Siriella brevicaudata species group

Diagnosis

Anterodorsal margin of carapace rather short, not protruding beyond tip of subrostral process. Telson rather short, less than 1.1 times as long as last abdominal somite and less than 2.5 times as long as wide. Telson anterior part with more than three (up to six) lateral spiniform setae. Antennular peduncle segment 3 of female with one medial seta. Appendix masculina conical. Male antennule inner flagellum normal, not dilated or meandering in proximal part. Labrum with short anterior spine, less than 0.3 times as long as rest of labrum. Pereopodal endopods about equally long and rather strong; anterior endopods slightly stronger than posterior. Pseudobranchia of male pleopods 2–4 spirally coiled; pleopod setae not modified. Proximal segment of uropodal exopod armed only with small number (three to six, rarely eight) of distolateral spiniform setae. Medial spiniform setae of uropodal endopod about equally long or slightly increasing in length distally, not forming groups of short and long setae.

Comparison

The Siriella brevicauda group is distinguished from other groups of the genus Siriella by a shorter labrum spine, which is in certain species very small and barely visible, while in others is rather welldeveloped, but never longer than 0.3 of the rest of the labrum (0.37–1.0 in other Siriella species groups). Species of the group also have a shorter telson, which is as long as or typically slightly shorter than the last abdominal somite, while in some species it can be slightly longer (1.1) than the last abdominal somite (never shorter than the last abdominal somite in other groups; 1.0–1.8). The telson is also less than 2.0 times as long as wide in the majority of brevicaudata group species, while it can be slightly more in some species (up to 2.5 in S. bassi sp. nov.); this value is never less than 2.0 in other groups of the genus. The brevicaudata group is most similar to the brevirostris subgroup of Siriella thompsonii group, from which it differs, apart from the labrum anterior spine length, by (1) a larger number of telson anterolateral spiniform setae (three to six as opposed to one or two in the brevirostris subgroup) and (2) pereopodal endopods being rather strong and nearly similar in length (slender and with middle pereopodal endopods clearly longer than anterior and posterior ones in the brevirostris subgroup).

Distribution and habitat

West Indo-Pacific (Figs 1–2). Shallow-water, not deeper than 100 m (upper sublittoral).

Remarks

We modified the original definition of the group given by Murano & Fukuoka (2008) after the inclusion of six more species into the group. In particular, we modified the description of the anterodorsal margin of the carapace, because in S. gibbosa, S. occulta sp. nov., S. bassi sp. nov., S. spinula, S. talbotae sp. nov. and S. vincenti it is angular, as opposed to broadly rounded in other species. The telson is not always less than twice as long as broad in the anterior part, sometimes being slightly more than twice as long as broad in S. bassi sp. nov., S. spinula, S. talbotae sp. nov. and S. vincenti, and quite variable in this characteristic between species. In most of the other groups of the genus Siriella, the spiniform setae of the uropodal endopod are different in length along the medial margin, forming groups of shorter and longer ones, which is not the case for the brevicaudata group. However, a simpler pattern of uropodal endopod armature is not unique for the brevicaudata group. Additional characters common for all species of the group include: telson anterior armature and antennular setation and shape of processus masculinus and flagellae, which have different states in some species from other Siriella groups.

Composition

The group currently consists of 12 species: S. bassi sp. nov., S. brevicaudata, S. gibbosa, S. hanseni, S. lacertilis Talbot, 2009, S. lingvura, S. muranoi sp. nov., S. occulta sp. nov., S. spinula, S. tabaniocula sp. nov., S. talbotae sp. nov. and S. vincenti.

Key to the species of the Siriella brevicaudata group

1. Last (terminal) pair of posterolateral spiniform setae of telson less than twice as long as subterminal. Uropodal endopod with more than nine spiniform setae, occupying entire or almost entire inner margin of ramus.............................................................................................................2

– Last (terminal) pair of posterolateral spiniform setae of telson three times as long as subterminal. Uropodal endopod with three spiniform setae, occupying proximal part of the ramus ....................... ...................................................................................................................... S. gibbosa Ledoyer, 1970

2. Telson apically with slight emargination..........................................................................................3

– Telson apically straight or convex....................................................................................................4

3. Telson with 4 to 7 posterolateral spiniform setae, with last pair 0.4–0.5 times as long as telson ............................................................................................................ S. brevicaudata Paulson, 1875

– Telson with 9 to 13 posterolateral spiniform setae, with last pair 0.2 times as long as telson ............. ................................................................................................................................. S. occulta sp. nov.

4. Anterodorsal margin of carapace angular, apically rounded, with straight or slightly concave lateral sides. Terminal spiniform seta on medial margin of uropodal endopod reaching apex of ramus .................................................................................................................................................5

– Anterodorsal margin of carapace smoothly or nearly smoothly rounded. Terminal spiniform seta on medial margin of uropodal endopod not reaching apex of ramus...................................7

5. Telson rather long and narrow, 2.35–2.5 times as long as wide (at maximum anterior width); its last (terminal) pair of posterolateral spiniform setae 0.07–0.08 times as long as telson. Male carapace with cephalic tubercle .............................................................................. S. bassi sp. nov.

– Telson <2.1 times as long as wide; its last (terminal) pair of posterolateral spiniform setae 0.11–0.14 times as long as telson. Male carapace without cephalic tubercle...............................6

6. Carapace without cephalic tubercle. Terminal pair of posterolateral spiniform setae of telson longer than subterminal pair in male ......................................................... S. spinula Panampunnayil, 1995

– Female carapace with cephalic tubercle. Terminal pair of posterolateral spiniform setae of telson shorter than or as long as subterminal pair in both sexes ................ S. vincenti W.M. Tattersall, 1927

7. Telson as long as or longer than last abdominal somite. Last (terminal) pair of posterolateral spiniform setae of telson <0.13 times as long as telson.........................................................................8

– Telson shorter than last abdominal somite. Last (terminal) pair of posterolateral spiniform setae of telson> 0.13 times as long as telson............................................................................................9

8. Last three to six pairs of posterolateral spiniform setae of telson nearly equal in length. Anterior spine of labrum very short, barely visible, <0.1 times as long as rest of labrum. Uropodal endopod with 10 to 13 medial spiniform setae ............................. S. hanseni W.M. Tattersall, 1922

– Last pairs of posterolateral spiniform setae of telson gradually increasing in length, with terminal pair clearly longer than subterminal. Anterior spine of labrum rather long, 0.25–0.3 times as long as rest of labrum. Uropodal endopod with 17 or 18 medial spiniform setae ...... S. talbotae sp. nov.

9. Cephalic tubercle absent. Telson with 5 or 6 anterolateral spiniform setae. Uropodal exopod 3.6–3.7 times as long as wide ........................................................................... S. lingvura Ii, 1964

– Cephalic tubercle present. Telson with 3 or 4 anterolateral spiniform setae. Uropodal exopod 3.9–4.2 times as long as wide........................................................................................................10

10. Last (terminal) pair of posterolateral spiniform setae of telson 0.25 times as long as telson. Last terminal and subterminal spiniform setae of telson about twice as long as preceding posterolateral. Uropodal exopod proximal segment 1.4–1.5 times as long as distal segment .................... ...................................................................................................................... S. lacertilis Talbot, 2009

– Last (terminal) pair of posterolateral spiniform setae of telson 0.13–0.19 times as long as telson. Last terminal and subterminal spiniform setae of telson 1.2–1.4 times as long as preceding posterolateral. Uropodal exopod proximal segment 1.7–1.9 times as long as distal segment.............11

11. Terminal pair of posterolateral spiniform setae of telson about as long as subterminal pair and 0.13–0.14 times as long as telson. Labrum spine short, but well-established, about 0.2 times as long as rest of labrum ....................................................................................... S. muranoi sp. nov.

– Terminal pair of posterolateral spiniform setae of telson clearly longer than subterminal pair and 0.18–0.19 times as long as telson. Labrum spine very short, barely visible, <0.1 times as long as rest of labrum ................................................................................. S. tabaniocula sp. nov.