Abyssorchomene De Broyer 1984

Abyssorchomene De Broyer, 1984

(Fig. 1)

Abyssorchomene De Broyer, 1984: 198.— Barnard & Karaman, 1991: 507 (in part).— Lowry & Stoddart 2003: 281 (catalogue).

Type species. Orchomenopsis chevreuxi Stebbing, 1906, original designation.

Included species. Abyssorchomene includes eleven species: A. abyssorum (Stebbing, 1888); A. charcoti (Chevreux, 1912); A. chevreuxi (Stebbing, 1906); A. distinctus (Birstein & M. Vinogradov, 1960); A. gerulicorbis (Shulenberger & Barnard, 1976); A. musculosus (Stebbing, 1888); A. nodimanus (Walker, 1903); A. pelagicus (Birstein & M. Vinogradov, 1960); A. plebs (Hurley, 1965); A. rossi (Walker, 1903); A. scotianensis (Andres, 1983).

Diagnostic description. Antenna 1 peduncle article 1 without anterodistal lobe; accessory flagellum with an elongate article 1 (at least twice as long as article 2) partially covering callynophore. Antenna 2 with brush setae. Mandible molar setose with a triturating surface. Maxilla 1 outer plate a well developed 7/4 crown. Maxilla 2 inner plate slightly to significantly shorter than outer plate. Gnathopod 1 subchelate or parachelate; coxa 1 large, about as long as coxa 2, subrectangular with concave anterior margin or adze-shaped; ischium short (length less than 2 × breadth); carpus compressed; propodus margins subparallel. Uropod 2 inner ramus not constricted. Telson moderately to deeply cleft.

Remarks. Abyssorchomene is most similar to Koroga. Both genera have subchelate first gnathopods with a compressed carpus. However, Abyssorchomene has a moderately to deeply cleft telson (notched in Koroga). Both genera are scavengers with wide-spread distributions.

D’Udekem d’Acoz & Havermans (2012) include A. plebs and A. rossi in the tryphosine genus Pseudorchomene Schellenberg, 1926 based on molecular evidence. Abyssorchomene plebs and A. rossi are morphologically dissimilar to P. coatsi (Chilton, 1912), the type species of Pseudorchomene, Schellenberg, 1926, but very similar to species usually assigned to Abyssorchomene. D’Udekem d’Acoz & Havermans (2012) did not test the molecular affinities of the other ten species of Abyssorchomene against Pseudorchomene before moving these species. Transferring A. rossi and A. plebs to Pseudorchomene confounds the morphological concept of both genera. Therefore based on the maxilla 1 setal-tooth arrangement and the morphology of the first gnathopods we prefer to retain these species within Abyssorchomene until such time as affinities of the entire Abyssorchomene / Pseudorchomene complex have been tested.

Abyssorchomene abyssorum Stebbing, 1888, A. gerulicorbis (Shulenberger & Barnard, 1976) and A. scotianensis (Andres, 1983) are extremely similar morphologically. The only character difference we see is the first coxa which is rectangular in A. gerulicorbis and subtlety adze-shaped in A. scotianensis. We leave them as separate species for the moment. The species recorded as A. abyssorum by Barnard & Ingram, 1990 and by Birstein & Vinogradov, 1960 from the tropical Pacific Ocean look more similar to each other than either does to the A. abyssorum of Stebbing, 1888 from the south-western Atlantic.

Distribution. Cosmopolitan.

Key to Abyssorchomene species

[A. abyssorum, A. gerulicorbis and A. scotianensis are difficult to separate in this key]

1. Gnathopod 1 parachelate............................................................................... 2

- Gnathopod 1 subchelate................................................................................ 3

2. Gnathopod 1 coxa rectangular with straight anterior margin; ischium not enlarged; propodus palm slightly obtuse. Pereopod 7 basis posterior margin with distal concavity........................................................ A. charcoti

- Gnathopod 1 coxa adze-shaped; ischium enlarged; propodus palm transverse. Pereopod 7 basis posterior margin entire.............................................................................................. A. nodimanus

3. Uropod 3 innerramusabout 2/3 aslongasouterramus.................................................. A. plebs

- Uropod 3 inner ramus slightly shorter (reaching base of article 2) or subequal to outer ramus........................ 4

4. Epimeron 3 posteroventralcornersubquadrate.......................................................... A. rossi

- Epimeron 3 posteroventralcornerbroadlyrounded.......................................................... 5

- Epimeron 3 posteroventralcornernarrowlyrounded........................................................ 6

5. Gnathopod 1 propodus palm convex............................................................. A. distinctus

- Gnathopod 1 propodus palm straight........................................................... A. musculosus

6. Urosomite 1 with straight boss not projecting over urosomite 2....................................... A. pelagicus

- Urosomite 1 withroundedbossslightlyprojectingoverurosomite 2............................................. 7

7. Gnathopod 1 coxa weakly adze-shaped. Uropod 3 inner ramus just reaching base of outer ramus article 2... A. scotianensis

- Gnathopod 1 coxasubrectangular. Uropod 3 ramisubequalinlength........................................... 8

8. Gnathopod 2 palmstraight............................................................................. 9

- Gnathopod 2 palmexcavate........................................................................... 10

9. Maxilla 2 innerplatesignificantlyshorterthanouterplate................................................................................................. A. abyssorum (of Barnard & Ingram 1990, Birstein & Vinogradov 1960)

- Maxilla 2 inner plate slightly shorter than outer plate............................... A. abyssorum (of Stebbing 1888)

10. Gnathopod 2 dactyluswithoutsetalbasket......................................................... A. chevreuxi

- Gnathopod 2 dactyluswithsetalbasket......................................................... A. gerulicorbis