(Figs 7, 8, Pl. 3H)

Type material. Holotype, male, 14.6 mm, AM P 70663; Watsons Bay, Lizard Island (14°39.41’S 145° 27.14°E), tunicate Styela sp., patch reef, 5 m, J. K. Lowry, 24 February 2005 (QLD 1638). Paratype, female, 12.7 mm, AM P 71221; Casuarina Beach, Lizard Island (14°40.38’S 145°26.69’E), 11 tunicates from rock & rubble, sandy bottom with rubble, algae & sparse seagrass, 2 m, S. LeCroy, 2 March 2005 (QLD 1770).

Additional material examined. 1 male, AM P 70631 (QLD 1621); 2 females, AM P 70589 (QLD 1622); 1 female AM P 70647 (QLD 1634); 1 female, AM P 70662 (QLD 1638); 4 females, AM P 70736 (QLD 1643); 1 male, AM P 70754 (QLD 1645); 1 female, AM P 70761 (QLD 1652); 1 male, AM P 70764 (QLD 1659); 4 females, AM P 70918 (QLD 1681); 3 females, 2 juveniles, AM P 70970 (QLD 1687); 1 female, AM P 70885 (QLD 1692); 1 female, AM P 17028 (QLD 1713); 1 juvenile, AM P 71175 (QLD 1723); 1 female, AM P 71184 (QLD 1730); 2 females, AM P 71100 (QLD 1733); 1 juvenile, AM P 71168 (QLD 1751); 1 female, AM P 71260 (QLD 1757); 1 female, AM P 71230 (QLD 1760); 3 females, 1 male, AM P 71281 (QLD 1770); 4 females, AM P 71377 (QLD 1788); 4 females, 1 male, AM P 71354 (QLD 1789); 1 male, AM P 71380 (QLD 1791); 1 juvenile, AM P 71372 (QLD 1798); 1 female, AM P 71358 (QLD 1803); 1 male, AM P 71427 (QLD 1808); 1 male, AM P 71416 (QLD 1816); 1 female, AM P 71492 (QLD 1819); 1 female, AM P 71441 (QLD 1820); 1 female, 2 juveniles, AM P 71562 (QLD 1823); 1 female, 3 juveniles, AM P 71532 (QLD 1823); 1 female, 1 male, AM P 71564 (QLD 1823); 2 females, AM P 71579 (QLD 1829); 1 female, 1 male, AM P 71522 (QLD 1829); 1 male, 4 females, GCRL2887 (SEL /LZI-2-1); 1 male, 1 female, GCRL2885 (SEL /LZI-2-3); 1 male, 1 juvenile, GCRL2884 (SEL /LZI-3-2); 2 females, GCRL2886 (SEL /LZI-2-7); 2 females, GCRL2888 (SEL /LZI-4-1).

Type locality. Watsons Bay, Lizard Island, Queensland, Australia (14°39.41’S 145° 27.14°E).

Etymology. After the Greek ‘ epidemos ’, meaning ‘ prevalent, common to a large number ’ and referring to the high numbers of this species collected at Lizard Island, Australia.

Description. Based on holotype, male, 14.6 mm, AM P70663.

Head. Head length less than pereonite 1+2, anterior margin rounded, anterodistal margin rounded without cusp, ventral cephalic keel rounded, rostrum small to medium; eyes with 10 or more ocelli, posteriorly concave. Antenna 1 0.3 x body length; flagellum 8-articulate, peduncle width less than 2 x article 2. Antenna 2 0.3 x body length, shorter than antenna 1; flagellum 3-articulate. Mandibles lacking molars; palp 3-articulate, ratio of articles 1-3, 1.0:4.2:2.5, article 2 with 17 – 18 distal setae, article 3 with 2 distal setae, incisors strongly dentate; left mandible lacinia mobilis large, strongly toothed, with 12 accessory setae, right mandible lacinia mobilis small, with 7 accessory setae. Upper lip asymmetrically lobate, anterior margin setose. Lower lip inner lobes fused, bare; outer lobes with moderate gape, anterior margins weakly setose. Maxilla 1 palp 2- articulate with 4 distal setae; outer plate with 7 distal spines and 4 proximal setae. Maxilla 2 inner plate with 5 distal and 3 proximal setae; outer plate with 3 robust and 20 thin distal setae. Maxilliped inner plates fused, apical margin with v-shaped indentation, with short spines, long setae and a dense coating of facial setae; outer plate smooth, reduced, reaching half of palp article 1, with 3 proximal and 3 distal setae and 1 distal spine; palp 4-articulate, article 4 slender, slightly recurved.

Pereon. Coxae 1–4 relative widths 1.0:1.1:0.8:1.4. Gnathopod 1 coxa smooth, bare, anterodistal corner produced, truncate, distal margin straight, posterodistal margin subquadrate, with 1 facial seta; basis slightly constricted proximally, anterior margin with 12 setae, posterior margin bare; ischium bare; carpus and propodus distally tapered; carpus length 8.9 x width, proximal margin with 3 serrate rows, distal margin setose; propodus straight, palm serrate with 6 proximal setae; dactylus smooth, reaching 0.4 x propodus length. Gnathopod 2 coxa length subequal to width, slightly wider than coxa 3, smooth, bare, anterodistally rounded, distal margin evenly rounded, posterior margin slightly concave, facial setae absent; basis widened anterodistally, with 3 distal tubercles, anterior margin with 12 – 14 setae, posterior margin bare; ischium with 3 anterodistal, 3 distal and 2 posterodistal setae; carpus 0.3 x propodus length, curved, distally truncate, spoonlike, anterior margin smooth; propodus posterior margin without spines/serrations, with 1 mediofacial setal row displaced to midline, reaching 0.8 x propodus length, 1 row of 14 submarginal setae, palm convex with 4 major projections; dactylus curved, proximal margin smooth, anterior margin distally subacute, reaching 0.7 x propodus length. Pereopod 3 coxa length 1.5 x width, smooth, bare, anterodistal corner over-riding distal face of coxa 2, not extending below it, anterior margin evenly rounded, distal margin oblique, posterior margin straight, facial setae absent. Pereopod 4 coxa smooth, bare, anterior and distal margins evenly rounded, posterior margin excavate, facial setae absent. Pereopods 5–7 coxa facial setae absent; basis width length ratio 1:1.2, 1:1.2, 1:1.2, posterior margins smooth, bare.

Pleon. Epimera 1+3 bare. Epimeron 2 with 4 ventral setae. Epimeron 3 posteroventral margin weakly produced. Uropods 1–3 relative lengths 1.0:0.8:1.0; inner and outer ramus with robust setae. Uropod 1 peduncle subequal in length with inner ramus. Uropod 2 peduncle 0.8 x inner ramus length; outer ramus 0.7 x inner ramus length. Uropod 3 peduncle 1.3 x inner ramus length. Uropod 1+3 outer ramus subequal to inner ramus length. Telson 2.6 x longer than wide, apex tridentate.

Female (sexually dimorphic characters). Based on paratype, female, 12.7 mm, AM P 71221. Gnathopod 2 carpus not spoon-like, anterior margin serrate; propodus mediofacial setal row less setose; submarginal setal row with 8 setae. Pereopod 3 coxa with marginal setae.

Habitat. Inside branchial chamber of ascidians Polycarpa aurata (Quoy & Gaimard, 1834), Atriolum robustum Kott, 1983 and Styela Fleming, 1822 sp., in algae, coral rubble and the sponge Spirastrella vagabunda (Ridley, 1884).

Remarks. Leucothoe epidemos sp. nov. shares the spoon-like gnathopod 2 carpus with Leucothoe alata J.L. Barnard, 1959, Leucothoe lihue J.L. Barnard, 1970 a, Leucothoe spinulosa Chevreux, 1927, Leucothoe tridens Stebbing 1888 and L. rudicula sp. nov. The profusely displaced mediofacial setal row is found in Leucothoe commensalis Haswell 1879, Leucothoe wuriti Thomas & Klebba, 2007 and Leucothoe thula sp. nov. Leucothoe epidemos sp. nov. differs from L. commensalis in its single mediofacial setal row, rounded ventral cephalic keel and wider pereopods 5 – 7 basis. Leucothoe epidemos, L. wuriti, Leucothoe pollexa sp. nov. and L. thula share the facial seta on coxa 1. This species may be a sister taxon to L. commensalis, which has been reported frequently from this region, with each description showing slight variations. It is likely that L. commensalis is a cryptic species complex due to its isolated lifestyle in ascidian hosts.

Distribution. Australia. Queensland: Lizard Island (current study).