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Volume 81, 1953
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Studies on the New Zealand Amphipodan Fauna No. 5. Pleonexes lessoniae, a New Species of the Family Amphithoidae*

[Read by title before Otago Branch on Tuesday, June 9, 1953; received by Editor, June 22, 1953.]


Pleonexes lessoniae, a new species of amphipod of the Family Amphithoidae, is described from Island Bay, Wellington. This is the first species of this family recorded from New Zealand A key is given to genera of the family.

Introduction and Acknowledgments

Until recently, no amphipods of this family had been recorded from New Zealand. However, in some algae collected from Island Bay, Wellington, several amphipods were noted on Lessonia variegata and passed over to me by Dr. J. G. Gibbs for examination. Two of these specimens proved to belong to the genus Pleonexes, and are described below.

I wish to express my thanks to Dr. Gibbs for making this material available, and to Prof. L. R. Richardson, under whose supervision this work was carried out at Victoria University College.

Family Amphithoidae Stebbing
Amphithoidae Stebbing, 1899: 211
Ampithoidae Stebbing, 1906: 631.

“Head, lateral lobes not very prominent. Sideplates regular; 4th with hind margin not excavate; 5th with broad front lobe, as deep as 4th. Antenna 1, with 3rd segment short; accessory flagellum wanting or small. Lower lip with front lobes deeply notched. Mandibular palp stout, slender or wanting; molar usually well-developed. Mouthparts otherwise as in Aoridae. Gnathopods 1 and 2 not simple, 2nd usually the larger, usually larger in male than in female, and more or less differently shaped. Peraeopods 1 and 2 glandular. Peraeopod 5 the longest. Uropods 1–3 biramous. Uropod 3 with short rami, the outer uncinate. Telson simple.”

[N.B.—To include the genus Exampithoe Barnard (1925), the family definition has been widened by omitting the phrase “Peraeopod 3 reverted”.]

Key to Genera of Amphithoidae
Based on Stebbing (1906) and Barnard (1940).
1 Mandible with palp 2
   Mandible without palp Sunamphitoe

[Footnote] * This study is part of an investigation carried out at the Zoology Department, Victoria University College, Wellington, with the and of a New Zealand University Research Fund Fellowship.

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2. Antenna 1 without accessory flagellum 3
   Antenna 1 with accessory flagellum 6
3. Peraeopods 3–5: 6th segment not apically widened to form distinct palm 4
   Peraeopods 3–5; 6th segment apically expanded to form distinct palm 5
4 5th peraeopod enormously expanded, flattened, oai-like Macropisthopous
   5th peraeopod not enormously expanded Ampithoe
5. Sideplates shallow, telson without hooks Exampithoe
   Sideplates fairly deep, telson with 2 hooks Pleonexes
6. Uropod 3, outer ramus with 1 hook Amphithoides
   Uropod 3, outer ramus with 2 hooks Paragrubia
   Uropod 3, outer ramus with 3 hooks Cymadusa

As will be seen from this key and from comparison with the description and figures which follow, the specimens are distinguished as Pleonexes by their possession of a mandibular palp, lack of an accessory flagellum, the apical widening of the 6th segment of peraeopods 3–5. and the presence of hooks on the telson. They are further separated from Exampithoë, in that the latter has the dactylos of peraeopod 3 reverted. They appear to come closest to Ampithoe from which they are distinguished by the apical broadening of the propod of peraeopods 3–5.

The only recent paper dealing with the genus Pleonexes is one by Miloslawskaja (1939) which I have been unable to obtain. It deals with P. gammaroides. Chevreux and Fage (1925) list two species of Pleonexes, P. gammaroides and P. ferox The Island Bay specimens appear to belong to neither of these but come closer to P. gammaroides.

The distribution of these two species, as also of the three doubtful species which Stebbing (1906) lists, is confined to the northern hemisphere, with the exception of P. dubius from Mauritius, in the Indian Ocean, and this Stebbing (1888) considers more likely a species of Sunamphitoë Barnard has recorded four genera from South Africa, Cymadusa, Macropisthopous, Ampithoë and Exampithoë. Sunampithoë has been recorded from New Guinea (Ruffo, 1948). Sheard (1937) lists Ampithoe and Grubia species from Australia, most of which Pirlot (1939) places in the synonymy of Cymadusa filosa. But none of these genera can be made to include the Island Bay specimens. Probably the discontinuity of Pleonexes is due to lack of collecting in the intervening areas.

It will be noticed that there are variations in the spelling of the root form of the family name and generic names formed from it. It is regrettable that all three variants must be accepted, unless one is to disregard the rules of nomenclature.

Genus Pleonexes Bate
Stebbing, 1906: 642.

“Like Ampithoë, except that in peraeopods 3–5 the 6th joint is subchelately widened at the apex. Front lobes of lower lip bifid. Mandibular palp narrow, 3rd joint setose only at apex Peraeopods 1 and 2 have the 2nd joint rather broadly oval.”

There are 2 accepted species of this genus (excluding P. lessoniae), and 3 doubtful ones.

Pleonexes lessoniae n.sp. (Text-figs. 1, 2.)

Description of Male.

Length, 9 mm.; depth, 2 mm.; width, 2 mm. Eyes absent. Colour in spirit, white.

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Text-fig. 1—Pleonexes lessoniae, n. sp. 1—Antenna 1, ♂ 2—Antenna 2, ♂. 3—Maxilliped. 4—Maxilla 1. 5—Maxilla 2. 6—Upper lip. 7—Lower lip. 8—Left mandible, male. 9—Right mandible, male 10—Left mandible, female 11—1st pleopod and epimeral plate 12—2nd pleopod and epimeral plate. 13—3rd pleopod and epimeral plate. 14—Coupling spine from pleopod peduncle 15—Uropod 1. 16—Uropod 2. 17—Uropod 3. 18—Uropod 3, dorsal margin of outer ramus 19—Telson, side view in male 20—Telson, dorsal view in female.

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Antennae. First: Length, 3 ½ mm. Flagellum longer than peduncle, of 23 successively longer and narrower segments, each with pair of setae on superior angle, group of setae and 2 long flaccid sensory setae on inferior; accessory flagellum absent. Peduncle, 1st segment slightly longer and wider than 2nd; 3rd ½ length 2nd; a few surface setae, several on end margins. Second: Length, 3 mm. Flagellum shorter than peduncle, of 10 or 11 segments, the proximal 3 apparently in state of division; segments with 4 groups of setae on end margins, terminal segment tufted. Peduncle, 2nd segment small, gland-cone small; 5th segment 4 times and 4th 3 times length of 3rd, which is broader than long, has numerous setae on surface and inferior margin; 4th and 5th with groups of setae on margins and surface.

Mouthparts. Upper Lip: Distally setose; lateral margins medially notched. Lower Lip: Inner lobe almost as long as outer but much narrower, heart-shaped, distally bristled. Distal margin of outer lobe more or less horizontal; outer distal angle produced to short, sharp bristled lobe; inner distal angle to less obvious but more extensively bristled lobe; margin bristled between these lobes and outside outer angle. Outer proximal angle produced as largely inwardly and backwardly projecting curved mandibular process. First Maxilla: Outer plate the larger; the whole maxilla more like normal mandible. Outer plate narrows to end group of 10 strong, toothed spines; palp of 2 segments, 2nd segment twice length 1st, 3 or 4 end setae, palp not reaching as far as outer plate does. Inner plate small, subrectangular, barely reaching base of palp, single seta midway along inner margin. Second Maxilla: Inner plate the narrower, inner and end margins of both fringed with long plumose setae. Mandibles: Cutting edge, upper article a long incised plate, lower a small somewhat fan-shaped incised plate; molar process very weak, round and small, guarded proximally by 4 or so spines. Left mandible has spine row of 5 toothed spines, right has 6. Palp, 3rd segment missing, seems to arise on inner face rather than outer. Maxilliped: Palp 4-segmented. Inner plate subrectangular, reaching a little past merus base, strong spine on inner distal angle, long plumose setae on end and inner lateral margins. Outer plate ovate, reaches a little past propod base; inner margin has row of 14 setose stout teeth right to end; set back from them a row of about 5 medium-length setae, a few setae on inner margin proximally below teeth; distal ⅓ of outer margin has 9 medium-length plumose setae. Inner distal angles of basos and subrectangular ischium have 4 plumose spines. Merus proximally wide, sub-triangular, outer margin convex, 2 plumose setae on angle, inner margin sinuous. Carpus inner margin convex, with strong rows of plumose setae, twice length of own outer margin and as long as merus outer margin. Propod slightly narrower, margins convex, outer as long as merus outer margin. Inner and end margins of propod with long finely-combed spines; 2 or more on outer surface; group of plumose setae on propod outer distal angle. Curved dactylos has strong end nail; ½ propod length, surface has combs of minute bristles.

Gnathopods. First: Sideplate subrectangular, ventral angles rounded, a strong seta on posterodistal angle, short setae on ventral margin. Basos proximally constricted, width ⅓ length, single seta on anterodistal angle, convex posterior margin has about 6 long plumose setae medially Ischium narrower, as long as basos is wide, plumose setae on posterodistal angle. Merus subtriangular, posterior margin as long as ischium, with long plumose setae distally; anterior margin contiguous with proximal ⅓ of carpus posterior margin. Carpus subtriangular,

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⅔ basos length, plumose setae on distal angles. Propod subrectangular, length more than 3 times width, longer than carpus, anterior margin has 4 groups of plumose setae; posterior has long single and grouped plumose setae. Palm transverse, ½ propod width, plumose setae either side, a single spine on one side at inner base of dactylos, a similar spine at end of palm on other. Dactylos longer than propod is wide, more than twice length of palm; inner margin has at least 9 sharp serrations, marked off distally by a strong seta; surface has row of many minute bristles. Second: (Left side) Subchelate. Sideplate trapezoid; a large seta at posterodistal angle, small setae ventrally. Basos proximally constricted, greatest width about ⅔ length; sinuous anterior margin has 6 or so single spines, convex posterior has about 5 long plumose setae medially. Ischium much narrower, subsquare, long setae on posterodistal angle. Merus as large as ischium, anterior margin contiguous with proximal ½ of carpus posterior margin; posterior margin convex, almost right-angled, row of long plumose setae with cup-shaped bases on distal ⅓, small combs of minute setae just above them. Carpus subtriangular, as wide as basos, as wide as long; anterior margin convex, distal ½ and anterior surface with many long plumose setae; posterior free margin lobed, lobe proximally has area of fine combs of minute setae; end and margins have long plumose setae which are so finely plumose that setae appear simple. Propod ovate; as long and as wide as basos; oblique sinuous palm defined by large stout spine at end; margins and surfaces have rows of long finely-plumose setae except for area down the middle. Posterior free margin of propod less than ½ length of anterior margin. Right gnathopod: Palm much shorter, concave, with distinct sharp angle, defined by angle and spine. Dactylos over-reaches palm, which is ½ propod width, inner margin has large strong serrations forming teeth.

Peraeopods. First: Sideplate ovate, ventral margin rounding to anterior, small setae ventrally, posterior margin straight. Basos almost oval, greatest width about ⅔ length; posterior margin has 3 or 4 long setae ½ along; a few minute setae on angle. Ischium subrectangular, posterior margin as long as merus and ¼ basos length; merus as wide distally as long, anterior margin slightly convex, anterodistal angle produced in narrow process ½ along carpus with 4 or so setae terminally; posterior margin has setae distally. Carpus slightly shorter and narrower than merus, 1 or 2 small setae anterodistally, posterior margin has long setae. Propod ⅓ basos length, tapering to dactylos; long setae medially and distally on anterior margin and distal ⅔ of posterior. Stout curved dactylos ⅔ propod length. Second: Similar, shorter. Third: Sideplate, ovate anterior lobe has convex ventral margin; ovate posterior lobe extremely small. Basos ovate, almost circular, 1 or 2 setae on anterior margin, short single setae around posterior; distal posterior surface almost reaches merus. Ischium small, 2 or 3 setae on anterodistal angle. Merus nearly twice ischium width, width ½ length; proximally constricted, posterodistal angle produced bluntly downwards a little, setae on margins. Ovate carpus not quite merus length, much narrower, width ½ length, 1 or two setae ½ along each margin, several on angles. Propod subchelate, as long as merus, widening distally to form palm, distal width nearly ½ length, 2 or 3 groups of setae on margins. Palm less than ½ propod width, anterodistal angle with spine, produced a little distally; several long setae immediately below palm; strong curved finger much longer than propod is wide, greatly overlapping palm. Fourth: Basos anterior margin concave, posterior margin straight, narrowing distally; greatest width ⅔ length; posteroproximal angle bluntly rounded

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Text-fig. 2.—Pleonexes lessoniae, n.sp. 1—Gnathopod 1, male. 2—Gnathopod 1, female, propod. 3—Gnathopod 2, male, right propod 4—Gnathopod 2, male, left gnathopod. 5—Gnathopod 2, female, last 4 segments. 6—Peraeopod 1, male. 7—Peraeopod 2, male. 8—Peraeopod 3, male. 9—Peraeopod 4, male. 10—Peraeopod 5, male.

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and produced slightly upwards; a few small setae on margins, spine on posterodistal angle. All segments much longer in proportion to width than Pr. 3; otherwise similar. Fifth: Longer than Pr. 4; basos and other segments proportionately narrower; otherwise similar.

Epimeral Plates. Ovate, small. First and second almost semicircular.

Pleopods: Inner ramus slightly the longer, rami longer than peduncle, of about 16 plumosely-setose segments; 2 arrow-headed coupling spines on peduncle, 1 to 6 plumose spines on each margin.

Uropods. First: Rami subequal, ½ peduncle length. Peduncle outer dorsal margin has 3 to 6 short strong seta-tipped spines distally; broken row of setae along inner margin proximally; spine on ventrodistal angle. Outer ramus widest medially, 4 or 5 seta-tipped spines dorsally, 3 short spines and 1 long one on end; ramus margins convex; inner ramus dorsally naked, about ½ width of outer, upper margin concave, 1 long and 2 short spines at end. Second: As long as 1st; in situ reaching past 1st and 3rd. Peduncle stout, over twice length outer ramus; 8 or 9 seta-tipped spines dorsally; produced in small lobe superodistally. Outer ramus the longer, 4 or 5 seta-tipped spines dorsally, 1 long and 3 short spines at end; margins convex so ramus widens proximally, narrows distally; dorsal and end margins minutely toothed most of length. Inner ramus ½ width of outer, upper margin concave; 1 short, 1 long spine and a seta at end. Third: Inner ramus more than ½ length outer; both much less than ½ peduncle length. Peduncle broad, dorsal margin has 3 or 4 groups of long sparsely-plumose setae; ventral margin convex proximally, 2 or 3 plumose setae at ventrodistal angle; series of fairly long plumose setae on inner distal margin along base of inner ramus. Inner ramus sub-circular, about 9 plumose setae distally; outer ramus, ventral margin convex, 2 large uncinate spines terminally, dorsal margin straight with about 9 curved strong conical teeth, these the most prominent of combs of similar teeth on either side of margin and extending to end, those furthest from dorsal margin the smallest. Telson: Two large uncinate spines at end; telson in side view ovate, almost spherical; 2 small setae dorsal to, and at base of, spines; in dorsal view telson subtriangular, narrowing to the terminal spines which give telson anchor-like appearance when flattened by mounting; slightly notched between spines.

Description of Female.

Length, 5 ¾ mm.; depth, 1 ¾ mm.; width, 1 ¾ mm.; carrying at least 9 ova.

Antennae. First: The longer, about 15 segments, length, 3 ¾ mm. Second: Reaching ⅔ along flagellum of 1st; length, 3 mm.

Gnathopods. First: As in male, carpus a little broader comparatively. Second: Sideplate, anterior margin rounding more broadly to ventral than in male and destroying quadrate appearance; posterior slightly concave. Carpus and propod differ slightly from male, most noticeably in lack of profuse setation on anterior margins, there being only a distal tuft on carpus and about 5 rows of up to about 6 setae on propod. Carpus about ½ propod length; palm oblique, straight, defined by spines, like that of right gnathopod of male more than left.

Locality. Island Bay, Wellington, on Lessonia variegata, coll. J. G. Gibbs, ⅛/50.*

Type Slides 90, male; 91, female. Personal collection.

[Footnote] * I have since recorded this species from Otago Harbour —D E. H.

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I have been able to obtain the original description of only one of the doubtful species listed by Stebbing (1906: 643), that of P. dubius which is given by Stebbing (1888: 167) in a resume of Templeton's original paper (1836). Stebbing makes it clear that P. dubius does not agree with my specimens—e.g., it differs in having a minute accessory flagellum which Templeton described but did not figure. Papers describing P. validus (Czern.) and P. virescens (Stimpson) are not available. P. ferox Chevreux, described and figured by Chevreux and Fage (1925: 336, figs. 345), differs very obviously in the gnathopods, the male 2nd gnathopod having the palm defined by a large tooth, and in the telson which is “arrondi, avec 2 paires de soies submarginales et 2 petites épines crochues”. In most features, especially the telson, P. lessoniae comes close to P. gammaroides, but from Sars' figures (1895: plate 207) it is evident that there are differences in the 1st maxilla palp which has the 2nd segment exceedingly long in P. gammaroides; in the lower lip which has the outer distal angle very strongly produced distally; and in the 6th segment of peraeopods 3–5, which has a very distinct spine on the palm near the base of the dactylos in P. gammaroides. As well, both P. ferox and P. gammaroides possess eyes; P. lessoniae appears to have none. Accordingly, I am describing these specimens as a new species, distinguished particularly by the nature of the 1st maxilla palp, the telson, the 6th segment of peraeopods 3–5, and the absence of eyes.

Literature Cited

Barnard, K. H., 1925. Contributions to the Crustacean Fauna of South Africa. No. 8. Further Additions to the list of Amphipoda. Ann. S. African Mus. vol. 20: 319–380, pl. 34.

— 1940. Contributions to the Crustacean Fauna of South Africa. XII. Further Additions to the Tanaidacea, Isopoda and Amphipoda, together with Keys for the Identification of the hitherto Recorded Marine and Freshwater Species Ann. S. African Mus. vol. 33 (5): 381–543, text-figs. 1–35.

Chevreux, E. and Fage, L., 1925. Amphipodes Faune de France, vol. 9:1–488, text-figs.

Miloslawskaja, N., 1939. Zum Studium der Amphipoda Gammaroidea des Schwartzen und Azovschen Meeres. Trav. Stat. Sci. nat. Karadagh. 5: 69–151, figs.

Pirlot, J. M., 1939. Amphipoda dans: Resultats scientifiques des Croisières du Navire-école Belge “Mercator.” Mem. Mus. Hist. nat. Belg. (2), 15: 47–80, 7 figs.

Ruffo, S., 1948. Studi sui Crostacei Anfipodi. XVII. Gli Anfipodi del Museo Civico di Storia Naturale di Genova. Ann. Mus. Civico Storia Nat. Genova. vol. LXIII: 205–217, fig. 1–3.

Sars, G. O., 1895. An Account of the Crustacea of Norway. 1. Amphipoda. Text and plates.

Sheard, K., 1937. A Catalogue of Australian Gammaridea. Trans. Roy. Soc. S. Austr. vol. lxi: 17–29.

Stebbing, T. R. R., 1888. Amphipoda. Report “Challenger” Expedition. vol. 29.

— 1899. Revision of Amphipoda. Ann. Mag. Nat. Hist. ser. 7 (4): 205–211.

— 1906. Amphipoda Gammaridea. Das Tierreich. Lief. 21, pp. i-xxxix, 1–806, text-figs.

Templeton, Robert, 1836. Descriptions of some undescribed exotic Crustacea. Trans. Entom. Soc. Lond. vol. 1 (3): 185–194.