Heteromysis (Olivemysis) sixi, Wittmann & Abed-Navandi, 2021

Wittmann, Karl J. & Abed-Navandi, Daniel, 2021, Four new species of Heteromysis (Crustacea: Mysida) from public aquaria in Hawaii, Florida, and Western to Central Europe, European Journal of Taxonomy 735, pp. 133-175 : 166-170

Heteromysis (Olivemysis) sixi sp. nov

urn:lsid:zoobank.org:act:A5C4DF45-0137-4A0F-AAB5-4C71E081755C

Figs 12–13

Diagnosis

Only male holotype known (ranges of meristic characters are here indicating differences between left and right side of the body). Rostrum ( Fig. 12A ) triangular with narrowly rounded tip, ⅔ length of terminal segment of antennular trunk. Cornea ( Fig. 12 A–B) occupies distal third of eye surface. Eyestalks with smooth surface except for prominent tooth on disto-mesial edge. Antennular trunk ( Fig. 12 C–E) with blade-like, subapically flagellate spine in addition to setae on disto-mesial edge. Distal 16–24% of this spine with rugged, obliquely truncate margin. Antennal scale ( Fig. 12A ) undivided, extending shortly beyond antennal peduncle; scale length 2.4–3.0 times maximum width. Thoracic sternites ( Fig. 12H ) with short median processes. Flagellum of thoracic exopod 1 with 8 segments, exopods 2–8 with nine segments ( Fig. 13A ). Carpopropodus of thoracic endopods 1–8 with 2, 2, 2, 4, 6, 6, 6, and 6 segments, respectively. Thoracic endopod 3 ( Fig. 12 I–M) without tooth-like extension on disto-mesial edge of merus; carpopropodus length 2.5 times maximum width. Distal 44–64% of male carpus 3 with total of six flagellate spines along inner margin; propodus without paradactylary setae. Penes ( Fig. 13A ) cylindrical, apically blunt, ending in five blunt lobes; size ¾ length of ischium 8; small setae present at half length of penis. Pleopods ( Fig. 13 B–G) rudimentary, unsegmented, with residual differentiation of endopod (pseudobranchial lobe). Male pleopods 1, 3–5 normal, without spines. Pleopod 2 with large smooth spine (stylet) on apex and three minute flagellate spines in series along distal half of outer margin of elongate distal portion. Exopod of uropods ( Fig. 13H ) 1.2–1.3 times length of endopod; endopod with only one small spine near statocyst; distal spine-free portion ⅔ length of endopod. Each lateral margin of telson ( Fig. 13I ) with six spines on distal half, proximal portion smooth. U-shaped apical cleft penetrates ¼ telson length; proximal 5/6 of cleft with 26 laminae. Disto-lateral lobes each with two spines on narrowly truncate apex. The outer apical spines are 16–18% telson length; inner apical spines are 0.2–0.3 times length of outer ones.

Etymology

The species name is a masculine noun in genitive singular, dedicated on the occasion of his retirement to Franz Six, citizen scientist and 55 year-long promoter of the ‘Haus des Meeres’, a public marine aquarium in Vienna.

Material examined

Holotype HAWAII • ♂ ad., BL 3.0 mm (on slides); Honolulu, Waikiki Beach , Waikiki Aquarium ; Jan. 2020; Gwen Lentes leg.; NHMW 26964 .

Description

All features of the diagnosis. Cephalothorax comprises 30% of body length, pleon (without telson) 55%, and carapace (without rostrum) 32%. Abdominal somites 1–5 measure 1.0, 0.9, 0.8, 0.9, and 0.9 times the length of somite 6, respectively.

CARAPACE ( Fig. 12A ). Carapace covers 75% of cephalothorax dorsally. Rostrum reaches to proximal third of artificially straight forward-oriented eyestalks (without cornea). Cervical sulcus short but strong. Posterior margin leaving 1–2 posterior thoracic somites mid-dorsally exposed.

EYES ( Fig. 12 A–B). Eyestalks and cornea dorsoventrally compressed ( Fig. 12B ). Eyestalks without scales. In dorsal view ( Fig. 12A ) the cornea appears calotte-shaped, measuring 4/5 length of eyestalk

(cornea not included). In lateral view ( Fig. 12B ) the cornea appears roughly trapezoid with well-rounded edges, length 1.4–1.5 times maximum width.

ANTENNULAE ( Fig. 12 C–E). Trunk extends 56–62% its length beyond eyes, 12–14% beyond antennal scale. Measured without apophysis along dorsal midline, the basal segment is 47–48% trunk length, median segment 10–18%, and terminal segment 34–43%. Basal segment on basal half of its outer face with two small, stout, barbed setae as in Fig. 2D . Dorsal apophysis with three barbed setae and one whip seta ( Fig. 12E ). Lateral apophysis with two barbed setae and one smooth seta. Median segment dorsally with large apophysis bearing three barbed setae and one whip seta. Terminal segment 1.3–1.4 times as long as wide. Its mid-dorsal apophysis with four small, barbed setae. Outer antennular flagellum is thicker than inner one by factor of 1.5–1.7 when measured near basis of flagella. Male lobe setose, inserts ventrally close to terminal margin of antennular trunk, length is 22–31% width of terminal segment of trunk, its width 22–25%.

ANTENNAE ( Fig. 12A ). Three-segmented antennal peduncle with basal segment 20% length of peduncle, second segment 47–48%, and third segment 33%. Sympod with tongue-like, terminally broad process on ventral face (as in Fig. 10E ).

MOUTHPARTS ( Fig. 12 F–G). Mandibular palp ( Fig. 12 F–G) three-segmented. Proximal segment without setae, 10–15% length of palp. Length of median segment 1.6–2.3 times its maximum width and 61–68% length of palp. Inner margin of median segment with three sparsely barbed setae in subapical to subbasal position, and with 4–6 short setae on proximal third. Basal half of these setae thickened; with unilateral series of stiff barbs; distal half smooth ( Fig. 12G ). Outer margin of median segment all along with setae; most setae barbed on basal third. No scales present on median segment. Terminal segment strongly setose (most setae below drawing plane in Fig. 12F ), this segment 27–28% palp length. Pars molaris with well-developed grinding surface in both mandibles. Pars incisiva with four teeth, digitus mobilis with three teeth, and pars centralis with 4–5 spiny teeth. Labium as described above for H. smithsoniana sp. nov. Maxillula and maxilla as normal in this genus. Outer margin of exopod of maxilla all along with 16 plumose setae, two apical setae larger than remaining ones.

THORACIC STERNITES ( Fig. 12H ) with short median processes, in sternites 1–3, 8 terminally rounded, in sternites 4–7 ending in small triangular denticles.

THORACOPODS (general; Figs 12 I–S, 13A). Length of flagella as well as of basal plates increase from exopod 1 to 5–6, and decrease from exopod 6‾8. Basal plates ( Fig. 13A ) expanded, length 1.7–2.1 times width. Outer edge of plates angular, tip narrowly rounded in exopods 1–2, and pointed in remaining exopods. The first thoracopods with large, leaf-like, smooth epipod. Plumose seta on intersegmental joint between sympod of thoracopod 2 and the corresponding sternite. Basis of endopods 4–8 with a small, lappet-like apophysis on rostral face below endopod ( Fig. 13A ). Total length increases in endopods 1‾6 and decreases in 6‾8. Ischium becomes longer and more slender from endopods 1 to 4, while both features remain subequal among endopods 4–8. Ischium shorter than merus in endopods 1–3 ( Fig. 12I ) but longer than merus in endopods 4–8 ( Fig. 13A ). Thoracic endopods with claw 3 longest; claws 1, 4–8 about half length of claw 3; claw 2 miniaturized. Claws 1–4 smooth ( Fig. 12 N–Q), claws 5–8 subapically unilaterally serrated ( Fig. 12 R–S). Claws 1, 4 straight ( Fig. 12N, Q ), claws 2–3 weakly bent ( Fig. 12 O–P), claws 5–8 strongly bent ( Fig. 12 R–S). Penes with one barbed plus one smooth seta in about central position ( Fig. 13A ).

MAXILLIPEDS. First maxilliped as described above for H. smithsoniana sp. nov. Basis of second maxilliped with large, distinctly medially projecting endite. Combined praeischium plus ischium 0.9 times length of merus, carpopropodus plus dactylus 1.1 times merus. Dactylus very large, with dense brush formed

by large numbers of normal setae and about ten modified setae, the latter apically bent, bearing two symmetrical series of denticles (stiff barbs) on each side in subbasal to median portions.

GNATHOPODS (thoracic endopods 3; Fig. 12 I–M). Ischium 1.7‾2.0 times as long as wide; merus 2.8‾3.3 as long as wide and 1.7‾2.3 length of ischium. Carpus 0.6–0.7 times length of merus, 0.9–1.1 times ischium. Claw 2.6–3.6 times length of dactylus, and 44–52% carpopropodus. Distal half of ischium with 3–4 small whip setae ( Fig. 12M ) on inner margin; these setae inserted on short, terminally rounded projections. Six to eight such setae intermixed in series of larger smooth setae along 60–80% of inner margin of merus, these setae not inserted on projections. Series of five unilaterally barbed setae ( Fig.12L ) near outer margin on distal ¾ of merus. These setae with series of normal barbs (cilia) along median to subapical portions, basal portions thickened. Carpus with proximal two spines unpaired, distal spines arranged in two pairs; only distal 2–4 spines with rugged anterior margins.

PLEOPODS ( Fig. 13 B–G). Length increases in series of pleopods 1, 3 = 4, 5, 2. For presence and numbers of spines on pleopod 2, see ‘Diagnosis’ above. All setae of pleopods 1–5 plumose or barbed.

TAIL FAN ( Fig. 13 H–I). Exopod of uropods reaches with 14–19% its length beyond endopod and 46–49% beyond telson. Endopod reaches with 23–26% its length beyond telson. Statoliths mineralised with fluorite. Statoliths discoidal, diameter 40–42 µm (n = 2). Telson length 1.4 times maximum width, 0.9 times endopod of uropod, 0.7 times exopod of uropod, and 1.3 times last abdominal somite. For further details of telson, see ‘Diagnosis’ above.

FOREGUT ( Fig. 12 T–U) essentially as described above for H. smithsoniana sp. nov. Dorsolateral infoldings with two comparatively large, centrally serrated spines ( Fig. 12T ); posterior part of lateralia with 2–3 smaller, centrally serrated spines ( Fig. 12U ). The examined gut was almost empty.

Distribution

The species is so far known only from a service tank of the ‘Waikiki Aquarium’, Honolulu, Hawaii. Origin most likely in coastal marine waters of the Central Pacific ( Table 1 ).

Figures

Fig. 12. Heteromysis sixi sp. nov., holotype, ♂ with body length 3.0 mm (NHMW 26964). A. Anterior body region, dorsal view; right antennula, left antenna, and setae of antennal scale omitted. B. Detail of panel (A) showing right eye in lateral view. C. Right antennula, dorsal. D–E. Details of antennula (C) showing flagellate spine (D) and an example for whip setae (E) found here and on other appendages. F. Palp of left mandible, caudal. G. Detail of panel (F) showing barbed seta from outer margin of median segment. H. Thoracic sternites 1–8 with median processes. I. Right thoracic endopod 3, rostral. J–M. Details of endopod 3 (I) showing modified spines (J–K) of carpus, barbed seta (L) of merus, and whip seta (M) from ischium. N–S. Series of dactyli with nail in thoracic endopods 1–5, 8; setae omitted. T–U. Modified spines from dorsolateral infoldings (T) and lateralia (U) of foregut.

Fig. 13. Heteromysis sixisp. nov., holotype, ♂with body length 3.0 mm (NHMW26964). A. Thoracopod 8 with penis, caudal. B–F. Series of male pleopods 1–5, caudal. G. Detail of male pleopod 2 (C) showing flagellate spine. H. Uropods, ventral; with setae and ventral wall of statocyst omitted. I. Telson.

Fig. 2. Heteromysis smithsoniana sp. nov., paratypes, ♂♂ with body length 4.8 mm (A–D, I–K, R–S: NHMW 26971), 4.2 mm (E–H, L–N: NHMW 26972), and ♀ 4.2 mm (O–Q: NHMW 26970). A. Male antennula, dorsal. B–D. Details of antennula (A) showing modified spines (B–C) and setae (D). E. Antenna with antennal gland, dorsal. F. Carapace expanded on slide. G–H. Details of carapace (F) showing pore groups in front of cervical sulcus (G) and behind cardial sulcus (H). I. Palp of left mandible, rostral. J–K. Details of mandibular palp (I) showing three scale groups (J) and one modified seta (K) from inner margin of median segment; most setae of terminal segment are below drawing plane. L. Right male thoracic sympod and endopod 3, rostral. M–N. Details of male endopod 3 (L) showing modified spine (M) of carpus and modified seta (N) of merus. O. Left female thoracic endopod 3, rostral. P–Q. Details of female endopod 3 (O) showing modified spine (P) of carpus and modified seta (Q) of merus. R–S. Dactyli 1 and 3, with claw, setae omitted.

Fig. 10. Heteromysis waikikensis sp. nov., holotype, ♂ with body length 3.9 mm (N: NHMW 26965), paratypes, ♂ 4.2 mm (A–E, L–M, O–P: NHMW 26967) and ♀ 4.2 mm (F–K: NHMW 26966). A. Male antennula, dorsal.B–D. Details of male antennula (A) showing modified spines. E.Antenna with antennal gland, dorsal. F. Palp of left mandible, caudal. G–H. Setae from outer margin of median segment of mandibular palp in ♀ (G) versus ♂ (H). I. Left female thoracic endopod 3, rostral. J–K. Details of female endopod (I) 3 showing modified spines of carpus. L. Left male thoracic sympod and endopod 3, rostral. M. Detail of male endopod 3 (L) showing modified seta of merus. N. Scutellum paracaudale, lateral. O–P. Modified spines from dorsolateral infoldings (O) and lateralia (P) of foregut.