Macrolabrum tangaroa, Błażewicz-Paszkowycz & Bamber, 2012

Błażewicz-Paszkowycz, M. & Bamber, R. N., 2012, The Shallow-water Tanaidacea (Arthropoda: Malacostraca: Peracarida) of the Bass Strait, Victoria, Australia (other than the Tanaidae), Memoirs of Museum Victoria 69, pp. 1-235 : 74-78

Macrolabrum tangaroa sp. nov.

Figures 48–50

Material examined. 1 (J56366), holotype, Stn BSS202 , Western Bass Strait, 39º00.2'S 144º33.9'E, 74 m depth, 23 November 1981, sandy shell, coll. R. S. Wilson .

Description of male (limited by dissection of half of only available specimen). Body ( Fig.48A) typical of a pagurapseudid, small, holotype about 1.85 mm long. Cephalothorax apparently naked, rostrum ( Fig. 49A) convex, smooth. Eyelobes distinguished with anterior pointed apophysis, eyes present as group of black-pigmented ocelli. Epistome not visible dorsally. Pereonites 1, 3 and 5 subequal, about 0.6 times as long as cephalothorax; pereonites 2 and 4 subequal, 1.15 times as long as pereonite 3; pereonite 6 longest, 1.45 times as long as pereonite 3. Pleon with five free subequal pleonites, each pleonite about half as long as pereonite 6. Pleonites 1 and 2 only bearing pleopods. Pleotelson semicircular, about as long as last two pleonites together, with plumose lateral seta and simple posterior seta on each side ( Fig. 50I).

Antennule ( Fig. 49B) proximal peduncle article 2.25 times as long as wide, margins without denticulation or apophyses inner margin with simple setae, outer margin with simple setae and two penicillate setae in distal half; second peduncle article 0.44 times as long as first with simple mesial and distal setae; third article 0.6 times as long as second, fourth article half length of second. Main flagellum of two segments, with single aesthetascs on each segment; accessory flagellum of one segment, distally just reaching distal edge of second segment of main flagellum.

Antenna ( Fig. 49C) with two basal articles fused into wide proximal peduncle article with complex inner denticulation and single inner and outer-distal plumose setae; second article not as long as wide, with small inner distal spine; third peduncle article 1.5 times as long as second, fourth 2.5 times as long as second, both with distal penicillate setae. Flagellum of two very short segments, distal segment with one short and one very long distal setae, longer seta 1.5 times as long as distal three peduncle articles together.

Labrum (not figured) bilobed, rounded, sparsely setose, epistome present, not reaching anterior margin of carapace. Left mandible ( Fig. 49D) with quadricuspid pars incisiva, narrow, bicuspid lacinia mobilis, setiferous lobe with four variously crenulate setae, pars molaris round, blunt, with distal marginal crenulations; palp ( Fig. 49 D') of three articles, basal article with single ventrodistal plumose seta, second article with five ventral plumose setae in distal half, third article with six plumose setae in distal third, these setae progressively longer distally such that proximal seta half length of article, distal seta more than twice as long as article; right mandible not seen. Labium ( Fig. 49G) typically marginally setose, palp with two distal setae. Maxillule ( Fig. 49E) inner endite with four distally-setulose distal setae, no outer apophysis; outer endite with 9 distal spines, outer and inner margins densely setose; palp ( Fig. 49 E') of two articles with distinct articulation, distally with four simple setae. Maxilla ( Fig. 49F) outer margin naked, outer lobe of moveable endite with two subdistal and five distal simple setae, inner lobe with five simple setae; fixed endite outer lobe with five simple, one sabre-like, one trifurcate and two bilaterally denticulate distal spines and subdistal bilaterally denticulate spine, inner lobe with two longer distally denticulate setae and rostral row of 15 setae; one large inner distally denticulate seta. Maxilliped ( Fig. 49H) basis with two inner plumose setae, three inner marginal denticulations, outer margin with small setose apophysis and two fine setae; proximal palp article with coarsely denticulate inner and outer margins and with one inner and one outer plumose seta; second article with coarsely denticulate inner and outer margins extended into large teeth inner-distally, and with six submarginal plumose setae and four plumose setae along inner margin, single outer plumose seta; third article with four setae on slight inner apophysis, each with fine denticulation in distal half; fourth article with six distal and two outer subdistal simple setae, each with fine denticulation in distal half; endite not seen. Epignath ( Fig. 49I) large, marginally densely setose, inner lobes inconspicuous, distal spine setose.

Cheliped ( Fig. 50A) with compact basis 1.2 times as long as wide, dorsally naked, ventrally with proximal seta, mudventral spine and six distal setae; exopodite absent. Merus subtriangular, with ventral plumose setae and ventrodistal denticulate triangular apophysis. Carpus unusually wide, 1.8 times as long as wide, widening distally to form cuff into which reflexed propodus could sit, cuff lined with crenulations; dorsally with sparse proximal plumose and simple setae, ventrally with sparse simple setae. Propodus robust, as long as wide with four ventral setae; fixed finger only half length of body of propodus (“palm”), with three ventrodistal and five dorsal setae, saw-like row of small teeth distally on cutting edge; moveable finger stout, curved, with three distal setae and one seta on cutting edge.

Pereopod 1 ( Fig. 50B) longest pereopod, with stout basis 2.2 times as long as wide, dorsal margin bearing nine plumose setae but no apophyses, two subdistal submarginal outer plumose setae, ventral margin with three simple setae and plumose ventrodistal seta; exopodite present, large, distal article with fifteen plumose setae. Ischium 0.2 times as long as basis, with naked dorsal margin, simple ventral seta and single ventrodistal plumose seta. Merus 0.6 times as long as basis, naked, ventral margin with six plumose setae and nine shorter denticulate setae. Carpus just shorter than merus, with two ventral and three ventrodistal denticulate setae and longer ventrodistal simple seta, dorsodistally with two simple and two denticulate setae. Propodus as long as merus, with three ventral spines and two ventrodistal spines with adjacent simple setae. Dactylus curved, longer than propodus, with fine ventral setae, unguis slender, sharp, 0.6 times as long as dactylus.

Pereopods 2 to 6 similar to each other, each about one-half as long as pereopod 1. Pereopod 2 ( Fig. 50C) basis stout, 1.75 times as long as wide, with two plumose ventral seta; ischium with four plumose ventrodistal setae. Merus, carpus and propodus bearing “sucker-like” spines, generally in three ventral rows, and plumose setae as figured. Merus just shorter than carpus; propodus 0.3 times as long as carpus, with minutely denticulate distal spine; dactylus and unguis not fused, together longer than propodus. Pereopod 3 ( Fig. 50D) with more setae on basis, only three plumose setae on ischium, carpus proportionately longer. Pereopod 4 ( Fig. 50E) with only two plumose setae on ischium, carpus shorter than merus. Pereopod 5 ( Fig. 50F) similar to pereopod 3, fewer sucker-like spines on merus. Pereopod 6 ( Fig. 50G) with only one plumose seta on ischium.

Pleopods ( Fig. 50H) only present on pleonites 1 and 2, biramous, reduced; basis with two dorsal but no ventral plumose setae; exopod with five distal plumose setae, endopod almost circular, with nine distal and inner plumose setae. Uropod ( Fig. 50I) biramous, basis with one simple and one plumose distal setae; endopod longer than basis, of two segments, first segment shorter than basis, naked, second segment with three robust distal setae and one penicillate seta; exopod of two segments, first segment shorter than proximal endopod segment, second segment reaching half length of distal endopod segment, with two distal setae.

Female. Unknown. Etymology. The RV Tangaroa was one of the vessels used on the Bass Strait Survey between 1979 and 1984 (noun in apposition).

Remarks. Unusually for a Macrolabrum species, the epistome of M. tangaroa sp. nov. does not exceed the anterior margin of the carapace (a condition also found in M. distonyx , a species with a three-segmented uropod endopod). The only other Macrolabrum species to have two segments in both uropod rami is M. aenigmaticus (known only from a juvenile), but that species differs from M. tangaroa in having a four-segmented main flagellum and a two-segmented accessory flagellum on the antennule, a propodus on pereopod 2 not shorter than the carpus, and distinct setation on the pereopod 1 basis, inter alia. The extremely wide, almost oval, cheliped carpus of M. tangaroa appears to be unique in the genus, but may represent a sexual dimorphism (males are not known for all species). The very short propodus on pereopod 2 is also an unusual and characterizing feature of this species.

The single specimen of Macrolabrum tangaroa was taken at 74 m depth in the western Bass Strait.