Heteromysis (Heteromysis) octopodis Wittmann & Griffiths

Wittmann, Karl J. & Griffiths, Charles L., 2017, Three new species of Heteromysis (Mysida, Mysidae, Heteromysini) from the Cape Peninsula, South Africa, with first documentation of a mysid-cephalopod association, ZooKeys 685, pp. 15-47 : 25-28

Heteromysis (Heteromysis) octopodis Wittmann & Griffiths sp. n. Figs 6B, 9, 10, 11

Type series.

All materials from Miller’s Point, on the False Bay coastline of the Cape Peninsula, South Africa, leg. Craig Foster. Holotype, adult male with 7.0 mm body length, in vial at NHMW-25907; paratypes immature female 6.2 mm and immature male 6.1 mm, in vial at SAM-MB-A067552; dissected paratypes adult male 6.8 mm, subadult female 10.8 mm, immature female 7.8 mm, on slides at NHMW-25908, associated with Octopus vulgaris inside den in 3 m depth, 34°13.77'S, 18°28.45'E, 15°C, 26 March 2017; paratype adult male 8.8 mm (dissected, slides at NHMW-25909) from euhaline intertidal pool, 34°13.79'S, 18°28.43'E, 10 Aug. 2014; paratypes 2 subadult females (9.4 mm entire, 9.0 mm in 2 parts, both in vial at SAM-MB-A067553) plus damaged subadult female (estimated original size 8.8 mm, head missing, dissected, slides at NHMW-25910), from same pool as before, at a few cm depth upon low tide, 8 Sept. 2015.

Diagnosis.

Based only on adult males and subadult females. Carapace produced into well-projecting, (sub)triangular rostrum with rounded apex. Eyes well-developed; cornea occupies 60-70% of eye surface; eyestalks with inconspicuous, distally-directed, blunt extension of (obliquely anterior facing) inner margin. Antennular trunk with a number of smooth and barbed setae, but no particularly modified setae; inner distal corner of its terminal segment with apophysis carrying two medium-sized, smooth setae. Antennal scale stout, extending to 0-20% below tip of antennular trunk; outer margin convex. First thoracic sternite with anteriorly-projecting, terminally-rounded median lobe; sternites 2-8 without lobes. Carpopropodus of thoracic endopods 1-8 with 2, 2, 2-3, 4-6, 5-7, 5-8, 6-8, or 6-8 segments, respectively. Third thoracic endopod without any spines or spine-like setae; carpus not swollen with respect to merus. Carpus with series of three weakly-modified setae near outer margin; these setae subbasally furnished with bristles. Merus (sub)terminally with two subbasally toothed setae. Fourth thoracic endopod with series of 2-3 such toothed setae near outer margin of carpopropodus; no such modified setae in endopods 5-8. Penes long and slender, 1.6-1.7 times length of merus of eighth thoracic endopod; tip with three rounded lobes, each wider than long. Pleopods reduced to small setose, bilobate plates, without any spines. Exopods of uropods extend distinctly beyond endopods. Endopods with only one spine on inner margin, in subbasal position near statocyst. Telson subtriangular, terminally truncate; lateral margins weakly sigmoid, along their distal 46-53% furnished with almost continuous series of 13-17 spines each. Telson with apical cleft forming a proximally rounded ‘V’. Cleft slightly deeper than wide, its depth 17-23% telson length. Cleft densely furnished with a total of 26-37 acute laminae all along its margins. Two latero-apical lobes of telson show narrow transverse apical margins, each bearing a large latero-apical spine, plus much smaller medio-apical spine.

Description.

As described above for H. cancelli sp. n. unless stated otherwise in the following. Cephalothorax comprises 37-44% of body length without telson, pleon 54-63%, and carapace 32-38%, when measured along dorsal median line. First thoracic sternite with median lobe showing smooth rounded apex. Each of first to fifth abdominal somites measure 0.8-1.1 times length of sixth somite. Terminal margin of sixth pleonite with lateral shields triangular, with tip more rounded in subadult females (Fig. 10L) than in males (Fig. 10K).

Carapace (Figs 6B, 9, 10A). Non-dimorphic, antero-lateral edges evenly rounded. Cervical sulcus well marked, cardial sulcus weak but distinct. Anterior pore group of carapace with 29-32 pores in strongly flattened ‘U’ -shaped arrangement; posterior group with 12-15 pores (n = 6).

Eyes (Figs 6B, 9, 10A). Anterior and posterior margins of eyestalks densely covered by scales, but not so the basal portions. Ocular symphysis with subtriangular to sinusoid, in any case terminally rounded, smooth, subrostral process (dashed line in Fig. 10A).

Antennulae (Fig. 10A, B). Basal segment 41-49% length of trunk, middle 14-18% and terminal segment 34-44%. Trunk on the average stouter in males, with basal segment 0.9-1.4 times longer than broad, compared to 1.2-1.5 times in subadult females. Terminal portion of basal segment with bifid dorsal apophysis bearing 4-5 barbed setae on its more median projection, plus two smooth setae on its outer projection. Outer ventral lobe of basal segment bears four plumose setae at its tip plus a small barbed seta in subbasal position. Anterior margin of median segment dorsally with apophysis bearing a smooth seta together with several barbed setae; inner margin anteriorly with medium-sized to large barbed seta together with smooth seta. Two small barbed setae antero-ventrally close to outer margin (not visible in Fig. 10A). Terminal segment ventrally with 1-2 large, obliquely forwards-directed, plumose setae and 0-2 additional plumose setae dorsally at inner distal corner in both sexes. Lobe with four medium-sized barbed setae plus a dense series of short bristles in about median position on anterior margin of terminal segment. Outer antennular flagellum 1.3-1.5 times as thick as inner flagellum, when measured near basis.

Antennae (Fig. 10A). Length of antennal scale 2.8-3.2 times its maximum width. Apical segment contributing 5-7% to total scale length. Basal segment 21-28% length of peduncle, second 36-40% and third 34-40%.

Mouth parts (not figured). Pars incisivus of mandible with 3-4 large teeth, and digitus mobilis with 3-4 large plus 2-3 small teeth. Pars centralis with 3-4 spiny teeth. Distal segment of maxillula terminally with 9-12 weakly serrated spines, subterminally with a transverse row of 5-6 barbed setae. Endite of maxillula with three large, distally spinose setae, and 18-28 smaller, smooth or barbed setae.

Thoracopods in general (Figs 10 D–H; 11 A–F). Sizes increase from exopod 1 to (4-6) and decrease from 6 to 8. Flagellum of first to eighth exopods with 8, 9, 9, 9, 9, 9, 9, 9 segments. Exopods with basal plate 1.3-2.2 times longer than broad. Claws of endopods 1, 3, 4 more strongly serrated in male versus subadult female, not so in endopods 5-8.

Maxillipeds (Fig. 10C, D). Sympod of first endopod with hairs on outer half. First thoracic epipod large, leaf-like, with small field of minute scales near insertion with sympod, and (sub)basally with 1-2 large, sparsely barbed setae (Fig. 10C). Dactylus of first endopod with strong, subapically, bilaterally-microserrated claw (Fig. 10D). Dactylus of second endopod with dense brush of setae, among these 16-19 modified ones.

Gnathopods (Figs 10E, 11 A–C). Endopod with comparatively slender carpopropodus, 4.9-5.5 times longer than broad; length 0.8-1.0 times that of merus, and 0.9-1.0 times that of ischium.

Pereiopods (Figs 10 F–H, 11 D–F). Fourth endopod with moderately-small dactylus bearing a long, weakly-bent claw, microserrated on two opposite sides of its subapical portions (Fig. 10F). Fifth to eighth endopods equipped with again smaller dactylus bearing much shorter claw that shows a stronger, distally increasing curvature; this claw unilaterally microserrated only in median portions of inner margin (Fig. 10G, H).

Penes (Fig. 11F) long, facing obliquely in anterior direction up to basis of fourth thoracopod. Five to six small, barbed setae scattered all along the penis.

Oostegites. The subadult females already have eggs in the ovaries, visible in Fig. 9A by yellow complexion through the semi-transparent, essentially red carapace (best visible at the rostrum). The subadults show well-formed but still immature marsupial plates on the seventh and eighth thoracopods, plus rudimentary oostegites on the sixth thoracopods.

Pleopods (Fig. 10J). The seta at inner, terminal edge of endopod weakly barbed, remaining setae well-barbed or plumose. Part of plumose setae of exopod with wave-like series of weak constrictions along shaft. Total length of pleopod 5 is 173-207% that of pleopod 1 (n = 7). Starting with pleopods 1 versus 2, the length increase between subsequent pleopods is 14-31%, 3-12%, 0-15%, and 21-58%, respectively.

Uropods. The exopods reach with 15-38% of their length beyond endopods and 33-43% beyond telson, endopods 6-17% of their length beyond telson. Exopod length 3.8-4.2 times maximum width. Statoliths composed of fluorite; diameter 120-196 µm (n = 12); statolith formula 2 + 3 + (0-1) + (8-16) + (12-19) = 27-39.

Telson (Fig. 11G). Length 1.2-1.5 times that of ultimate abdominal somite, or 0.7-0.9 times exopod of uropods. Length of telson 1.5-1.6 times its maximum width. Laminae of cleft show about 0.5-0.7 times average length of lateral spines. Basal half of outer margins smooth. Length of lateral spines distally (somewhat discontinuously) increasing in size by a factor of 1.3-1.7.

Colour (Figs 6B, 9). General appearance of living specimens red-orange to blazing red. Cornea brown to yellow-golden; eyestalks red, except for a white ribbon along posterior, dorsal portions of the inner margin of the cornea (best visible in Fig. 6B). Chromatophores could only partly be discerned individually due to their strong expansion (stronger in Fig. 6B than in Fig. 9A) in three specimens micro-photographed alive. Red spots scattered over eyestalks, antennae, carapace, pleon, uropods, and telson. Transverse double series of spots near posterior margin of each pleomere, not well distinguishable in thoracomeres; a broader posterior red band on pleomere 6. Uropods and telson most intensively red. Compared with that of the male in Fig. 6B, the more orange tinge of the female thorax in Fig. 9A comes from yellow yolk in the ovarian tubes. Persistence of colours as in H. cancelli .

Etymology.

The species name octopodis is a noun in genitive singular, derived from the substantive octopus by using the third declension of New Latin.

Type locality.

Octopus den (Fig. 9B) in 3 m depth at Miller’s Point, on the False Bay coastline of the Cape Peninsula, South Africa, 34°13.79'S, 18°28.43'E. This is closely adjacent to the type locality of H. fosteri sp. n. (above).

Microdistribution.

Schools of this mysid were encountered during daytime in shallow sublittoral waters inside dens occupied by Octopus vulgaris (in case of Fig. 9B together with a crab and an additional, undetermined mysid species). Association with octopus appears a regular phenomenon as H. octopodis was found there nine times between 14 May 2016 and 26 March 2017. Nonetheless, this mysid species was also found upon low tide in a few cm depth in rocky tide pools (Figs 6B, 9A). Also this kind of microdistribution appears to be a regular phenomenon since it occurred there upon various excursions, namely 10 Aug. 2014, 14 Aug. 2014, and 8 Sept. 2015.