(Figs 7–12)

Type material. Holotype, male, 10 mm, AM P80880, Ferriers Creek, Lizard Island, Queensland, Australia (14°40.05’S 145°27.13’E), detritus and dead wood at outer edge of mangroves (Rhizophora stylosa), mangrove creek, by hand, intertidal, C. Serejo & A. Myers, 25 February 2005 (QLD 1675). Paratypes: female, 11 mm, AM P80881; 9 specimens, AM P70883; 1 specimen, AM P80442; 1 specimen, AM P70924 (QLD 1675).

Additional material examined. 4 specimens, AM P70926, Ferriers Creek, Lizard Island, Queensland (14°39.95’S 145°27.05’E), wood pieces from mangrove creek (Rhizophora stylosa), S. LeCroy, 25 February 2005 (QLD 1677).

Type locality. Ferriers Creek, Lizard Island, Queensland, Australia (14°40.05’S 145°27.13’E), detritus and dead wood at outer edge of mangroves (Rhizophora stylosa), mangrove creek.

Etymology. Named for the Mrs Watson who lived beside the creek named for her son, Ferrier. Both died tragically after being marooned on Howick No. 5 Island in 1881.

Description. Holotype, male, 10 mm, AM P80880.

Head. Eye medium (1/5–1/3 head length) size. Antenna 1 reaching beyond end of antenna 2 peduncle. Antenna 2 up to half body length; peduncular articles narrow; peduncular article 5 longer than article 4. Mandible left lacinia mobilis 4-dentate. Maxilliped palp article 2 without distomedial lobe; article 4 distinct, small.

Pereon. Gnathopod 1 not sexually dimorphic; subchelate; posterior margin of merus, carpus and propodus each with lobe covered in complex setae in both male and female; propodus subrectangular; palm transverse; dactylus subequal in length to palm. Gnathopod 2 sexually dimorphic; subchelate; coxal gill lobate, smaller than gill 3; basis slightly expanded; posterior margin of merus, carpus and propodus each without lobe covered in complex setae; carpus triangular, reduced (enclosed by merus and propodus), posterior lobe absent, not projecting between merus and propodus; palm acute, weakly toothed, incised, lined with robust setae; with bimodal shelf near dactylar hinge; with small midpalmar sinus; posterodistal corner with socket; dactylus subequal in length to palm; dactylus not attenuated distally. Pereopods 2–4 coxae wider than deep. Pereopods 3–7 simplidactylate. Pereopod 3 coxal gill lobate, similar size to gill 4. Pereopod 4 subequal or slightly shorter than pereopod 3; coxal gill slightly lobate, larger than gill 5; carpus similar in length to pereopod 3 carpus, dactylus similar to pereopod 3 dactylus. Pereopod 5 coxal gill lobate, similar in size to gill 6; propodus distinctly longer than carpus. Pereopod 6 not sexually dimorphic, slightly shorter than pereopod 7; coxa posterior lobe inner view posteroventral corner with small cusp, posterior margin (inner view) oblique to ventral margin, posterior lobe without ridge, with 1–2 marginal setae; coxal gill lobate. Pereopod 7 slightly sexually dimorphic; basis lateral sulcus absent, posterodistal lobe absent; distal articles (merus and carpus) slightly expanded; merus posterior margin evenly rounded; carpus subrectangular.

Pleon. Pleopods 1–3 well developed; peduncles not expanded. Pleopod 1 peduncle without marginal setae; biramous, outer ramus subequal in length to peduncle; inner ramus subequal in length to outer. Pleopods 2–3 biramous, outer ramus longer than peduncle. Epimeron 2 subequal in length to epimeron 3. Epimeron 3 posterior margin smooth, without setae, posteroventral corner with small subacute tooth; ventral margin robust setae absent. Uropod 1 not sexually dimorphic; peduncle with 8 robust setae, distolateral robust seta very large (1/3 to 1/2 length of outer ramus), with simple tip, row of ventral setae absent; rami without ventral robust setae; inner ramus subequal in length to outer ramus, with 3 marginal robust setae; outer ramus without marginal robust setae. Uropod 2 not sexually dimorphic; peduncle with 3–4 robust setae; inner ramus subequal in length to outer ramus, with 2 marginal robust setae; outer ramus without marginal robust setae. Uropod 3 peduncle with 2 robust setae; ramus shorter than peduncle, ramus linear (narrowing), ramus without marginal setae, without marginal setae, with 2 apical setae. Telson longer than broad, apically incised, dorsal midline vestigial or absent, with apical robust setae only, telson with 3 robust setae per lobe.

Female (sexually dimorphic characters) based on paratype, female, 11 mm, AM P80881. Gnathopod 2 mitten-shaped; basis slender; posterior margin of merus, carpus and propodus each with lobe covered in complex setae; carpus well developed (not enclosed by merus and propodus); palm obtuse, smooth; dactylus shorter than palm. Pereopod 7 distal articles (merus and carpus) slender. Oostegites longer than wide, weakly setose, setae with multifurcated tips.

Habitat. Among detritus and dead wood in mangroves.

Remarks. Among the species of Microrchestia, M. watsonae appears to be the most unusual. The male second gnathopod looks nothing like those of M. bousfieldi and M. macrochela. The palm has a shallow distal plate and the main palmar area is lined with robust setae. Microrchestia watsonae and M. macrochela both have a small subacute tooth on the posteroventral corner of epimeron 3. Beyond the second gnathopod, differences between species are mainly based on the shape of the posteroventral corner of epimeron 3, and the number of robust setae on the uropods.

Microrchestia watsonae differs from M. similis in the posteroventral corner of pereopod 6 coxa (rounded in M. watsonae and forming a small cusp in M. similis); in the posterior margin of epimeron 3 (without setae in M. watsonae, setae present in M. similis); and in the telson which is longer than broad in M. watsonae and broader than long in M. similis).

Distribution. Australia. Lizard Island, northern Great Barrier Reef.