. nov. P1 enp-1 1.7 times as long as exopod and 6.9 times as long as wide.................................. A. problematica .