Arenopontia breviarticulata Mielke, 1975

Sak, Serdar, Karaytuğ, Süphan & Huys, Rony, 2024, Review of Neoleptastacus Nicholls, 1945 (Harpacticoida, Arenopontiidae), including an updated key to species and proposal of Phreatipontia gen. nov., Zootaxa 5525 (1), pp. 1-66 : 46-48

Arenopontia breviarticulata Mielke, 1975

Arenopontia (Neoleptastacus) breviarticulata Mielke, 1975 : Bodin (1979)

Pararenopontia breviarticulata ( Mielke, 1975) Bodiou & Colomines (1986)

Arenopontia (Pararenopontia) breviarticulata Mielke, 1975 : Bodin (1997)

Original description. Mielke (1975):110–112; Abb. 74 (♂ only).

Type locality. Germany, Schleswig-Holstein, Isle of Sylt, List ; in front of old “Litoralstation List/Sylt”; medium to coarse sandy beach .

Body length. 710 μm (♂).

Remarks. The description of Arenopontia breviarticulata is based on a single male that displays a 2-segmented P1 exopod (exp-2 and -3 fused) and a reduced P5 bearing only one spiniform and two setiform elements. As pointed out by Sak et al. (2008) it remains unconfirmed whether the inner spine of the male P5 (or spinous process when incorporated in the segment) is genuinely absent or was overlooked.The latter is unlikely given that A. breviarticulata , with a recorded male body size of 710 μm, is by far the largest arenopontiid to be described so far. Sak et al. (2008) offered an alternative explanation by comparing Mielke’s (1975) illustration with the P5 observed in the copepodid IV stage of N. indicus ( Rao 1967: fig. 3-22) which shows a similar underdeveloped condition. This may suggest that A. breviarticulata has a paedomorphic morphology which is further substantiated by its 2-segmented P1 exopod, a segmentation pattern that is displayed in copepodid II–IV of N. indicus before a third segment is added at the next moult ( Rao 1967). Conversely, A. breviarticulata has retained the plesiomorphic armature pattern on P2 endopod and P4 exopod. Mielke’s (1975) illustration of the P3 is problematic since it shows an inner seta on exp-3 and two very long setae on enp-2. Such features have not been observed in any other arenopontiid, raising the suspicion that Mielke (1975) did not observe the real P3 but duplicated P4 instead. The only difference between his illustrations of P3 and P4 is the extreme disparity in length of the outer basal seta.

Mielke (1975) described the anal operculum with two lateral “Zacken” (jags, sharp projections) which can be regarded as the positional homologues of the paired lateral spinous processes on the anal somite in the acanthus - group of Neoleptastacus . The very long outer spines on exp-2 of P2 and P4 (and possibly P3) in A. breviarticulata differentiates it from all members of this species group.

Bodin (1979) listed A. breviarticulata under the subgenus A. ( Neoleptastacus ) while Mielke (1975) considered it a member of the subgenus A. ( Arenopontia ). Bodiou & Colomines (1986) placed the species in their new genus Pararenopontia , the validity of which was dismissed by Martínez Arbizu & Moura (1994) on the grounds that it was an artificial taxon uniting species with reduced leg segmentation. Sak et al. (2008) favoured a relationship with the Mesopontia - Onychopontia - Neoleptastacus lineage based on the armature formula of P1 enp-2, displaying one geniculate seta and one outer distal spine.Although the morphology of the anal somite suggests that A. breviarticulata is probably nested within the acanthus -group of the genus Neoleptastacus , it is here treated as a species incertae sedis in the Arenopontiidae and not in Neoleptastacus as proposed by Sak et al. (2008).

Key to species

Redefinition of the generic boundaries in the Arenopontiidae ( Sak et al. 2008) and addition of new species have outdated the keys by Lang (1965), Bodiou & Colomines (1986), Karanovic (2000) and Wells (2007). Neoleptastacus accraensis and N. secundus (both species inquirendae) are included in Table 2 but not in the updated key below. Identifications made with this key must be confirmed by reference to the original descriptions in the literature.

1. Anal somite with paired dorsolateral spinous spurs or processes................................................2.

Anal somite without paired dorsolateral spurs or processes...................................................13. 2. Abdominal somites with integumental pattern of rectangular plates dorsally and ventrally; P2 enp-2 without inner seta.....3.

Abdominal somites without conspicuous surface sculpturing; P2 enp-2 with inner seta..............................4. 3. P3 enp-2 with two distal elements; P5 of both sexes with four articulating setae/spines................... N. ornamentus .

P3 enp-2 with one distal element; P5 of both sexes with three articulating setae/spines................... N. reductaspina . 4. P3 enp-2 with two distal elements........................................................................5.

P3 enp-2 with one distal element.........................................................................7.

5. P1 exp-3 with three setae/spines.................................................................... N. huysi . P1 exp-3 with four setae/spines..........................................................................6.

6. P1 endopod clearly longer than exopod; P3 enp-2 about half the length of enp-1; P5 of both sexes with one seta and two short spines between inner spinous process and outer basal seta............................................ N. acanthus . P1 endopod as long as exopod; P3 enp-2 only slightly shorter than enp-1; P5 of both sexes with one seta and one spur between inner spinous process and outer basal seta...................................................... N. chaufriassei .

7. Sternal plates of somites bearing P1–P4 with midventral hook-like processes..................... N. emendatus sp. nov. Sternal plates of somites bearing P1–P4 without such posteriorly directed processes................................8.

8. Paired spinous processes on anal somite straight and backwardly directed.........................................9. Paired spinous processes on anal somite dorsally recurved....................................................10.

9. P1 endopod 1.15 times as long as exopod; P5 elongate in both sexes, length/maximum width ratio 3.0 (♀) and 3.5 (♂), respectively................................................................................ N. longiremis . P1 endopod 1.30 times as long as exopod; P5 much shorter in both sexes, length/maximum width ratio 1.7 (♀) and 2.25 (♂), respectively............................................................................. N. rectus sp. nov.

10. P1 endopod shorter than exopod........................................................ N. abbreviatus sp. nov. P1 endopod at least as long as exopod....................................................................11.

11. P1 endopod as long as exopod; spinous process on caudal ramus relatively short, about 35% of ramus length.................................................................................................. N. chilensis sp. nov. P1 endopod 1.25–1.30 times as long as exopod; spinous process on caudal ramus longer, about 45–55% of ramus length..12.

12. P1 enp-1 1.25 times as long as enp-2; P 5 ♀ twice as long as maximum width; terminal process of caudal ramus dorsally recurved.................................................................................... N. gussoae . P1 enp-1 1.65 times as long as enp-2; P 5 ♀ 2.7 times as long as maximum width; terminal process of caudal ramus not dorsally recurved.................................................................................... N. indicus .

13. P1 exopod 2-segmented...............................................................................14. P1 exopod 3-segmented...............................................................................16.

14. P5 inner spine at least partly delimited at base; P 6 ♂ with one seta.............................................15. P5 inner spine fused at base, forming spinous process; P 6 ♂ with two elements.................. N. supersetosus sp. nov.

15. P2 exp-2 outer spine shorter than exopod; P2 endopod shorter than exp-1, flask-shaped; P4 exp-3 four times as long as maximum width, with inner seta arising from distal fifth of inner margin......................................... N. trisetosus . P2 exp-2 outer spine distinctly longer than exopod; P2 endopod as long as exp-1, subrectangular and slightly tapering in distal third; P4 exp-3 2.75 times as long as maximum width, with inner seta arising from middle third of inner margin............................................................................................ N. panamensis sp. nov.

16. P3 endopod 1-segmented, with one distal element..........................................................17. P3 endopod 2-segmented, enp-2 with two elements (outer one fused at base).....................................19.

17. P1 exp-3 with four elements; P2 endopod 2-segmented; P4 exp-3 without inner seta....................... N. australis . P1 exp-3 with three elements; P2 endopod 1-segmented; P4 exp-3 with inner seta.................................18.

18. Inner spinous process of P 5 ♀ distinctly curved outwardly (unknown in ♂); terminal process of caudal ramus dorsally recurved........................................................................ N. angolensis comb. nov. Inner spinous process of P 5 ♀ / ♂ and terminal process of caudal ramus straight........................... N. africanus .

19. P4 exp-3 without inner seta.................................................................... N. pacificus . P4 exp-3 with inner seta...............................................................................20.

21. Urosome (except anal somite) with distinct surface ornamentation consisting of elongate rectangular plates............................................................................. N. clasingi . Urosome without conspicuous surface ornamentation........................................................22.

22. Caudal ramus with dorsolateral spur near base of seta VII....................................................23. Caudal ramus without dorsolateral spur near base of seta VII..................................................24.

23. Lappets of abdominal hyaline frills semi-incised obtusidigitate; inner seta of P2–P3 enp-2 longer than endopod; P5 three times as long as wide, with naked spinous process........................................................ N. spicatus . Lappets of abdominal hyaline frills denticulate; inner seta of P2–P3 enp-2 shorter than endopod; P5 about 2.5 times as long as wide, with pinnate spinous process.......................................................... N. spinicaudatus .

24. P1 endopod distinctly longer than exopod...................................................... N. ishikarianus . P1 rami equally long............................................................ N. pseudishikarianus sp. nov.

Genus Phreatipontia gen. nov.

https://zoobank.org/ 90EF26CD-AACB-474A-BA67-1D246C2F5E75

Diagnosis. Arenopontiidae . Urosomites without conspicuous surface ornamentation. Anal somite without paired dorsolateral spinous processes. Anal operculum not modified. Hyaline frills of abdominal somites with rectangular digitate lappets. Caudal ramus without dorsolateral spur or raised spinular row near medial margin; seta IV with long setule on outer margin. P1 exopod three-segmented; exp-1 with outer spine; exp-3 with two spines and two geniculate setae. P1 endopod not prehensile, slightly longer than exopod; enp-2 with two geniculate setae. P2–P4 endopods two-segmented. P2–P3 endopods with one apical seta. P4 endopod with well developed outer distal element. Armature formula as follows:

P3 endopod ♂ not sexually dimorphic, two-segmented. P5 with outer basal seta and four discrete elements; innermost element fused to segment forming spinous process; length of process sometimes sexually dimorphic. P 6 ♂ with two setae/spines.

Type species. Arenopontia (Neoleptastacus) speluncae Cottarelli, Bruno & Venanzetti, 1994 [by original designation].

Other species. Arenopontia (Neoleptastacus) phreatica Cottarelli, Bruno & Venanzetti, 1994 = Phreatipontia phreatica ( Cottarelli, Bruno & Venanzetti, 1994) comb. nov.

Etymology. The genus name is derived from the Greek φρέαρ (phréar), meaning well, spring, and πόντος (pόntos), meaning sea, and refers to the low salinity habitat preference of its members. Gender: feminine.

Remarks. The two members included in this genus, P. phreatica comb. nov. and P. speluncae comb. nov., differ from all Neoleptastacus species in the presence of two geniculate setae on the distal segment of the P1 endopod (instead of an outer spine and an inner geniculate seta). This character state is shared with two other genera in the Arenopontiidae , Psammoleptastacus and Onychopontia , both of which display sexual dimorphism on the P3 endopod which is not expressed in Phreatipontia gen. nov. Psammoleptastacus additionally differs from the new genus in (1) the presence of a dorsolateral spur on the inner margin of the caudal ramus, (2) P1 endopod being distinctly shorter than the exopod, (3) the presence of two distal elements on P2–P3 enp-2, and (4) the absence of an inner spinous process on the P 5 in both sexes. Onychopontia can be differentiated from Phreatipontia gen. nov. by (1) the characteristic deeply incised hyaline frills on the abdominal somites, (2) the absence of the inner serrate seta on P2 enp-2, (3) the very short P2–P3 enp-2, and (4) the absence of an inner spinous process on the female P5. Both species of Phreatipontia gen. nov. display a reduced armature on P2–P3 endopods with only one apical seta on the distal segment. This condition is shared with members of the trisetosus -group of Neoleptastacus , however in this lineage the endopods are only 1-segmented ( Table 2). Finally, both P. phreatica comb. nov. and P. speluncae comb. nov. exhibit a characteristic caudal ramus seta IV which has a long setule on the outer margin (indicated by arrows in Figs 12C ; 16C ) which can be considered an autapomorphy for the genus. Current records suggest that, unlike most arenopontiids, both species favour low salinity environments, including phreatic and water table habitats ( Cottarelli et al. 1994, 1996; Bruno et al. 1998; this study).

Within the family, only Arenopontia cf. subterranea was previously recorded from habitats with a strong freshwater influence in various localities in Abruzzi, Lazio and Tuscany in Italy ( Cottarelli 1969; Cottarelli et al. 1994; Cottarelli & Venanzetti 1989), representing a second but independent incursion into low salinity environments.

Figures

FIGURE 12. Phreatipontia phreatica (Cottarelli, Bruno & Venanzetti, 1994) comb. nov. (♀): (A) habitus, lateral; (B) habitus, dorsal; (C) anal somite and caudal rami, dorsal [arrow indicating modified seta IV]: (D) anal somite and right caudal ramus, lateral.

FIGURE 16. Phreatipontia speluncae (Cottarelli, Bruno & Venanzetti, 1994) comb. nov. (♀): (A) habitus, dorsal; (B) habitus, lateral; (C) anal somite and caudal rami, dorsal [arrow indicating modified seta IV]; (D) anal somite and right caudal ramus, lateral.