Maliithipon aberrans Cottarelli & Bruno 2021

Maliithipon cf. aberrans (Mielke 1984a) = Apodopsyllus aberrans Mielke 1984a in Packmor et al. (2015) and Packmor & George (2016)

(Figs. 6–11)

Material examined. Two male and four female specimens, each one undissected and mounted on one slide labelled respectively: Apodopsyllus aberrans, adult female. Prainha, ST 1/3 (Madeira Island); Apodopsyllus aberrans, adult female, Prainha, ST 1/4 (Madeira Island); Apodopsyllus aberrans, adult male. Prainha, ST 1/6 (Madeira Island); Apodopsyllus aberrans, adult male. Calheta, ST1/1 (Madeira Island); Apodopsyllus aberrans, adult female, Puerto Santo Island East, ST 3/6; Apodopsyllus aberrans, adult female, Puerto Santo Island East, ST 3/6. Details on the collecting methods, collecting sites and dates in Packmor et al. (2015).

Description of female. Habitus (Fig. 6A). Body unpigmented, elongate, cylindrical and slightly depressed dorsoventrally, naupliar eye absent. Length 228 μm, n = 4, mean = 229 μm. Last urosomites tapering posteriorly. Free pedigerous somites without any lateral or dorsal expansions, all connected by well-developed arthrodial membranes. Ventrolateral margin of first, second, and fourth free thoracic somites with row of denticles (Fig. 6B). Integument weakly sclerotized, without cuticular pits. First pedigerous somite and dorsal cephalic shield fused forming cephalothorax representing about 26% of total body length. Cephalothorax and somites with few sensilla and several pores on dorsal, lateral and ventral surfaces (pores were difficult to observe, their pattern could not be precisely determined). Second and third urosomite completely fused to form genital double-somite. Penultimate body somite without sensilla, carrying a fine, well developed lobate pseudoperculum (Figs. 6A, 7C). Anal somite small, with pair of dorsal sensilla (Fig. 7B); anal operculum not visible. Genital field (Fig. 6C–D) small, ellipsoidal with two small lateral lobes, located mid-ventrally and at mid-length of the genital double-somite, surrounded by eight pores of different size; P6 not discernible (morphology probably as in M. wellsi).

Caudal rami (Fig. 7 °–B). Parallel, slightly longer than the last urosomite; length: 14 μm; length/width: 1.8. Approximately rectangular in shape, slightly tapering posteriorly, the distal outer corner is prolonged in a ventral cuticular outgrowth; armature represented by six setae (seta I apparently missing): seta II short and very thin, seta III stout and pointed, unipinnate along approximately one/fifth of the distal outer margin; seta IV unipinnate, seta V bare and very reduced; seta VI as seta III but shorter; seta VII bare, bi-articulate at base and arising distally at two-thirds of the ramus length.

Rostrum (Fig. 6A, 8A). Small, approximately triangular, with round tip, fused to cephalic shield, armed with two sensilla.

Antennule (Fig. 8A–B). Short, robust, 8-segmented. First segment longest, without armature; second segment with ten setae, four of which pinnate, the one on the outer distal corner longest. Third segment provided with two pinnate and five bare setae. Inner distal corner of fourth segment forming a sub-cylindrical process carrying a long and thick aesthetasc fused at base with one long bare seta, and two short, basally-enlarged setae, the outer one pinnate. Fifth segment smallest, with one bare seta. Sixth segment with two bare setae; seventh segment with one bare and one pinnate seta. Eighth segment with seven bare setae and one pinnate and acrothek formed by a slender aesthetasc and a seta fused at base. Armature formula: 1–[0], 2–[6 bare + 4 pinnate], 3–[5 bare + 2 pinnate], 4–[1 bare + 1 pinnate + (1 + ae)], 5–[1 bare], 6–[2 bare], 7–[1 bare + 1 pinnate], 8–[8 bare + 1 pinnate + acrothek].

Antenna (Fig. 8C–D). 4-segmented; coxa small and bare; basis approximately twice longer than wide, without any surface ornamentation; exopod (Fig. 8D) 1-segmented, with two bare setae laterally, one pinnate seta and one short spine apically. Endopod 2-segmented, proximal segment with one pinnate abexopodal seta; distal endopodal segment armed with two subdistal bare spiniform setae, distal margin carrying three bare geniculate setae and three bare normal setae, one of which very short.

Mandible (Fig. 8E). Coxa long and narrow, with well-developed gnathobase bearing one bare seta at dorsal corner; cutting edge provided with three apical needle-like teeth, one long pointed tooth on inner and on outer corner, one tooth has double the size of the other. Palp reduced, 2-segmented, comprising roundish basis, 1-segmented endopod with one seta, one pinnate spiniform seta, and one spinule distally, and one lateral unipinnate spiniform seta.

Maxillule (Fig. 8F). Difficult to discern. All setae bare. Praecoxal arthrite roundish, with one seta at half of the lateral margin (marked with asterisk in Fig. 8F), two thin and two stronger setae at distal margin. Coxa prolonged in a cylindrical endite with two apical setae; basis with one apical seta. Exopod and endopod missing.

Maxilla (Fig. 9A). Overall very large compared to the other mouthparts (in lateral view, it projects from the cephalothoracic margin); syncoxa with three endites, proximal one small with one apical pinnate seta, median endite small with one apical pinnate seta, distal coxal endite with two apical bare setae. Allobasis prolonged into strong denticled claw; endopod fused to basis and armed with four strong setae distally.

Maxilliped (Fig. 9C). 3-segmented, comprising syncoxa, basis, and 1-segmented endopod; syncoxa bare, basis slightly longer than coxa and bare; endopod short, distally one normal and two slightly geniculated bare setae.

P1 (Fig. 9D). Intercoxal sclerite long and thin, slightly concave; coxa bare, well-developed; basis without outer seta, with bare inner seta at half length of the inner margin; exopod and endopod 2-segmented, exopod slightly shorter than endopod. First exopodal segment longer than the second one, with spinular row along outer margin, strong pinnate seta on distal outer corner; second segment with spinular row along outer margin, four distal pinnate setae, the innermost is the longest. First endopodal segment bare, 2.5 times longer than the second one, with spinules along outer margin; second segment with spinules along the distal part of the outer margin, two long geniculate apical setae.

P2–P4. (Figs. 9E–F, 10A). Intercoxal sclerites long and thin. P2 basis with small bare outer seta and with a row of spinules on inner margin; P3 and P4 basis with outer pinnate seta. Exopods 3-segmented, first, second and third segments with spinular row along outer margin and outer unipinnate, curved spine at distal corner, longest in P4 exp-2; third segment also with apical long geniculate pinnate seta. Endopods missing. Setal formula provided in Table 1.

P5 (Fig. 6C). Reduced to two adjacent lobed plates laterally inserted; the outermost plate with a pinnate seta (ancestral basal seta), the innermost plate, roundish, with three setae, the longest middle one unipinnate.

Description of male. General body shape and ornamentation as in female, but slightly smaller and more slender, body length 220 μm; prosome to urosome ratio: 0.85. Last urosomites tapering posteriorly. Cephalothorax representing about 25% of total body length. Antenna, Mdb, Mxl, Mx, Mxp, P1–P4 (Figs. 10D, 11–C) as in female. Sexual dimorphism in separation of genital somite, A1, P5, and P6, ventrolateral margin of free thoracic somites without row of denticles; P2 basis ornamentation, P3–P4 basis armature; P4 exp-2 armature.Anal somite small, anal operculum and dorsal sensilla of the anal somite not visible.

Caudal rami (Fig. 11E). Shape and armature as in female but proportionally longer and thinner (length/width = 2).

Antennule (Fig. 10C). Strong, short, subchirocer, 7-segmented. First segment without distal seta, as in female; second segment with long lateral seta, as in female, two short pinnate lateral setae, five bare setae. Third segment with four bare setae of different lengths, one short pinnate seta. Fourth segment small, with two short setae; fifth segment enlarged, square, with long aesthetasc fused at base with long seta. Fifth segment with lateral tubercle carrying one thick esthetasc and two setae of same length; inner distal corner produced in a tubercle carrying one bare seta and three shorter ones, the middle one is enlarged in the first half and pinnate. Sixth segment with one long and three short setae, one of which pinnate; seventh segment with eight bare setae and one small seta fused to aesthetasc to form apical acrothek. Armature formula: 1–[0], 2–[6 bare + 2 pinnate], 3–[4 bare + 1 pinnate], 4–[2 bare], 5–[4 bare + 1 pinnate + (1 + ae)], 6–[3 bare +1 pinnate], 7–[8 bare + acrothek].

P2 (Fig. 11A). Basis without inner row of spinules.

P3 (Fig. 11B). Basis with outer bare seta.

P4 (Fig. 11C). Basis with outer bare seta; exp-2 with distal spine of normal size.

P5 (Fig. 10E). Similar to female but the two lobes are merged, smaller, and with bare setae; the exopod is merged with the somite and carries two setae (one less seta than in the female).

P6 (Fig. 11D) represented by two asymmetrical (the right one is larger) roundish plates with one seta, the one on the right is longer.

Spermatophore (Fig. 11D). Ellipsoidal, approximately as long as the somite carrying the P5.

Variability. In one female paratype the maxillary distal endite is longer and carries three setae instead of two (Fig. 9B). In a second female paratype the P5 exopod carries one seta less (Fig. 10B) and setae IV and V on the caudal rami are normally developed (Fig. 7C). In one male seta V is normal on the right caudal ramus, and extremely reduced on the left caudal ramus (Fig. 11F).

Remarks. Since the description of the new genus Maliithipon is based exclusively on morphological characters, M. cf. aberrans can be included in the new genus (Table 1). Females and males of M. cf. aberrans can be distinguished from those of M. wellsi sp. nov. by easily-observable characters, e.g. the caudal rami, P5 and P6. The distinction of M. cf. aberrans from M. aberrans comb. nov. is more difficult, mainly due to the lack of males in the type series of M. aberrans. As a consequence, the males collected in the Azores, initially listed as A. aberrans in Packmor et al. (2015) and Packmor & George (2016), cannot be compared with males of the type series, and could potentially belong to a different taxon. However, because the females collected in the Azores share the main discriminant features with M. aberrans, but differ in some minor features which are more difficult to observe (Table 1), we have chosen to be more conservative in the attribution of the taxonomic status to this population of Maliithipon, and not to follow Packmor et al. (2015) and Packmor & George (2016). The discovery of new populations, or the availability of new knowledge on the taxonomy of the existing populations (based for instance on scanning and/or confocal laser scanning microscopy; developmental study, molecular analysis) will hopefully allow to attribute a final taxonomic status to this taxon.