Acidostomatidae, Stoddart & Lowry, 2012

Stoddart, H. E. & Lowry, J. K., 2012, Revision of the lysianassoid genera Acidostoma and Shackletonia (Crustacea: Amphipoda: Acidostomatidae fam. nov.), Zootaxa 3307 (1), pp. 1-34 : 3-4

Acidostomatidae fam. nov.

Diagnostic description. Head exposed, slightly deeper than long or about as long as deep, without cheek notch. Antennae calceoli absent. Antenna 1 with callynophore in male. Antenna 2 peduncular article 3 without distal hook. Epistome and upper lip fused; epistome with proximal portion not produced. Mouthpart bundle conical. Mandible incisors well developed, symmetrical, left convex, smooth, right convex, smooth; left lacinia mobilis absent; accessory setal row with less than 5 robust setae, without distal setal tuft; molar large or small, flap-like, setose to non-setose, or vestigial or absent; palp inserted approximately midanteriorly. Maxilla 1 inner plate with 2 or less apical pappose setae; outer plate with 9–12 setal-teeth in modified 7/4 arrangement, setal-teeth small; palp large, moderate or vestigial, with or without apical robust setae. Maxilla 2 inner plate significantly shorter than (1/2 to 3/4 length) outer plate, inner plate without oblique row of facial setae. Maxilliped outer plate medial setae vestigial or absent, without apical slender setae; palp 4-articulate, article 4 well developed, reduced or vestigial.

Gnathopod 1 simple to weakly subchelate; coxa large (length more than 1.9 × breadth); ischium short (length up to 2 × breadth); carpus short (length up to 2 × breadth); propodus small; dactylus slightly curved. Gnathopod 2 coxa large, subequal in size to coxa 3. Pereopods all simple; distal spurs absent. Pereopod 4 coxa with welldeveloped posteroventral lobe. Pereopod 5 coxa anterior and posterior lobes subequal. Pereopod 6 coxa posterior lobe slightly or much deeper than anterior lobe.

Uropod 2 peduncle with well-developed lateral flange. Uropod 3 biramous. Telson cleft, notched or entire.

Type genus. Acidostoma Lilljeborg, 1865 .

Generic composition. Acidostoma Lilljeborg, 1865 ; Shackletonia K.H. Barnard, 1931 .

Remarks. Acidostomatids have a modified 7/4 setal-tooth arrangement where ST1–ST4 and STA are maintained in an unmodified position, but the remaining setal-teeth are reduced in size and situated down the medial face of the outer plate. Other families with a 7/4 setal-tooth arrangement include the Aristiidae , Hirondelleidae , Lepidepecreellidae , Pachynidae , Scopelocheiridae and Trischizostomatidae . The aristiids, lepidepecreellids and scopelocheirids also have weakly subchelate or simple first gnathopods and only the scopelocheirids also have an unmodified first coxa. Only the acidostomatids have the combination of a 7/4 setaltooth arrangement, a weakly subchelate or simple gnathopod 1 with a large coxa and a conical/subconical mouthpart bundle. Conical/subconical mouthpart bundles appear 12 times within lysianassoid suprageneric groups including four taxa with 7/4 setal-tooth arrangements. We think that this adaptation has most likely evolved independently in each case.

Shackletonia robusta appears to be the least derived of the acidostomatids based on the fully developed 2- articulate palp on maxilla 1, plumose setae on the rami of uropod 3 and a telson that is much longer than broad. Within Acidostoma , A. australis and A. merimbula , both from the South Pacific Ocean, appear to be the least derived taxa in the genus. Acidostoma hancocki , A. molarifera and A. pectinata form a North Pacific species group which all have a posterodistal spine on epimeron 3. Acidostoma ortum , also from the North Pacific, appears to be more similar to the North Atlantic species group, all of which have a vestigial or absent palp on maxilla 1 and all, except A. laticorne , have the incised outer plate on the maxilliped, first described by Dahl (1964).

Distribution. North Polar Sea, North Atlantic Ocean, Mediterranean Sea, North and South Pacific Oceans and Southern Ocean; from shallow continental shelf to abyssal depths.

Key to species of Acidostomatidae

1. Telson notched or emarginate............................................................................ 2

– Telson cleft.......................................................................................... 3

2. Mandibular molar a distinct, triangular flap. Maxilliped outer plate without a subapical notch. Epimeron 3 posteroventral corner produced into a small spine................................................................. A. laticorne

– Mandibular molar vestigial. Maxilliped outer plate with a subapical notch. Epimeron 3 posteroventral corner produced, rounded................................................................................... A. neglectum

3. Maxilla 1 palp large, 2-articulate, as long as outer plate. Uropod 3 rami with plumose setae................... S. robusta

– Maxilla 1 palp small, 1-articulate, vestigial or absent, shorter than outer plate. Uropod 3 rami without plumose setae...... 4

4. Epimeron 3 posteroventral corner produced into a distinct spine................................................ 5

– Epimeron 3 posteroventral corner produced, rounded......................................................... 7

5. Maxilla 1 with small palp. Uropod 1 peduncle with more than 10 dorsolateral robust setae. Uropod 3 rami longer than peduncle................................................................................................... 6

– Maxilla 1 without a palp. Uropod 1 peduncle with less than 10 dorsolateral robust setae. Uropod 3 rami shorter than peduncle........................................................................................... A. hancocki

6. Mandible without molar. Maxilla 1 palp small, 2-articulate. Uropod 2 peduncle with dorsolateral margin weakly serrate................................................................................................ A. pectinata

– Mandible with triangular molar. Maxilla 1 palp small, 1-articulate. Uropod 2 peduncle with dorsolateral margin strongly castellate..................................................................................... A. molarifera

7. Maxilla 1 palp small, with apical robust setae.............................................................. 8

– Maxilla 1 palp absent or vestigial and without apical robust setae............................................... 9

8. Pereopods 5 to 7, merus longer than broad. Telson cleft about 60%.................................... A. merimbula

– Pereopods 5 to 7, merus about as broad as long. Telson cleft about 80%.................................. A. australis

9. Mandible with triangular molar................................................................... A. ortum

– Mandible without a molar............................................................................. 10

10. Maxilla 1 palp vestigial. Uropod 3 rami longer than peduncle........................................... A. obesum

– Maxilla 1 palp absent. Uropod 3 rami shorter than peduncle......................................... A. namibiensis