Bathycalanus Sars, 1905

Bradford-Grieve, Janet M., Blanco-Bercial, Leocadio & Boxshall, Geoffrey A., 2017, Revision of Family Megacalanidae (Copepoda: Calanoida), Zootaxa 4229 (1), pp. 1-183 : 74-76

Genus Bathycalanus Sars, 1905

Differential diagnosis. As for Megacalanidae plus following character states: Rostral filaments usually parallelsided, sausage-shaped (tapering to point in Ba. unicornis ). Anterior head with small rounded protrusion on which sits pair of spine-like processes or single anteriorly-directed spine-like process. Genital double-somite with variable length: width characteristics, widest width, in dorsal view at variable positions anterior to posterior. Pedigerous somite 5 posterolateral corners of variable shape. Female antennule ancestral segment XXIII with aesthetasc, spines may be present along anterior and posterior borders of some segments in some species; female and male ancestral segments I, II, III, and V with dorsal surface hair sensillum, usually with adjacent macula cribrosa. Right male antennule segments XIV–XV and XXI–XXIII fused, segments XIX, XX, XXI and sometimes XVIII with fused gripping elements. Antennal exopod ancestral segments I–III with vestigial seta, seta on segment IV extends short of distal border of exopod. Mandibular gnathobase with ventral tooth set at right angles to main plane of gnathobase therefore appearing tapering, similar to other teeth; endopod segment 1 with 4 setae (distal seta very small), endopod segment 2 with 9 long setae and 2 vestigial setae. Maxillulary setation species specific. Setae of maxilla endopod curled completely in on themselves and with row of fine, long dense setules along distal half of concave surface; long setae extending anteriorly beyond rostrum, inner proximal seta of endopod segment 2 vestigial. Maxilliped with some endopod setae of reduced size. Leg 1 basis without anterodistal hook-like process, exopod segments 1 and 2 without outer border spines, exopod segment 3 with 1 distal outer border spine.

Description. Female. Anterior margin of head in dorsal view usually with pair of small spine-like processes ( Ba. unicornis with 1 spine-like process). Rostrum extending into two long, ventrally-directed, parallel-sided, sausage-shaped filaments. Urosome of four free somites. Genital double-somite in dorsal view usually as long as wide (three species longer than wide), widest width in dorsal view variable in location anterior to posterior, and paired seminal receptacles present ( Barthélémy 1999). Posterolateral borders of pedigerous somite 5 usually rounded but may be irregularly shaped ( Ba. bucklinae n. sp., Ba. adornatus n. sp., and Ba. pustulosus n. sp.). Caudal rami with seta I absent, setae II and III lateral, setae IV–VI terminal (seta V longest), seta VII inserted at inner distal corner on small projection.

Antennule with ancestral segments II–IV, X–XI fused, XXVII and XXVIII separate. One macula cribrosa at base of each aesthetasc: most setae of modified type (ms) or aesthetascs (a), longest modified setae on at least segments I, V, IX, XXV, and XXVI. Setation of segments as follows: I—1 ms, 2ss (vestigial), 1a; II to XXI—2ms, 1a; XXII to XXIII—1ms, 1a; XXIV to XXV—1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII—3ms, 1a, 1ss. Oblique row of small setules on posterodistal border of segments V–IX; segments I, II, III, and V with dorsal surface hair sensillum and usually adjacent macula cribrosa; ventral surfaces of ancestral segments XIV to XVII without teeth, and anterior and posterior borders of segments smooth or with teeth.

Antenna with separate coxa and basis; coxa with 1 short inner plumose seta and inner tuft of setules, basis with 2 inner setae one bordered by 2 rows of short setules. Exopod about same length as endopod; endopod 2-segmented although line of fusion between segments 2 and 3 visible on posterior surface; segment 1 with 2 inner setae (one naked and one plumose) and short longitudinal row of outer distal setules; terminal segment with 9+7 setae and lined with long outer setules; exopod with arthrodial membrane present between ancestral segments I and II, arthrodial membrane between ancestral segments II–III, III–IV and IV–V not developed although signs of demarcation between ancestral segments III and IV and IV and V may be visible; ancestral segments VI, VII and VIII expressed and segments IX and X fused. Terminal segment IX–X with 1 + 3 terminal setae, segments V–VIII each with long plumose seta, ancestral segments I–III each with 1 vestigial seta and segment IV with short seta bearing short setules extending, at most, just beyond exopod segment VIII.

Mandible coxal gnathobase with five complex teeth with opaline tips, largest tooth set at right angles to main plane of gnathobase, ventrally situated, tapering when viewed at right angles to broad plane of gnathobase, separated from adjacent tooth by wide gap; 4 following teeth progressively decreasing in size, but 2 teeth immediately adjacent to large ventral tooth not in same plane, one being more anterior, other posterior appearing to lie nearly on top of one another when mounted; 3 simpler teeth without opaline tips follow, dorsal-most tooth longest; and finally, 1 lash-like element situated dorsally, bordered by wide setules. Basis with 4 inner setae; endopod 2-segmented, segment 1 with large inner lobe and 4 (including 1 sometimes vestigial) distal inner setae, segment 2 with 9 + 2 vestigial terminal setae; exopod 5-segmented with 1, 1, 1, 1, 2 setae, respectively.

Maxillule praecoxal arthrite with 13–15 setae, 2–4 posterior surface setae present and 2 distoanterior surface seta; coxal endite poorly developed, usually without seta (1 seta in both Ba. tumidus n. sp. and Ba. pustulosus n. sp.), coxal epipodite with 7 long and 2 short setae; basal endites 1 and 2 with 2 and 2–4 setae respectively, exite with 1 seta; basis and endopod segments 1 and 2 fused, segment 3 expressed; endopod with 2–3, 2, 5 + 1 small posterior surface seta; exopod with 11 setae.

Maxilla praecoxal endites 1 and 2 with 6+1 small and 3 setae, respectively; coxal endites 1 and 2 with 3 setae each, coxal epipodite with 1 small seta; basal endite longest with 4 setae; endopod segment 1 with short lobe bearing 3 vestigial + 1 large seta, endopod segments 2–4 apparently with 3 (proximal seta vestigial), 2, 2 setae, respectively. Inner surfaces of praecoxal endite 2, coxal endites 1 and 2 and basal endite each with 1 shorter seta lined with 2 rows of spinules along distal half whereas other setae ornamented with one row of spinules; coxal endite 2 usually with short setae, only 1 of them curled distally; basal endite and endopod segments 1–4 with very long setae, distally very thin, curling completely on themselves distally, lined along more than distal half with very fine spinules. None of setae on maxilla with auxiliary spinules although inner distal seta of endopod segment 2 has row of spinules along convex, distal-facing surface. Longest setae extend beyond rostrum when maxilla directed ventroanteriorly, and appear to function as mechanical tangling devices.

Maxilliped directed ventrally so setae on syncoxa and basis directed into animal’s midline; syncoxal endites with 1, 2, 4, 4 setae; basis with 3 setae and elongate patch of setules on anteroproximal surface. Endopod segment 1 mostly separate from basis and bearing 2 setae, 1 longer seta with long spinules along one edge, short seta with smooth, wide basal part then rapidly tapering and lined distally by 2 rows of long spinules; endopod segment 2 with 4 setae, 3 setae bordered on one side by long spinules, 1 seta with much shorter spinules; endopod segments 3–6 with 1 large + 3 small, 1 large + 2 small, 1 large + 2 small + 1 small outer seta, 2 large + 1 small + 1 small outer seta, respectively.

Legs 1–5 biramous, each ramus usually 3-segmented (apart from leg 1 exopod in Ba. richardi ) with following setal formula (Roman numerals indicate spines, Arabic numerals setae, outer border setation listed to left in each group separated by ‘;’):

Leg 1 (Coxa 0-1. Basis 1-1. Exopod 0-1; 0-1; I,1,4. Endopod 0-1; 0-2; 1,2,3);

Leg 2 (Coxa 0-1. Basis I-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,4);

Leg 3 (Coxa 0-1. Basis I-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,4);

Leg 4 (Coxa 0-1. Basis 1-0. Exopod I-1; I-1; III,1,5. Endopod 0-1; 0-2; 2,2,3);

Leg 5♀ (Coxa 0-0. Basis 1-0. Exopod I-0; I-1; II,1,4. Endopod 0-1; 0-1; 2,2,2).

Leg 1 basis inner distal seta almost straight, hook-like process absent, outer distal seta rudimentary, macula cribrosa on anterior surface adjacent to insertion of exopod segment 1 although sometimes difficult to see, exopod segments 1 and 2 without outer distal spine, segment 3 with 1 outer distal spine, distolateral corner of endopod segment 1 rounded. Legs 2–4 with coxal seta on inner margin; pore openings located on anterior surface at base of outer edge robust setae of exopod segments 1–3, outer border of exopod segment 2 and outer distoproximal border of exopod segment 3 extending into thin flange bordered by short setules; endopod segment 1 outer distal corner rounded. Leg 5 with outer distal corner of endopod segment 1 rounded; pore openings on anterior surface at base of outer border spines; outer border of exopod segment 2 and outer distoproximal border of exopod segment 3 extending into thin flange bordered by short setules.

Male. Anterior margin of head and rostrum similar to that of female. Urosome of five free somites, urosomite II of various proportions relative to other somites.

Antennule asymmetrically developed, geniculate on right. Aesthetascs larger than in female with stiffened posterior border. Left antennule with ancestral segments II–IV and IX–XI fused, XXVII and XXVIII separate with aesthetascs doubled on segments, III, V, VII, IX, XI–XIV. Most setae of modified type (ms) or aesthetascs (a). One macula cribrosa present at base of each aesthetasc. Setation of segments as follows: I—1 ms, 2ss (vestigial), 1a; II– IV—6 ms, 4a; V—2 ms, 2a; VI—2 ms, 1a; VII—2 ms, 2a; VIII—2 ms, 1a; IX– XI—6 ms, 5a; XII to XIV—2ms, 2a; XV to XXI—2ms, 1a; XXII to XXIII—1ms, 1a; XXIV to XXV— 1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII— 3ms, 1 enlarged ss, 1a. Right antennule geniculate between segments XX and XXI, with ancestral segments II–IV, IX–XI, XXI–XXIII fused, XIV–XV fused or separate, XXVII and XXVIII separate. Setation of segments as follows: I—1 ms, 2ss (vestigial), 1a; II– IV—6 ms, 4a; V—2 ms, 2a; VI—2 ms, 1a; VII—2 ms, 2a; VIII—2 ms, 1a; IX– XI—6 ms, 5a (clavate seta not observed — damaged?); XII to XIV—2ms, 2a; XV to XVIII—2ms, 1a; XIX—1ms, 1 fused gripping element, 1a; XX—1 fused gripping element, 1ms, 1a; XXI–XXIII—2 fused gripping elements, 1a, 1 small fused gripping element, 1ms; XXIV to XXV— 1+1ms, 1a; XXVI—1+1ms; XXVII—1+1ms; XXVIII—2ms, 2a, 1ss. Segments 1, II, III, and V with dorsal surface hair sensilla and adjacent macula cribrosa.

Antenna, mandible, maxillule, maxilla, maxilliped and legs 1–4 as in female. Leg 5 with following setal formula (Roman numerals indicate spines, Arabic numerals setae, outer border setation listed to left in each group separated by ‘;’):

Leg 5♂ left (Coxa 0-0. Basis 1-0. Exopod I-0; I-1; II,1,0. Endopod 0-0; 0-1; 2,2,2);

Leg 5♂ right (Coxa 0-0. Basis 1-0. Exopod I-0; I-0/1; II,1,0. Endopod 0-0; 0-1; 2,2,2).

Leg 5 almost symmetrical, apart from specialised seta on inner distal border of left exopod segment 2 and sometimes on right leg exopod segment 2 ( Ba. unicornis ). Outer distal corner of endopod segments 1 and 2 rounded; pore openings on anterior surface of exopod segments at base of outer border spines; outer border of exopod segment 2 and outer proximal border of exopod segment 3 extending into thin flange. Left leg 5 exopod segment 2, specialised seta composed of two sections: wider basal part and lash arising from distal border of basal part. Exopod segment 3 on both sides with analogue of terminal spine on other legs located on distal inner border; on left bordered by setules, on right with or without bordering setules.

Type species. Bathycalanus richardi Sars, 1905

Remarks. The phylogenetic analyses (see Figs 112 , 113 ) clearly identify two separate clades within what was previous called Bathycalanus . When Sars (1905) described this genus, Ba. richardi was the only included species. Thus, the name Bathycalanus is retained for the group of species without reduced numbers of setae on mouthparts of both sexes and with one outer border spine on leg 1 exopod segment 3. Here, we designate, by original monotypy (ICZN 1999, Article 68.3), Bathycalanus richardi Sars, 1905 as the type species of the genus Bathycalanus . This genus currently contains the following species: Ba. richardi Sars, 1905 ; Ba. bradyi (Wolfenden, 1905) ; Ba. unicornis Björnberg, 1968 and Ba. eximius Markhaseva, 1983 , to which are added a further six new species.

The morphology of some Bathycalanus , as defined here, supported by limited molecular data, suggests that this genus might be further subdivided in the future. In the morphology-based cladistic analysis (see Fig. 112 ), the genus Bathycalanus has little bootstrap support and relationships among species are unresolved. In the limited molecular data, both the topology of the phylogenetic tree, and the genetic distances of Ba. tumidus n. sp. from Ba. milleri n. sp., Ba. richardi , Ba. bradyi and Ba. adornatus n. sp. would support the subdivision of this genus (see Fig. 113 ; Tables 14–17). The setation of parts of the maxillule (see Table 18 ) indicates the range of combinations of setation on the posterior of the praecoxal arthrite, coxal endite, basal endite 2 and endopod segment 1 although it is not clear which characters would be aligned with a monophyletic genus containing Ba. tumidus n. sp.

Figures

FIGURE 112. Strict consensus (A), 50 % majority-rule (B) trees, of 84 trees, length 102, consistency index (CI) = 0.75, retention index (RI) = 0.90. After one round of successive reweighting (50 % majority rule of 12 trees), the topology of tree 1 is same as unweighted 50 % majority rule tree (B). The trees are rooted to the outgroup Calanus helgolandicus. Clade number above the line and bootstrap support below.

FIGURE 113. Reconstructed phylogeny trees of the Megacalanidae and out-groups, showing the topology (upper panel) and branch lengths (lower panel) corresponding to the ML analysis. Numbers at nodes indicate statistical support under ML / Bayesian / MP / NJ (bootstrap recovery when> 50 %; Bayesian Posterior Probability when> 0.8). On the lower panel, ML distances are showed for branches within node “ A ”, comprising the Megacalanidae. Each colour represents a different genus. All genera are recovered as monophyletic, and relationships between genera resemble the morphology-based phylogeny. ML = maximum likelihood; MP = maximum parsimony; NJ = neighbour joining.