Heteromysis (Olivemysis) hornimani sp. nov.

urn:lsid:zoobank.org:act: 5C14C755-FE9E-4FD0-9113-F587163584BB

Figs 4–8

Diagnosis

Rostrum triangular (Fig. 4A), 69–74% length of terminal segment of antennular trunk, apex acute to narrowly rounded. Cornea occupies distal third of eye surface. Disto-mesial edge of eyestalks with small tooth (Fig. 4C). Antennular trunk (Fig. 5 B–E) with one strong, apically barbed, spine-like seta on dorsal apophysis of basal segment, and with two subapically flagellate spines in addition to setae, namely one spine on disto-mesial edge of median segment, and a broader one on disto-mesial edge of terminal segment. Antennal scale (Fig. 5G) with small apical segment; total scale length is 3.0–3.7 times maximum width; antennal scale reaches 0–20% its length beyond antennal peduncle. Male thoracic sternites 1–8 (Fig. 4D) each with single mid-ventral process. Female sternites without such processes. Flagellum of thoracic exopods 1 and 8 with 8–9 segments, remaining exopods with 9 segments in both sexes (Fig. 6F). Carpopropodus of thoracic endopods 1–8 with 2, 2, 2, 4 (rarely 5), 5–8, 5–8, 7–9, and 7–9 segments, respectively. No visible difference between left and right thoracic endopods 3. Disto-mesial edge of merus 3 without tooth-like extension. Carpus 3 separated from propodus by a distinct suture. Length of carpopropodus 3 is 1.8–2.6 times maximum width in males (Fig. 5L), 2.1–3.1 in females (Fig. 5H). Carpus 3 with 6–7 flagellate spines (Fig. 5 I–J) along distal 55–78% of inner (mesial) margin in both sexes; propodus without paradactylary setae or spines. Penes tube-like, 73–94% length of ischium of endopod 8 (Fig. 6F). Pleopods (Fig. 7) rudimentary, unsegmented, with residual differentiation of endopod (pseudobranchial lobe). Female pleopods 1, 3–5, and male pleopods 1, 5 without spines. Pleopods 3–4 with clearly dimorphic structure, while only spine numbers differ between sexes in pleopod 2. Apical third of female pleopod 2 with series of 2–4 mostly robust, flagellate spines increasing in length distally. Male pleopod 2 with 4–5 robust spines in comparable sequence (i.e., increasing in length distally) and position (i.e., on apical third). Distal half of male pleopods 3–4 knife-shaped with obliquely truncate, somewhat convex terminal margin; pleopod 3 with 8–14 smaller (compared with pleopod 2), flagellate spines, densely set along the ‘blade’; pleopod 4 with 21–28 even smaller, flagellate spines along the ‘blade’. Exopod of uropods 1.2–1.4 times length of endopod (Fig. 4F); endopod with 2–3 spines near statocyst; distal spine-free portion is 65–71% length of endopod. Lateral margins of telson (Fig. 6I) all along with 16–20 spines in continuous series; U- to V-shaped apical cleft penetrates telson length by 19–27%; cleft with 18–22 laminae along basal 67–86% of its margins. Apical lobes of telson each with two spines; latero-apical spines are 12–15% telson length; medio-apical spines are 0.4–0.6 times length of latero-apical spines.

Etymology

The species name is a masculine noun in the genitive singular, referring to the Horniman Museum Aquarium, London.

Material examined

Holotype LONDON • ♂ ad., BL 4.0 mm (in vial); Horniman Museum Aquarium; Dec. 2019; Jamie Craggs leg.; NHMW 26973.

Paratypes LONDON • 11 ♂♂ ad., BL 3.7–5.4 mm, 3 ♀♀ ad., BL 4.7–6.1 mm, 11 subad., 5 imm., 1 juv. (in vial); same collection data as for holotype; NHMW 26974 • 1 ♀ ad., BL 5.2 mm (on slides); same collection data as for holotype; NHMW 26975 • 1 ♂ ad., BL 4.7 mm (on slides); same collection data as for holotype; NHMW 26976 .

Non-types

FRANCE • 4 ♂♂ ad., BL 4.0– 6.6 mm, 1 ♀ subad., BL 3.7 mm, 2 imm., BL 2.4–2.8 mm, 1 juv. (in vial); Aquarium de Paris; Sep. 2020; Anaïs Courtet leg.; NHMW 26977 .

FRANCE • 1 ♂ ad., BL 5.3 mm, 1 imm. (in vial); Brest, Oceanopolis Aquarium; 3 Feb. 2020; Dominique Barthelmy leg.; NHMW 26978 • 1 ♀ ad., BL 4.3 mm, 1 ♂ ad., BL 5.4 mm (in vial); same collection data as for preceding; NHMW 26979 • 1 ♂ ad., BL 6.3 mm (on slides); same collection data as for preceding; NHMW 26980 .

POLAND (WROCLAW) • 1 ♀ ad., BL 6.1 mm (in vial); Wroclaw Zoo; Jul. 2020; Jakub Kordas leg.; NHMW 26981 • 1 ♀ ad., BL 6.5 mm, 2 imm., 7 juv. (in vial); same collection data as for preceding; Aug. 2020; NHMW 26982 .

Description of types

All features of the diagnosis. Size of adult females 4.7–6.1 mm, males 3.7–5.4 mm. Cephalothorax comprises 32–37% body length, pleon (without telson) 48–50%, telson 10–14%, carapace (without rostrum) 22–34%, and rostrum 3–5%. Abdominal somites 1–5 measure 0.6–0.9, 0.7–0.9, 0.7–0.9, 0.6– 0.8, and 0.7–0.8 times the length of somite 6, respectively.

CARAPACE (Fig. 4A). Carapace covers 66–93% of cephalothorax dorsally. Rostrum reaches at most to distal ⅔ of artificially straight forward-oriented eyestalks (without cornea). Cervical sulcus distinct. Posterior margin leaves 0.5–2.5 posterior thoracic somites mid-dorsally exposed. Carapace shape as in Fig. 2F, but with longer rostrum as in Fig. 4A.

EYES (Fig. 4A, C). Eyestalks and cornea dorsoventrally weakly compressed. Eyestalks with scales along basal half to ⅔ of mesial margin. In dorsal view, cornea appears calotte-shaped, measuring 0.5–1.0 times length of eyestalk (cornea not included). In lateral view the cornea appears ovoid, length 1.1–1.3 times maximum width.

ANTENNULAE (Fig. 5 B–E). Trunk extends 53–65% of its length beyond eyes, 14–43% beyond antennal scale. Measured along dorsal midline, basal segment is 30–42% trunk length, median segment 17–24%, and terminal segment 37–51% (Fig. 5B). Basal segment on basal half of outer face with two small, stout, barbed setae as in Fig. 2D. Dorsal apophysis with 3–4 barbed setae, a smooth, flagelliform seta, and an apically barbed, spine-like, stout seta (Fig. 5C). Lateral apophysis with four barbed setae. Median segment dorsally with large apophysis with three barbed plus a smooth, flagelliform seta, medially with large plumose seta and a flagellate spine (Fig. 5D). Terminal segment 1.1–1.2 times as long as wide. Mid-dorsal apophysis with four small barbed setae. Disto-mesial edge of terminal segment with one large flagellate spine (Fig. 5E), one large, laterally directed smooth seta, and two plumose setae in females, only one in males. The flagellate spine distally with 2–4 rounded to tooth-like tubercles (Fig. 5E). Outer antennular flagellum thicker than inner one by factor of 1.2–1.8 when measured near basis of flagella. Male lobe well setose, inserts ventrally close to terminal margin of antennular trunk, almost spherical, length is 15–24% of width of terminal segment of trunk, its width 17–22%.

ANTENNAE (Fig. 5G). The three-segmented antennal peduncle with basal segment 17–24% peduncle length, second segment43–55%,and third segment 28–39%. Sympod ventrally with stout, subrectangular, terminally rounded process (dashed line in Fig. 5G). Its outer margin ends in an apically blunt to acute projection.

MOUTHPARTS (Fig. 5A). Labrum as given above for H. smithsoniana sp. nov. Mandibular palp threesegmented. Proximal segment without setae, 10–17% length of palp. Length of median segment 2.0–2.6 times maximum width and 57–69% palp length. Inner margin of median segment with 4–5 basally barbed setae in subbasal to terminal position. Both sexes with 8–10 smooth, shorter setae along proximal half; these setae increasing in length distally; their basal half thickened. Proximal 50–80% of outer margin all along with smooth setae, remaining distal portion with basally weakly barbed setae. Only males with small field of scales near inner basal edge on rostral face (this is below drawing plane in Fig. 5A); size and shape of scales as in Fig. 2J. Terminal segment strongly setose (most setae below drawing plane in Fig. 5A), 21–26% palp length. Pars molaris with well-developed grinding surface in both mandibles. Pars incisiva with 3–4 teeth, digitus mobilis with 3–4 teeth, and pars centralis with three spiny teeth. Labium as described above for H. smithsoniana sp. nov. Maxillula as normal in this genus; distal segment terminally with 11–13 smooth, in part weakly serrated spines; subterminally with 3–4 setae barbed on distal half; 2–4 pores closely adjoining outer (= most ventral) seta. Endite of maxillula terminally with three strong, distally spiny setae accompanied by 4–5 proximally thick barbed setae; inner and outer margins with numerous less thick setae. Maxilla as normal in this genus; terminal segment of endopod with maximum width 44–76% length. Basal segment of maxillary palp with field of scales. Outer margin of exopod all along with 17–19 mostly plumose setae, two apical setae larger than remaining ones.

MALE THORACIC STERNITES (Fig. 4D). Size of mid-ventral processes increases from processes 1–4, while processes 4–8 remain subequal.Anterior 3–4 processes linguiform, terminally rounded, the posteriormost 4–5 processes triangular with acute or narrowly rounded tip. Process 1 emerges from usual mid-rostral lobe of thoracic segment 1, remaining processes from middle to posterior third of their respective sternite.

THORACOPODS (general; Figs 5 H–P, 6C–G). Length of flagella as well as of basal plates increases from exopod 1 to 5 or 6, and then decreases to exopod 8. Basal plates expanded, length 1.4–1.9 times width in both sexes. Outer margin of plate 1 ends in well-rounded edge; plates 2–8 with subrectangular, narrowly rounded to acute edge. First thoracopods with large, leaf-like, smooth epipod. Characteristic pair of setae on intersegmental joint between sympod of thoracopod 2 and corresponding sternite as in H. smithsoniana sp. nov.; no such setae in remaining thoracopods. Total length of endopod increases in series of thoracopods 1, 2, 4, 3, 5–8. Length and slenderness of ischium increase from 1 to 5, and remain subequal among endopods 5–8. Basis of endopods 4–8 with a small, ovoid to lappet-like apophysis on rostral face below endopod, no such apophysis in endopods 1–3. Ischium shorter than merus in endopods 1–4 but longer than merus in endopods 5–8. Thoracic endopods with claw 3 longest; claws 1, 4–8 roughly half length of claw 3; claw 2, if any, not detected. Varying portions of claw 1 unilaterally (Fig. 5P) or bilaterally (Fig. 5O) serrated, claws 3–4 smooth (Fig. 6 C–D), claws 5–8 (Fig. 6E, G) centrally to subapically serrated on inner margin. Claw 1 slightly bent, claw 4 straight, claws 3, 5–8 strongly bent. When stretched anteriorly, endopod 8 reaches to maxillula, when stretched posteriorly to end of pleonite 5 up to mid of pleonite 6. Penes cylindrical, subbasally with 1–2 small setae.

MAXILLIPEDS. As described above for H. smithsoniana sp. nov., coxa of first maxilliped with small endite bearing one barbed, basally thick seta at tip. In both sexes, combined praeischium plus ischium of second maxilliped are 0.6–0.9 times length of merus, carpopropodus plus dactylus 1.0–1.2 times merus. Dactylus 2 with dense brush formed by large numbers of normal setae and 13–14 modified setae; modification as in above-quoted species.

GNATHOPODS (third thoracic endopods; Figs 5 H–M, 6C). Ischium 1.4‾2.1 times as long as wide; merus 2.5‾3.1 as long as wide and 1.5‾2.1 length of ischium. Carpus 0.6–0.8 times length of merus, 0.9–1.2 times ischium. Claw 1.9–2.9 times length of dactylus, 47–54% carpopropodus. Series of 4–5 unilaterally barbed, basally thickened setae along ⅔ of outer face of merus; distal 2–3 setae with most proximal barb thickened in males (Fig. 5M), normal in females (Fig. 5K). Distal half of ischium with 3–6 short whip setae on mesial margin, each whip seta on tip of a short rounded projection in both sexes. Proximal ⅔ of merus with 3–6 short whip setae on mesial margin, the whip setae alternating with longer smooth setae. Carpus 3 with proximal 2–3 flagellate spines (Fig. 5 I–J) in linear series, distal four spines arranged in pairs; all these spines with subapical flagellum positioned on their smooth posterior (proximal) face; 3–4 anterior (distal) spines rugged along their anterior face.

MARSUPIUM. As described above for H. smithsoniana sp. nov. Oostegite on thoracopod 7 near basis with 6–9 microserrated setae, larger oostegite on thoracopod 8 with 3–5 such setae. Ventral and rostral portions of outer face of only larger oostegite with total of 14–19 flagelliform setae which are shorter than barbed setae present in dense series along lower margin. Thoracopod 6 with rudimentary oostegite represented by small, rounded lobe terminally with three setae which are microserrated on distal half.

PLEOPODS (Fig. 7). Size increases in series of pleopods 1, 3, 4, 5, 2 in females, and 1, 5, 2, 3, 4 in males. For presence and numbers of spines on pleopods 2–4, see ‘Diagnosis’ above. All setae of pleopods 1–5 plumose or barbed in both sexes (not counting flagellate spines). Most specimens of both sexes, including holotype, show peculiar perpendicular orientation of only second pleopod (as in Fig. 8C).

TAIL FAN (Figs 4F, 6 H–I). Scutellum paracaudale triangular, tip acute or narrowly to well rounded. Exopod of uropods extends by 4–25% its length beyond endopod, 25–47% beyond telson; endopod 28– 43% its length beyond telson. Diameter of fluorite statoliths is 80–106 µm (n = 10 statoliths from five specimens). Statoliths discoidal with shallow fundus and distinct tegmen. Statolith formula 2 + 3 + (0–1)

+ (5–10) + (6–10) = 19–22. Telson length 1.2–1.5 times its maximum width and 0.9–1.2 times length of sixth pleonite. Spine length on lateral margins of telson decreases in continuous series from basis to half-length and from there increases up to tip. For further details of telson, see ‘Diagnosis’ above.

FOREGUT (Fig. 6 A–B) essentially as described above for H. smithsoniana sp. nov. Caudal portion of lateralia in H. hornimani sp. nov. with group of 3–5 small, apically pronged spines serrated along distal ¾ of shaft (Fig. 6B). Dorsolateral infoldings with group of three longer, apically smooth, subbasally to subapically serrated spines (Fig. 6A). Gut contents of three specimens examined were mostly unidentifiable material and mineral particles, a few fragments of filiform algae, no arthropod remains.

Colour

PHOTOS FROM BREST (Fig. 8 A–B). General appearance transparent with yellow to orange-brown portions, rendering contents of foregut and alimentary canal visible. Small orange to brown chromatophore spots scattered over the thorax, with greatest density above the foregut. The reflecting spots above the foregut may be due to small oil globules (usually disappearing in ethanol-preserved material). Eggs are green, as often found in species of Heteromysis .

PHOTO FROM LONDON (Fig. 8C). Compared with the figures from Brest, the figure from London shows an essentially similar density of chromatophores, but more ‘expanded’ ones, giving the animal a more intensive orange to red tinge. The differences between Fig. 8 A–B and Fig. 8C are well within the range of normal colour change in species of the family Mysidae .

Distribution

The species is so far known only from tanks in the ‘Horniman Museum Aquarium London’ (UK), the ‘Oceanopolis Aquarium’ in Brest (France), the ‘Aquarium de Paris’ (France), and the ‘Zoo Wroclaw’ (Poland). The data from Paris suggest a Caribbean origin (Table 1: # 4).

Eggs and larvae

MATERIAL FROM LONDON. Two females with empty brood pouch, while another female with 5.3 mm body length carried three postnauplioid larvae at substage P2 with 1.28–1.35 mm length, one female 6.1 mm carried three P2 with 1.23–1.35 mm. A crude estimate suggests that the larvae attain ¼ parent length shortly before the moult to the free-living juvenile stage.

MATERIAL FROM WROCLAW. Female with 6.1 mm body length carried one nauplioid larva at substage N1 (0.92 mm length); female 6.5 mm carried two eggs with diameter 0.49 mm and eight N1-larvae with 0.87–1.11 mm. The co-occurrence of eggs and N1-larvae in the same brood pouch is rarely found. This indicates that the embryos were asynchronously hatching from the egg membrane shortly before and during fixation. The observed strong size differences between the hatchlings are clearly due to the obligatory stretching of the abdomen during and shortly after hatching. Adoption of eggs or larvae from other mothers appears unlikely in this case: in experiments of Wittmann (1978) with mysids from other subfamilies, the breeding females did not introduce offered eggs (embryos) into their brood pouch, but a few mothers did so with up to two nauplioid larvae, never with more. The here studied N1-larvae show a smooth cuticle all around.