Maliithipon, Cottarelli & Bruno, 2021

Cottarelli, Vezio & Bruno, Maria Cristina, 2021, Interesting interstitial Paramesochridae (Copepoda: Harpacticoida): Maliithipon wellsi gen. et sp. nov. from the Philippines, M. aberrans (Mielke, 1984) comb. nov from Panama, and M. cf. aberrans (Mielke, 1984) from the Azores, Zootaxa 5051 (1), pp. 68-93 : 70-73

Genus Maliithipon gen. nov.

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Diagnosis. Body elongate, cylindrical, slightly depressed dorsoventrally and slightly tapering distally. No distinct separation between prosome and urosome. Cephalotorax approximately quadrangular, larger than somites, longer than the following two somites, with sensilla and several dorsal pores. Dorsal pores present also on the remaining somites. Genital complex of female: genital field small, transversally oval, ventrally at half length of the double genital somite which lacks dorsal suture; P6 reduced to two small arched chitinous plates, apparently not fused; genital pore near the distal margin of the genital field, below the P6. Penultimate somite with well-developed quadrilobate pseudoperculum. Caudal rami approximately cylindrical, tapering distally in lateral view, apparently with six setae. Seta I missing (or not discernible), seta III and VI enlarged and pointed, unipinnate in the distal third on the outer (seta III) or inner (seta VI) margin. Distal setae (IV and V) variable in shape, peculiarly transformed in short fusiform structures in some specimens; seta VII dorsally inserted at 2/3 of the ramus length, biarticulate at base. Rostrum small, triangular, fused to cephalothorax, with two sensilla. Antennule 8-segmented in female, with large aesthetasc on segment four and small easthetasc on segment eight. Antennule 7-segmented in male, short, subchirocer, aesthetasc on segments V and VII. Antenna 4-segmented; coxa and basis without ornamentation, enp-1 with strong pinnate abexopodal seta, enp-2 armed with eight elements; exopod 1-segmented with two lateral setae and one or two distal ones, the longest one pinnate. Mandible with uniramous and 2-segmented palp comprising basis, 1-segmented endopod with four setae or three setae and one spine. Coxal gnathobase long, with dorsal seta; cutting edge with a row of thin and pointed teeth along the distal margin, with a stronger tooth at the inner and outer corner. Maxillula: praecoxal arthrite with strong lateral seta and 5-6 distal spiniform setae; coxa with one or two distal setae, basis with one or two to three apical setae; exopod and endopod missing. Maxilla large, with reduced chaetotaxy; syncoxa with three endites: proximal and median ones with one seta, distal one with two or three pinnate setae; basis produced into strong claw; endopod fused with basipodite, carrying four setae, one or two of which are claw-like. Maxilliped: subchelate, syncoxa bare; basis elongate and bare; endopod 1- or 2-segmented. P1: coxa large and bare, basis without outer basal seta, with strong inner seta; rami 2-segmented, endopod slightly longer than exopod, or of about equal length; exp-1 with long pinnate outer seta, exp-2 with four setae; enp-2 with two geniculate apical setae. P2-P4 laterally displaced but coxa and basis not fused and not markedly produced laterally; basis with outer seta. P2 basis with inner setular row in females; exopods 3-segmented, with two spiniform setae on distal segment; endopods absent. P5 very reduced in both sexes, partly incorporated in somite. In females, baseoendopod represented by a lobe with a lateral seta, exopod reduced to a minute subrectangular lobe with two bare and one pinnate setae or exopod reduced to a lobe with one seta. In males, baseoendopodal lobe with one seta, exopodal lobe with two setae or reduced to a small lobe without armature. P 6 in male reduced to two asymmetrical round plates with one or no seta.

Type species. Maliithipon wellsi gen. et sp. nov. (by original designation).

Other species. Apodopsyllus aberrans Mielke, 1984a = Maliithipon aberrans ( Mielke, 1984a) comb. nov.; Apodopsyllus aberrans Mielke, 1984a in Packmor et al. (2015) and Packmor & George (2016) = Malliithipon cf. aberrans ( Mielke, 1984a) .

Etymology. The generic name is derived from two Tagalog (the official language of the Philippines) words: “maliit” meaning small, and “hipon” meaning shrimp. Gender: masculine.

Relationships. Based on the existing classifications, M. wellsi sp. nov. could have been included in the genus Apodopsyllus , as we did when we first examined our specimens, and as it had occurred for the specimens collected from the locus typicus and the Azores ( Packmor et al. 2015; Packmor & George 2016). In fact, M. wellsi sp. nov. lacks the endopods of swimming legs P2–P4, as is typical of Apodopsyllus . However, members of the latter genus have the coxa and basis of P2–P4 fused forming a fusiform segment that is much longer than wide. To our knowledge no author noticed that in A. aberrans the coxa and basis are not fused, are wider than long, and that the exopods are inserted laterally. The same conditions occur in the new genus; the P2–P4 of this type are morphologically and functionally different from those of the remaining species of Apodopsyllus , possibly providing a very different locomotion modality. A similar type of swimming legs are present in Leptopsyllus Scott, 1894 and Wellsopsyllus Kunz, 1981 , which include species similar to the new ones established in this work (see below). The presence of this type of legs could therefore be a synapomorphic character, shared with other genera, however, Maliithipon gen. nov. has a set of exclusive (autapomorphic?) characters related to the mouthparts, P5 and caudal rami, the most significant being the following:

i) Mandibular palp: uniramous and 2-segmented, i.e. composed of basis and endopod, armed with few setae. Cutting edge of coxal gnathobase with long, thin and pointed teeth, the innermost and outermost ones being longer and stronger.

ii) Maxillule of simple structure, coxa and basis with few setae, exopod and endopod missing.

iii) Maxilla with three endites with reduced armature, the proximal endite can be reduced to a seta; the endopod is 1-segmented and fused to the basis, which is unarmed, at least one of the apical endopodal setae is claw-like. The maxillae are very large compared to the other mouthparts; in lateral view, they typically protrude from the cephalothorax.

iv) P5 very reduced in both sexes: in M. aberrans comb. nov. (only females are known for this species) each P5 is represented by two small adjacent plates (one is the baseoendopod, the other the exopod), inserted laterally and carrying respectively one and three setae; in M. cf. aberrans the exopod has three setae in the female but only two in the male, and the endopodal lobe in the male is partly fused with the exopodal lobe to form a structure similar to the one of Wellsopsyllus (Intermediopsyllus) smirnovi . The P5 can be further reduced: in M. wellsi sp. nov. the female has only one lobe (with a small incision, probably representing the boundary between baseoendopod and exopod) with one seta (missing in some specimens), and the male has a single, smaller lobe without setae.

v) Pseudoperculum with multilobate posterior margin.

vi) Caudal rami always with six setae, with peculiar transformation of setae III and VI.

All the above-mentioned characters are absent in the remaining species of Apodopsyllus ( A. aberrans excluded), which share with Maliithipon gen. nov. only the morphology of P1, the absence of P2–P4 endopods, and the morphology of P2–P2 exopods. These similarities could be due to parallel evolution; however, the morphology of P2–P4 coxa and basis differs between the two genera. Both genera also share the number of setae on the caudal rami, but setae III and VI are not transformed in Apodopsyllus , which also has a different shape and ornamentation of the body, caudal rami, and genital field.

In summary, the unique combination of the above-listed characters does, in our opinion, characterize the new genus and separates it from Apodopsyllus ; as a consequence the two species of Apodopsyllus which indeed share such set of characters, i.e. A. aberrans and A. cf. aberrans , are transferred to the new genus Maliithipon as M. aberrans ( Mielke, 1984a) comb. nov. and M. cf. aberrans ( Mielke, 1984a) , respectively.

As regards the genera Wellsopsyllus and Leptopsyllu s, the affinities with Maliithipon gen. nov. can be found with some species whose generic attribution is uncertain, according to their authors. As we discussed for Apodopsyllus , there are two unusual species among these genera, i.e. Leptopsyllus (Leptopsyllus) platyspinosus and Wellsopsyllus (Intermediopsullus) smirnovi , which, according to us, have more morphological characters in common with Maliithipon gen. nov. than with their respective congeners. The same applies, but to a lesser degree, to Leptopsyllus (Leptopsyllus) typicus , Wellsopsyllus (Wellsopsyllus) antarcticus Kottmann & Veit-Köhler, 2013 , and Biuncus ingens . In Leptopsyllus (L.) platyspinosus the mandible has a uniramous, 2-segmented palp, including an unarmed basis and a 1-segmented endopod carrying four setae; the morphology of the coxal gnathobase and cutting edge are also similar to those of the new genus. The maxillule and maxilla are very similar to those of the new genus (see Mielke 1984b: Fig. 23). The maxillipedal endopod is 2-segmented, there is a pseudoperculum with a bilobate rather than a quadrilobate posterior margin, the P4 has a 1-segmented endopod, the P5 has a reduced baseoendopod and small exopod, carrying three setae, and the genital field and shape and armature of the caudal rami are very similar to those of Maliithipon gen. nov. Leptopsyllus (L.) typicus has not been redescribed according to modern standards, and only the females are known. Nonetheless, the mandibular palp is uniramous, being represented by the basis and the endopod, and the structure of the maxilla (endites with reduced armature, basis prolonged into a claw, a second claw originates from the partially fused endopod) and maxilliped (with 1-segmented endopod) are characters which indicate a degree of affinity with the new genus. The morphology of the maxilla, pseudoperculum and P5 are unknown for this species, the caudal rami differ in shape and ornamentation and the body size of this species is very large (700 μm). Wellsopsyllus (I.) smirnovi has the same transformation/reduction of the mouthparts as in Leptopsyllus (L.) platyspinosus , but the three maxillary endites have more setae than the taxa discussed so far, and the P5, “a small plate with three setae, middle lobe not noticeable” ( Kunz 1992), is morphologically similar to the one of the new genus but even simpler: it is reduced to a very small exopod, completely fused to the somite, carrying only two setae. Setae III and IV of the caudal rami are not characteristically transformed, the pseudoperculum seems to be missing but it could have been overlooked by Kunz (1992) who, for the male, described only the subchirocer antennule and did not provide any information on the P5 and P6. This species is remarkably long (510 μm). Wellsopsyllus (W.) antarcticus (1) has a 1-segmented antennary exopod, a uniramous (but 3-segmented) mandibular palp, a maxillule without endopod or exopod and the size and armature of the maxilla are similar to those of Maliithipon gen. nov. Differences with the new genus are expressed in the P2–P4 which have retained the endopod, the genital field with a different structure carrying two setae, the P5 which is not reduced in either sex, the absence of a lobate pseudoperculum and the morphology of the caudal rami. Biuncus ingens is a large species, known only from males, and was included by Huys (1995) in the group of species related to Leptopsyllus based on the mandibular palp (which in Biuncus is simple but retains the exopod, represented by a seta, and the endopod is 2-segmented), the structure and size of the maxilla, and the presence of a lobate pseudoperculum. The analysis of all the above-listed taxa, which are morphologically close to Maliithipon gen. nov., underlines that only the species of this genus share all the above-mentioned characters. These same characters can be present in other Paramesochridae , but never all of them combined in one species.

1 We underline that Kottmann et al. (2013) provided some interesting remarks on the “uncertainties” regarding the systematic and morphology of this genus and subgenera with which we fully agree.

Particularly relevant to solving some systematic issues in the Paramesochridae is the analysis of the morphology of mouthparts, P5 and caudal rami. This view is supported by Kottman et al. (2013) who, while discussing affinities among species of Wellsopsyllus , remarked how “at present, the classification is mainly based on the segmentation of the swimming legs, but there are other important characteristics to be considered (e.g., mouthparts, furcal rami)”. More recently, Back & Lee (2017a) discussed the genus Leptopsyllus , underlining how most of the recent classifications for this genus are based on leg characteristics (reduced or absent), and the complete descriptions of mouthparts are lacking for many species. Veit-Köhler (2004, 2005) gave similar remarks for the genus Kliopsyllus (now Emertonia ). Already Kunz (1981) stated that in very specialized Paramesochridae with reduced pereiopods, such as Apodopsyllus and Leptopsyllus , the original components of the mouthparts are still recognizable. Later on, Huys (1987) remarked that Wellsopsyllus , Leptopsyllus and Apodopsyllus have undergone paedomorphic evolution and “can easily be included in a morphological series with increasingly pronounced larval characters”. The new genus Maliithipon gen. nov. can be added to this “morphological series” but the species included in the latter and the above-discussed species to which it is most closely related (mainly Leptopsyllus (L.) platyspinosus ) can represent a further step which characterizes and separates them from the other components of the evolutionary lineages hypothesized by Huys (1987). Possibly, further studies based also on a molecular approach will allow including them in a phylogenetically-valid group.

The discussion of the taxonomic position of Leptopsyllus (L.) platyspinosus and W. (I.) smirnovi is outside the scope of this paper. We, however, suggest that these two species should be considered incertae sedis, as already proposed by Wells (2007) for W. (I.) smirnovi . When and if Wellsopsyllus (I.) smirnovi will be redescribed with more details (the original drawings of Leptopsyllus (L.) platyspinosus are still adequate), it will be possible to include these two species in a taxonomic unit close to Maliithipon gen. nov., or possibly in the same genus appropriately reviewed and divided into subgenera. In fact, it is difficult to suppose that complicated structures as those described above could have evolved independently but in a similar way in different genera. It is worthy to remember that Kunz (1992: 90), while discussing the affinities of W. (I.) smirnovi , stated that “these observations suggest to separate L. platyspinosus , S. smirnovi and S. minutus as a phylogenetically-related group”.