Pilarella compacta, Komeda & Adachi & Ohtsuka, 2021

Komeda, Sota, Adachi, Kenta & Ohtsuka, Susumu, 2021, A new species of Pilarella (Copepoda, Calanoida, Arietellidae) from the hyperbenthic layer of Japan, with a molecular phylogenetic analysis of some representative genera of the Arietellidae, ZooKeys 1038, pp. 179-194 : 179

Pilarella compacta sp. nov.

Types.

Holotype: ♀ 1.97 mm preserved in vial (NMST-Cr29010). Allotype: ♂ 1.69 mm, appendages mounted on glass slide, body in vial (NMST-Cr29011). Paratype: ♀ 2.01 mm, appendages mounted on glass slide, body in vial (NMST-Cr29012).

Description.

Adult female. Body (Fig. 1A, B ) compact; cephalosome separated from first pediger; fourth and fifth pedigers completely fused; posterolateral corners of prosome with small triangular process extending posteriorly. Rostrum (Fig. 1C, D ) produced ventrally, with pair of frontal filaments distally. Urosome (Fig. 1E-G ) 4-segmented; genital double-somite symmetrical with pair of seminal receptacles; pair of gonopores located ventrolaterally. Caudal rami (Fig. 1F, G ) symmetrical, approximately 1.5 times longer than wide; minute seta I placed ventrally, seta II reduced, setae III-VI long, short seta VII dorsally; inner margin of rami furnished with row of fine setules.

Antennules (Fig. 1H, I ) asymmetrical, left antennule approximately 1.5 times longer than right antennule. Right antennule (Fig. 1H ) 21-segmented, reaching middle of second pediger, fusion pattern and armature of segments as follows: I-IV-9+2ae, V-1+1ae, VI-2, VII-2+1ae, VIII-2+1ae, IX-1+1ae, X-1+1ae, XI-2+1ae, XII-2+1ae, XIII-2+1ae, XIV-2+1ae, XV-2+1ae, XVI-2+1ae, XVII-2+1ae, XVIII-2+1ae, XIX-1+1ae, XX-2+1ae, XXI-1+1ae, XXII-1, XXIII-1, XXIV-XXVIII-10+1ae. Left antennule (Fig. 1I ) 21-segmented, reaching posterior border of genital double-somite, fusion pattern and armature of segments as follows: I-IV-8, V-1, VI-2, VII-1+1ae, VIII-2+1ae, IX-2, X-2, XI-2+1ae, XII-2+1ae, XIII-1+1ae, XIV-2+1ae, XV-2+1ae, XVI-2, XVII-2+1ae, XVIII-2+1ae, XIX-1+1ae, XX-2, XXI-2+1ae, XXII-1, XXIII-1, XXIV-XXVIII-9.

Antenna (Fig. 2A ) with unarmed coxa; basis with inner distal seta; exopod 5-segmented; ancestral segments II-IV, V-VII and IX-X fused, setal formula of 0, 0-0-1, 1-1-1, 1, 0-2; endopod indistinctly 3-segmented and distal 2 segments partly fused, proximal segment with seta on middle of inner margin, middle segment with 3 inner setae, distal segment with 4 distal setae.

Mandible (Fig. 2B, C ) gnathobase with 4 well-chitinized teeth, dorsalmost bifurcate; row of setules on dorsal margin of gnathobase; basis unarmed; endopod rudimentary, unsegmented, with 2 setae; exopod 5-segmented, setal formula 1, 1, 1, 1, 1.

Maxillule (Fig. 2D ) with 5 long spines, 1 short spine and row of long setules on praecoxal arthrite; coxal endite with 1 seta; coxal epipodite with 6 setae; proximal and distal basal endites without setae; endopod unsegmented with 2 setae; exopod with 3 setae.

Maxilla (Fig. 2E ) with praecoxal and coxal endites having 2, 2, 2 and 2 setae, respectively; basis having 1 heavily-chitinized spine with row of long spinules midway; endopod 4-segmented, setal formula 1, 3, 2, 2 [Maxilla with praecoxal and coxal endites having 2, 2 setae, respectively; basal endites having 2, 2 setae, respectively; endopod 5-segmented, setal formula 1, 1, 3, 2, 2; first endopodal segment having 1 heavily-chitinized spine with row of long spinules midway (the homology by Ferrari and Ivanenko 2001, 2008)].

Maxilliped (Fig. 2F ) syncoxal endites with 0, 1, 0, and 2 setae, respectively; rows of long setules on inner margin of syncoxa and basis; basis with 2 setae midway; first endopodal segment partly fused to basis; first to sixth endopodal segments with 1, 4, 3, 2, 2, and 4 setae, respectively [Maxilliped praecoxal endites with 0, 1, and 0 setae, respectively; coxal endite with 1 seta; rows of long setules on inner margin of coxa and basis; basis with 2 setae midway, and 1 seta distally; first to fifth endopodal segments with 4, 3, 2, 2, and 4 setae, respectively (the homology by Ferrari and Ivanenko 2001, 2008)].

Seta and spine formula of legs 1-4 as shown in Table 2 . Leg 1 (Fig. 3A ) with medial setules on proximal 2 segments of exopod; proximal 2 segments of endopod with lateral setules. Legs 2 and 3 (Fig. 3B, C ) with medial setules on proximal 2 segments of exopod; distal 2 segments of exopod and all segments of endopod with lateral setules. Leg 4 (Fig. 3D ) with medial setules on proximal 2 segments of exopod; distal 2 segments of exopod and proximal 2 segments of endopod with lateral setules.

Leg 5 (Fig. 3E ) uniramous, 3-segmented; basis broad, approximately 0.6 times as long as wide, with long plumose seta medially and short seta laterally; exopod unsegmented, approximately 3 times longer than wide, with 3 medial setules, 1 lateral spine and 2 terminal spines.

Adult male. Body (Fig. 4A, B ) compact; cephalosome separated from first pediger; fourth and fifth pedigers completely fused; posterolateral corners of prosome with small triangular process extending posteriorly. Rostrum similar to that of female. Urosome (Fig. 4C ) 5-segmented; gonopore located on right side of genital somite; caudal rami similar to female, except for lacking inner setules.

Antennules (Fig. 4D, E ) asymmetrical. Right antennule (Fig. 4D ) 21-segmented, ancestral segments XXIV and XXV partly fused, exceeding middle of first pediger, fusion pattern and armature of segments as follows: I-IV-6+4ae, V-1+1ae, VI-2+1ae, VII-1+1ae, VIII-1+1ae, IX-1+1ae, X-1ae, XI-1+1ae, XII-1+1ae, XIII-1+1ae, XIV-1+1ae, XV-1+1ae, XVI-2+1ae, XVII-1+1ae, XVIII-1+1ae, XIX-1+1ae, XX-2+1ae, XXI-1+1ae, XXII-1, XXIII-1, XXIV-XXVIII-7+2ae. Left antennule (Fig. 4E ) damaged; ancestral segments XV-XXVIII not observed, fusion pattern and armature of I-XV as follows: I-IV-6+4ae, V-2+1ae, VI-1+1ae, VII-1+1ae, VIII-1+1ae, IX-1+1ae, X-2+1ae, XI-2+1ae, XII-1+1ae, XIII-1+1ae, XIV-2.

Antenna, mandible, maxillule, maxilla, maxilliped and legs 1-4 similar to those of female.

Leg 5 (Fig. 4F ) uniramous, 5-segmented; basis with plumose seta laterally; exopod 3-segmented, proximal 2 segments with lateral spine, distal segment with 2 terminal setae.

Etymology.

The specific name of the new species is derived from the Latin adjective Paraheterorhabdus compactus meaning “stocky” to denote the habitus of the present new species.

Remarks.

The present new species falls within the diagnosis of the monotypic Pilarella ( Alvarez 1985) except for the following features: (1) left antennule not reaching the caudal rami; (2) short caudal rami; (3) 6 spines on the maxillular praecoxal arthrite (5 in the previous diagnosis); and (4) 6 setae on the maxillular coxal epipodite (5 in the previous diagnosis). For the differences from other genera, see generic remarks in the present study.

Pilarella compacta sp. nov. differs from P. longicornis Alvarez, 1985 as follows: (1) the body is more compact (vs. more slender in P. longicornis ); (2) the left antennule of the female is short, and reaches the posterior border of genital double-somite (vs. reaching the posterior margin of caudal rami in P. longicornis ); (3) the caudal rami are short, approximately 1.5 times longer than wide (vs. long, 4.3 longer than wide in P. longicornis ), (4) the mandibular endopod has 2 setae (vs. 1 in P. longicornis ), (5) the coxal epipodite of maxillule has 6 setae (vs. 5 in P. longicornis ), (6) there is a basal lateral seta on leg 4 (vs. seta absent in P. longicornis ).

Phylogenetic analysis.

The Maximum Likelihood tree based on 16 species of the superfamily Arietelloidea is shown in Figure 5 . Both ML and BI trees showed two clades in Arietellidae . In clade I, Pilarella compacta sp. nov. and Metacalanus sp. were placed in the same clade (BP = 69%; PP = 0.948). Hyperbionyx athesphatos (HM997029) (belonging to the Hyperbionychidae ) and clade I formed a cluster with a low bootstrap value (BP = 54%; PP = 0.868). In clade II, four arietellid genera ( Paraugaptilus , Paramisophria , Sarsarietellus , and Arietellus ) were grouped in the same cluster with high bootstrap value (ML = 81%; PP = 0.991), and Arietellus was placed in the same clade as Sarsarietellus with a high bootstrap value (BP = 87%; PP = 0.997). The sequence of Paraugaptilus buchani (LC510293) in this study showed a lower identity of 96% to P. buchani (HM997028) in the DDBJ/EMBL/GenBank databases, which differed by five gaps and 19 single nucleotide polymorphisms (SNP).

Figures

Figure 1. Pilarella compacta sp. nov., adult female A lateral habitus (holotype) B dorsal habitus (holotype) C rostrum, ventral view (paratype) D rostrum, labrum and left paragnath, lateral view (paratype) E genital double-somite, ventral view (paratype) F urosome, ventral view (paratype) G urosome, lateral view (paratype) H right antennule (paratype) I left antennule (paratype). Scale bars: 0.1 mm.

Figure 2. Pilarella compacta sp. nov., adult female, paratype A right antenna B coxa of left mandible C palp of left mandible D left maxillule E right maxilla F right maxilliped. Scale bars: 0.1 mm.

Figure 3. Pilarella compacta sp. nov., adult female, paratype A leg 1, posterior side B leg 2, posterior side C leg 3, posterior side D leg 4, posterior side E fifth legs, posterior side. Scale bar: 0.1 mm.

Figure 4. Pilarella compacta sp. nov., adult male, allotype A lateral habitus B dorsal habitus C urosome, ventral view D right antennule E left antennule F Fifth legs, anterior view. Scale bars: 0.1 mm.

Figure 5. Maximum Likelihood tree based on partial 28 S nuclear ribosomal DNA sequences of the superfamily Arietelloidea. Left and right Arabic numerals on nodes indicate bootstrap values and posterior probabilities, respectively. Scale bar shows nucleotide changes per site. Asterisks indicate arietellid species.