Hexapleomera cf. robusta ( Moore 1894 )

Edgar, Graham J., 2008, Shallow water Tanaidae (Crustacea: Tanaidacea) of Australia, Zootaxa 1836 (1), pp. 1-92 : 23-29

Hexapleomera cf. robusta ( Moore 1894)

( Figs 10–12 )

Material examined. One male ( QM W25442), one female ( QM W25443), Mon Repos Beach (23.28°S 151.53°E), Qld, attached to test of nesting turtle ( Caretta caretta ), Coll. C. Limpus et al., 12 Nov 1977 .

Description. FEMALE

Body. Observed length to 4.7 mm.

Cephalothorax rounded and narrowing anteriorly in dorsal view, length similar to width and about 28% of body length.

Pereon occupies about half of total length; pereonites with isolated setae along anterio-dorsal and lateral margins; pereonite 1 length about one-fifth width; pereonite 2 1.5 times length of pereonite 1; pereonite 3 1.5 times length of pereonite 2; pereonites 4 and 5 similar, about 1.5 times length of pereonite 3; pereonite 6 about four-fifths length of pereonite 5.

Pleonites together one seventh as long as body; pleonites 1, 2 and 3 with plumose setae on lateral margins, pleonite 1 length about five times width, pleonite 2 slightly shorter than pleonite 1; pleonite 3 three-quarters length of pleonite 2; pleonite 4 and 5 similar, half length of pleonite 3, with simple lateral setae.

Pleotelson about length of pleonites 4 and 5 together; posterior margin with pair of long setae central between pair of short setae, two long plus two or three short setae adjacent to articulation with uropod.

Antennule ( Fig. 10 ). Article 1 about three times longer than wide, with three pappose setae two-fifths distance along margin, several simple and pappose setae distally; article 2 two-fifths length of article 1, with numerous simple setae and pappose seta distally; article 3 about three-quarters length of article 2, with three or four setae distally; article 4 greatly reduced and cap-shaped, with numerous long setae and about six aesthetascs.

Antenna ( Fig. 10 ). Article 1 as long as wide, lacking setae; article 2 length twice width, about twice length of article 1, with single proximal and single distal setae; article 3 half length of article 2, lacking setae; article 4 twice length of article 3, with several long simple setae distally; article 5 three-quarters length of article 4, about twice as long as wide, with several pappose and simple setae distally; article 6 reduced to small terminal cap with numerous long setae.

Mouthparts ( Fig. 11 ). Labrum distally carpeted by fine setules.

Left mandible with large digitate lacinia mobilis, large denticulate seta at base, pars incisiva roundly bifurcated at tip, pars molaris broad and corrugated; right mandible with lacinia mobilis present as peg-like structure, spiniform seta at base, pars incisiva broad and pointed at tip, pars molaris broad and corrugated.

Labium with inner and outer lobes covered by fine setae near distal margins; outer lobe rounded with rounded terminal process.

Maxillule endite with eight large terminal spiniform setae and cluster of fine setules on outer margin near base of spines; palp with five very long setae.

Maxilliped coxa and basis not joined medially; coxa with three long proximo-medial setae; basis with two long distal setae near attachment of palp; endite finely setulose around distal margin, with three spiniform setae on distal margin and two longer circumplumose setae medio-distally; palp with four articles, first as long as wide with two small setae on outer margin, article 2 longer than article 1 with about five long distal setae on inner margin and single seta distal on outer margin, article 3 similar in length to article 2, length about twice width, fringed with two rows of about six long setae along inner margin, article 4 shorter than article 3 and half width, with a seta two-thirds distance along outer margin, three long terminal setae, and two adjacent rows of about six setae distally on inner margin.

Epignath with terminal circumplumose seta and fringed by fine setules..

Cheliped ( Fig. 10 ). Basis longer than wide with single setae near distal dorsal and ventral margins; merus triangular with a seta two-thirds distance along ventral margin and paired setae near basis; carpus length similar to basis, slightly longer than wide, with two setae distal near ventral margin and group of about four setae distal on dorsal margin; propodus slightly longer than carpus, length twice width, fixed finger flattened into anvil-shaped process, with large terminal spiniform seta, about five setae midway along ventral margin, about eight setae near terminal seta along dorsal margin of fixed finger and group of three setae plus a lateral seta near articulation with dactylus; dactylus with row of small setae along ventral margin, unguis short and solid, about one eighth total length of dactylus.

Pereopod 1 ( Fig. 12 ). Coxa barely produced into anterior process with two setae; basis length approximately five times width, with simple seta and pappose seta one quarter distance along margin; merus about one-third length of basis, with small distal seta; carpus slightly longer than merus, with two distal setae; propodus almost twice length of merus, with several distal setae; unguis two-thirds length of dactylus.

Pereopods 2 and 3 ( Fig. 12 ). Similar; basis width one-third length; merus half length of basis, with short spiniform seta and two simple setae distal on ventral margin; carpus half length of merus, with paired spiniform setae and simple setae ventral and dorsal on distal margin; propodus 1.5 times length of carpus; unguis two-thirds length of dactylus.

Pereopods 4 and 5 ( Fig. 12 ). Similar; basis width about half length; merus with a pair of large ventro-distal spiniform setae; carpus with adjacent rows of three or four serrated spiniform setae along distal margin and four long simple setae on dorso-distal margin; propodus length 1.5 times merus length, with several setae along ventral margin and pappose seta plus two long simple setae near distal dorsal margin; unguis merged into claw-like dactylus, with comb-like lateral rows of about six flattened setae on the two sides.

Pereopod 6 ( Fig. 12 ). Similar to pereopod 5 except for propodus with comb-like row of about nine flattened denticulate setae near distal margin, and three long denticulate setae, two long simple setae and a pappose seta distal on dorsal margin; dactylus with lateral comb-like rows of about nine flattened setae.

Pleopods 1–3 ( Fig. 12 ). Similar; basal article with five plumose setae along outer margin and one plumose seta on inner margin; exopod with numerous plumose setae along outer margin; endopod with two plumose setae along inner margin, large terminal circumplumose seta and numerous plumose setae along outer margin.

Uropod ( Fig. 12 ). Four articles, basal article length 1.5 times width, six long setae and one short seta distally, article 2 two-thirds length of basis with several long simple setae and pappose seta distally, article 3 three-quarters length of article 2 with several long simple setae and pappose seta distally, article 4 three-quarters length of article 3 with about five long simple setae and a pappose seta.

MALE. Similar to female, other than

Body ( Fig. 10 ). Cephalothorax pear-shaped, slightly longer than wide, with slight anterior concavity in lateral margin.

Antennule ( Fig. 10 ). Article 1 about five times longer than wide, with three pappose setae one-quarter distance along margin.

Antenna ( Fig. 10 ). All articles more elongate than in female, article 2 over three times as long as wide.

Cheliped ( Fig. 10 ). All articles proportionately wider than in female other than dactylus; propodus length 1.5 times width, about twice length of carpus; dactylus relatively narrow with smoothly-curving ventral margin. More description of propodus/fixed finger shape, please.

Distribution. Mon Repos Beach, Qld; Hexapleomera robusta : Mediterranean, Galapagos, Atlantic coast of North and South America, Mediterranean, Atlantic coast of Africa ( Sieg 1980b). 0–2 m depth.

Remarks. Setation on the appendages of Queensland specimens, including maxilliped, cheliped, pereopods, pleopods and uropods, differed in numerous ways from patterns described by Sieg (1980b) for Hexapleomera robusta . Queensland animals generally possessed more setae, including spiniform setae on the distal margin of the maxilliped endite, three rather than two long setae proximal on the maxilliped coxa, and two rather than a single plumose setae on the inner margin of the pleopod endopod. They also lacked a strong tooth on the dorsal margin of the fixed finger of the male cheliped.

The differences in setation perhaps reflected the size of the Queensland tanaids, which were considerably larger (4.7 mm cf. 2.5-3.5 mm) than those examined by Sieg (1980b). More likely, the disparities reflected geographic differences between Australian populations and those in other regions. Whether these differences are sufficiently marked to warrant specific separation from H. robusta remains uncertain. Sieg (1980b) does not provide information on the locality of specimens he figured, although he apparently examined material from the Atlantic and Pacific Oceans and the Mediterranean Sea relating to three nominal species ( Hexapleomera robusta , Tanais testudinicola Dollfus, 1898 , and Hexapleomera schmidti Dudich, 1931 ), which he regarded as conspecific.

Hexapleomera robusta in its broadest concept is an almost cosmopolitan taxon that occurs in high densities as an ectoparasite on the tests of sea turtles. It can survive and propagate in large marine aquariums, where high population numbers have been reported as harmful to host turtles (Rob Jones, Melbourne Aquarium, pers. comm.).

Figures

FIGURE 10. Hexapleomera cf. robusta. Mon Repos Beach, Qld. Male dorsal, antennules (A1), antennae (A2), chelipeds (CH), pleotelson (Plt). Scale units 0.1 mm (appendages) and 1 mm (dorsal).

FIGURE 11. Hexapleomera cf. robusta. Mon Repos Beach, Qld. Mouthparts figured from male (QM W25442). Labrum (L), left mandible (Mdl), right mandible (Mdr), labium (La), maxillule (Mx), maxilliped (Mxp), epignath (E). Scale units 0.1 mm.

FIGURE 12. Hexapleomera cf. robusta. Mon Repos Beach, Qld. Appendages figured from male (QM W25442). Pereopods 1–6 (P1–P6), pleopod (Pl), uropod (U). Scale units 0.1 mm.