Heteromysis (Heteromysis) cancelli 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 : 16-20

Heteromysis (Heteromysis) cancelli Wittmann & Griffiths sp. n. Figs 1, 2, 3, 4, 5

Type series.

Holotype, adult male with 6.9 mm body length, in vial at NHMW-25903; paratypes in vial at SAM-MB-A067550: adult male 5.3 mm, adult female 6.1 mm, subadult female 6.0 mm, one immature female, one juvenile; dissected paratypes on slides at NHMW-25904: adult female 6.4 mm and adult male 6.9 mm; Roman Rock, off Simonstown, on the False Bay coastline of the Cape Peninsula, South Africa, 34°10.95'S, 18°27.50'E, 20 m depth; all eight specimens from the same gastropod shell inhabited by the hermit crab Cancellus macrothrix Stebbing, 1924, 3 May 2015, leg. Jannes Landschoff.

Diagnosis.

Carapace produced into well projecting, broadly-subtriangular rostrum with rounded apex. Eyes well developed; cornea occupies 40-55% of eye surface; eyestalks with small, 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 anteriorly directed apophysis carrying two large, smooth setae. Antennal scale moderately stout and short, extending shortly beyond distal half of terminal segment of antennular trunk; outer margin feebly convex. First thoracic sternite with anteriorly projecting, terminally rounded median lobe; sternites 2-8 without lobes in both sexes. Carpopropodus of thoracic endopods 1-8 with 2, 2, 2, 3-4, 5, 6, 6, or 5-6 segments, respectively. Third thoracic endopod not dimorphic, without any spines or spine-like setae; carpus not swollen (with respect to merus). Carpus of third thoracic endopod with series of 2-3 subbasally spiny (i.e. with modified barbs) and medially to subterminally pectinate setae near outer margin. Carpopropodus of fourth thoracic endopod with series of 2-4 subbasally more strongly spiny setae near outer margin, these ‘spines’ thick but not tooth-like; no such modified setae in endopods 5-8. Penes long and slender, twice 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 in both sexes. Exopods of uropods extend distinctly beyond endopods. Endopods with only one spine on inner margin, in subbasal position near statocyst. Telson subtriangular, though terminally transversely truncate; lateral margins weakly sigmoid, along their distal 56-60% furnished with continuous series of 18-21 spines each. Telson with apical cleft that forms a narrow, proximally rounded ‘V’, cleft four times deeper than wide, its depth 27-36% telson length. Cleft densely furnished with total of 29-33 acute laminae (= laminar processes) along basal 81-84% of its margins. Two latero-apical lobes of telson show transverse apical margins, each carrying 2-3 large spines of subequal size.

Description.

General appearance is that of mysids with intermediate proportions. Cephalothorax comprises 39-42% of body length without telson, pleon 58-61%, and carapace 33-37%, when measured along dorsal median line (Fig. 1). First thoracic sternite with median lobe showing a number of minute bristles on its well-rounded apex (Fig. 3B). Each of first to fifth abdominal somites measures 0.7-0.9 times length of sixth somite. Terminal margin of sixth pleonite with sinusoidal lateral shields covering the basis of uropods in females (Fig. 5F) versus (sub)triangular ones in males (Fig. 5E).

Carapace (Fig. 2A, D). Non-dimorphic, antero-lateral edges evenly rounded. Cervical sulcus well marked, no cardial sulcus visible. Posterior margin rounded, emarginated, leaving the ultimate and part of penultimate thoracic segment dorsally exposed. Carapaces dissected and mounted on slides in only a single specimen for each sex: 10-11 pores (Fig. 2D) of about 1 µm diameter are in roughly butterfly-shaped arrangement in front of the posterior margin, surrounding a larger, less distinct pore, as in the Mediterranean H. arianii Wittmann, 2000, and NE. Atlantic H. dardani Wittmann, 2008. An additional group with 18-20 pores (Fig. 2C), in strongly flattened ‘V’ -shaped arrangement, is in median position, closely in front of cervical sulcus.

Eyes (Figs 1; 2A, B). Thick, shaped in form of dorsoventrally compressed globoids. Cornea appears calotte-shaped to sub-reniform in dorsal and in ventral view, oval in lateral view. Comparatively large group of scales distributed along inner, obliquely anteriorly facing margin of eyestalks. Ocular symphysis with broadly-rounded, smooth, subrostral process (as dashed lines in Fig. 2A, B).

Antennulae (Fig. 2A, B, E). Basal segment 42-47% length of trunk, middle is 12-19% and terminal segment is 36-42%, when measured along dorsal midline of trunk (only Fig. 2E shows entire extension of basal segment). Trunk stouter in males, with basal segment 1.4-1.5 times longer than broad, compared to 1.7-1.8 in females. A small dorsal apophysis and a longer outer ventral lobe (exite) extend (obliquely) forwards from end of basal segment. Dorsal apophysis bears 1-2 smooth setae and a number of barbed setae, ventral lobe bears four plumose setae at its tip. Median segment obliquely truncate, its anterior margin dorsally with a smooth seta in median position and more laterally an additional smooth seta, together with several barbed setae. Two small barbed setae antero-ventrally near outer margin. These last setae not visible in dorsal view (Fig. 2A, B, E). Inner distal corner of terminal segment ventrally with a large, obliquely inwards-forwards directed, plumose seta, this seta larger in females compared to males, and dorsally with an apophysis as described in the diagnosis. In sublateral to submedian position on anterior margin of terminal segment there is a lobe with 3-5 medium-sized barbed setae and a dense series of short bristles. Only females with additional, large plumose setae, also obliquely inwards-forwards directed in Fig. 2B, on terminal segment of trunk, one half-way on inner margin, the second on ventral surface proximally from inner flagellum, at about 25% segment length from anterior margin of terminal segment. Appendix masculina terminally bilobate, forming a medium-sized, forwards directed extension distally on ventral surface of terminal segment, and bearing a large and dense brush of setae extending obliquely downwards (Figs 1; 2A, E). Outer antennular flagellum 1.3-1.5 times as thick as inner flagellum, when measured near basis.

Antennae (Fig. 2A, B). Length of antennal scale 3.2-3.9 times its maximum width; without spines, setose all around. A small apical segment with five plumose setae separated from basal part by an essentially transverse, though slightly-oblique suture; apical segment broader than long, contributing 4-7% to total scale length. Antennal sympod with forwards-projecting tongue-like, terminally rounded lobe; posteriorly with a broad, terminally weakly-bilobate lobe containing end sac of antennal gland. Peduncle three-segmented, clearly shorter than scale. Basal segment 17-22% length of peduncle, second 40-48% and third 34-38%.

Mouth parts (Figs 2 F–J, 3A). Labrum, labium, maxillulae and maxillae as normal in this genus. Mandibular palp normal, three-segmented. Median segment with normal setae along inner and outer margins. Pars molaris of both mandibles with strong grinding surface. Pars incisivus with 2-3 large teeth, and digitus mobilis with 2-3 large, plus two small, teeth. Pars centralis ('spine row’ in terminology of Tattersall and Tattersall 1951) with 5-7 spiny teeth. Distal segment of maxillula terminally with 7-10 strong, inconspicuously serrated spines, subterminally with transverse row of 4-5 weakly barbed setae. Endite of maxillula with two large, distally spinose setae, and total of 12-17 smaller, smooth or barbed setae.

Thoracopods in general (Figs 3 B–M; 4 A–F; 5A, B). Sizes increase from exopod 1 to 5 or 6 and decrease from 6 to 8. Homologous exopods larger in males than in females. Flagellum of first to eighth exopods with 8, 9, 9, 9, 9, 9, 9, 9 segments in males, or 8, 9, 9, 9, 9, 9, 9, 8 segments in females, respectively, not counting the large intersegmental joint between basis and flagellum. Exopods with basal plate 1.4-2.2 times longer than broad in males, versus 1.8-2.5 in the mostly less broad exopods of females. Lateral expansion distinct in both sexes, outer margin ending in rounded edge. Endopods: for segmental numbers of carpopropodus see diagnosis. Distinct dactylus present in all thoracic endopods. Size of dactylus (Fig. 3 E–M) decreases in order of endopod 2> 1> 3> 4> (5-8). Claws present in endopods 1 and 3-8, but absent in endopod 2. Claws do not differ between sexes.

Maxillipeds (first and second thoracic endopods; Fig. 3 B–F). First endopod with sympod bearing large fields of minute hairs, mainly on its outer half. Inner portions of sympod representing basis of endopod. First thoracic epipod large, leaf-like, without setae or hairs, but with small field of minute scales near insertion with sympod (Fig. 3B). Basis of first endopod with short, conical endite ending in one plumose, basally-thick seta. Basis with additional, large prominent endite, ischium and merus with feebly-projecting endites, carpus with again shorter, almost indistinct endite: these endites densely setose on their inner margins. Large endite of basis strongly hairy, that of ischium weakly hairy only at and near its inner margin, that of merus again less hairy in this position, that of carpus not hairy at all. Dactylus with strong, subapically, bilaterally microserrated claw (Fig. 3E). Basis of second endopod with weakly-projecting, subterminally setose endite (Fig. 3C). Merus slender, slightly longer than combined praeischium and ischium, but distinctly shorter than combined carpopropodus and dactylus. Dactylus without claw (Fig. 3F), bearing only a dense brush of setae (Fig. 3C). Among these setae are 9-13 modified setae, each bearing bilateral series of stiff, partly-acute micro-barbs in their subbasal to median portions (Fig. 3D).

Gnathopods (third thoracic endopod; Fig. 4A, B). Endopod weakly powerful, somewhat subchelate. Its carpopropodus in both sexes comparatively slender among species of Heteromysis , 4.4-5.7 times longer than broad; length 0.9 times that of merus, and 1.0-1.1 times that of ischium. Modified setae on outer margin of carpus as in diagnosis. Claw smooth, powerful, showing only weak curvature; microserrated in subapical portions of only its outer margin (Fig. 3G). Most setae of endopod smooth or barbed to different degrees.

Pereiopods (fourth to eighth thoracic endopods; Figs 3 H–M; 4 D–F; 5A, B) all moderately long and slender. Their claws microserrated on two opposite sides of their subapical portions (Fig. 3 H–M). Fourth endopod with modified setae on outer margin of its carpopropodus, as in diagnosis. Its moderately small dactylus bearing long, thin, weakly-bent claw (Fig. 3H). Fifth to eighth endopods equipped with again smaller dactylus bearing much shorter claw that shows a stronger, distally-increasing curvature (Fig. 3 J–M). Fifth endopod, when stretched, extends well beyond eyes.

Marsupium. Females with large marsupial plates on seventh and eighth thoracopods. Sixth thoracopod with rudimentary oostegite representing a small lobe, on its inner margin with three proximally weakly-barbed setae.

Penes (Fig. 5A). Shape roughly that of rod-like, straight tubes, facing obliquely in anterior direction up to basis of second thoracopod. Each penis very long, stiff, with smooth cuticle; series of three small, barbed setae subterminally on exterior face.

Pleopods (Fig. 4 G–J). Length and structure of pleopods do not differ between sexes, with rod-like exopodal portion and shorter lobe-like endopodal portion. The long seta at inner, terminal edge of endopod smooth or almost smooth, all remaining setae well barbed or plumose. Total length of pleopod 5 about twice (198-214%) that of pleopod 1 (n = 4). This increase is not continuous: starting with pleopods 1 versus 2, the length increase between subsequent pleopods is 22-27%, 3-5%, 5-7%, and 41-65%, respectively.

Uropods (Fig. 5C). Exopods reach with 12-21% of their length beyond endopods and 25-33% beyond telson, endopods 15-18% of their length beyond telson. Exopod length 4.1-5.0 times maximum width, inner margin more strongly convex than outer one. Endopods basally with large statocyst, containing discoidal, in dorsal view slightly ellipsoid, statolith of average size. Statoliths with indistinct fundus and distinct tegmen, composed of fluorite; diameter 130-143 µm (n = 4); statolith formula is 2 + 3 + 1 + (7-12) + (7-12) = 21-30. Uropods with densely setose lateral margins, except for their most basal portions.

Telson (Fig. 5D). Length 1.2-1.4 times ultimate abdominal somite, or 0.8-0.9 times exopod of uropods. Length of telson 1.7-1.8 times maximum width. Laminae of cleft show about half average length of lateral spines. Basal half of telson as well as distal portions of its cleft with smooth margins. Lateral spines with almost equal length, their size not increasing distally.

Colour (Fig. 1). General appearance of living specimens light red. Cornea brown-golden; eyestalks mainly light red, except for a light spot near inner anterior corner and a narrow white ribbon along posterior, dorsal portions of the inner margin of the cornea (best visible in left eye of upper male in Fig. 1B). Red chromatophore centres scattered over eyestalks, antennae, carapace, pleon, uropods, and telson. Transverse double series of chromatophores near posterior margin of each thoracomere 8 and pleomere 1-6; additional chromatophores on pleomere 6. Uropods with chromatophores over entire length. Telson with the greatest density of chromatophores. These colours disappeared within a few weeks of fixation, except for some dark brown pigment in cornea. White ribbon on eyestalks persisted longer than red colour on remaining parts of stalk.

Nauplioid stage (Fig. 3N). Breeding female of 6.4 mm body length carried three nauplioid larvae at late substage 2, length 1.2-1.3 mm (n = 3). The female of 6.1 mm showed 11 naupliods at substage 3, length 1.0-1.1 mm (n = 8). Besides features typical of the respective state of development, one finds small setae near tip of antennulae and antennae and a pair of cercopods flanking end of larval abdomen. Each cercopod bears 10-15 acute spines with apically-increasing size (n = 11). A number of additional very small spines (or bristles) present on terminal tip of body, a few also more anteriorly on ventral face of larval abdomen.

Etymology.

The species name is a noun in genitive singular, adopted from the hermit crab host Cancellus .

Type locality.

Sublittoral marine coastal waters at Roman Rock, off Simonstown, on the False Bay coastline of the Cape Peninsula, South Africa, 34°10.95'S, 18°27.50'E, 20 m depth. H. cancelli sp. n. was only found in a single gastropod shell inhabited by the hermit Cancellus macrothrix Stebbing, 1924, although four other Cancellus specimens and numerous other hermit crabs, mainly Paguristes gamianus (H. Milne-Edwards, 1836), were collected and examined from this and nearby sites.