Robustura
Gnanamuth, 1954

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Suggested Common Name: Beach-Cerberuses
Number of subordinate taxa: 33 species globally, 9 species in the region
Etymology: []. Common name refers to this genus' subcosmopolitan occurance in beaches, which is also unique to this genus (but see Notes).
Taxonomic History: Microcerberus Karaman, 1933 (in part); Robustura Gnanamuthu, 1954; Coxicerberus Wägele, Voelz & McArthur, 1995
Size Range: most species generally reach about 1mm, although some (such as R. minutus) are smaller
Description: Head rostrum absent. Pereonites 2-4 anterior margin 4-lobed, with medial lobes formed from tergites and lateral lobes formed from the fused coxal plates. Pereopods 2-4 coxae fused to tergites as coxal plates. Pleopod 1 absent. ♂ pleopod 2 protopod elongate-rectangular; endopod elongate, apically bifid or trifid. Pleopod 4 covered by pleopod 3, biramous, lacking articulations; protopod very short. Uropod exopod shorter than endopod, frequently minute.
Type taxon: Robustura predatoris Gnanamuthu, 1954
Notes: While Robustura is the sole cosmopolitan genus in Microceberidea, all consituent species are local endemics. Most species are poorly known, with many not being recorded past their type descriptions, although it appears that many species are common or even abundant within their ranges, with the lack of reports most likely occuring due to the difficulty of finding individuals of this genus. Robustura can be distinguished form other genera in having the coxae of pereopods 2-4 fused with the tergites as coxal plates, a feature unique among the entire suborder. The genus is also distinct ecologically, being one of the very few marine Microcerberid genera worldwide. The only other marine genera known are the monotypic Isoyvesia (see that account for more information) and the undescribed genus "Microcerberus" remyi (a poorly described Mediterranean endemic).
Robustura is further divided into at least 4 species groups by the shape and details on the ♂ pleopod 2 endopod. Two of these groups, the mirabilis group and the remanei group, are known to occur in our area. The mirabilis group contains 8 species and appears to be mainly endemic to the Caribbean, with only 3 species occuring elsewhere (mexicanus in the tropical East Pacific, tabai on the Canary Islands and boninensis in Japan). Conversely, the remanei group appears to be subcosmopolitan, with only 3 species out of the total 15 occuring in our area. The other two described groups have quite different ♂ pleopod 2 endopods: the anfindicus group has an elongate endopod with simple subparallel margin with an apical cannula grouped with 1-2 other lobes, while the machadoi group has the cannula partially free but complexly lobed (especially on the medial margin) and the tip of the endopod body being serrate along with also being complexly lobed. The anfindicus group is endemic to the Indopacific while the machadoi group is an southern hemisphere amphiatlantic group (Baldari & Argano, 1984).
It appears that niche partitioning may occur in some members of this genus. The species littoralis, mirabilis and renaudi occur in the same general region but have been reported from fairly different habitats. See those species for more information.
For a while this taxon was refered to Coxicerberus Wägele, Voelz & McArthur, 1995, with Robustura remaining an obscure synonym. However, since Robustura was described over 40 years prior to Coxicerberus and is still a valid name (i.e. not being a junior homonym, a nomen nudum, or similar), it has priority over Coxicerberus and therefore becomes the valid name for this taxon.

Subordinate taxa: Robustura abbotti, Robustura littoralis, Robustura mexicana, Robustura minuta, Robustura mirabilis, Robustura nunezi, Robustura renaudi, Robustura simplex, Coxicerberus syrtica

Key to species
1 a. ♂ pleopod 2 endopod usually simple, lamellar, not lobed or few-lobed, cannula entirely free, running along entire length of endopod and frequently far exceeding endopod (Fig. 1) (remanei group) --> 2
b. ♂ pleopod 2 endopod complex with lobes and processes, cannula partially or entirely fused with endopod (Fig. 2) (mirabilis group) --> 4

2 (1) a. Pereonite 2-4 coxal plates lateral projections shorter than medial projections; Lucayan Archipelago --> Robustura renaudi
b. Pereonite 2-4 coxal plates lateral projections much longer than medial projections --> 3

3 (2) a. Antenna 1 6-segmented; intertidal in California and Washington--> Robustura abbotti
b. Antenna 1 5-segmented; subtidal around the Lucayan Archipelago --> Robustura littoralis

4 (1) a. Pereonites 2-4 tergal lobes apparently  absent; pereonites 1-2 fused to head, pereonite 1 lacking sutures, pereonite 2 with lateral sutures defining segment; W Mexico-- > Robustura mexicanus
b. Pereonites 2-4 tergal lobes present; pereonites 1-2 free; Caribbean --> 5

5 (4) a. ♂ pleopod 2 endopod medial lobe broad, curved to point laterally --> Robustura minutus
b. ♂ pleopod 2 endopod medial lobe narrower, directed medially or posteriorly --> 6

6 (5) a. ♂ pleopod 2 endopod lateral lamella free, immensely extended into a hooked appendage slightly shorter than the apical and medial lobes, proximoapical lobe lanceolate, about as long as distoapical lobe (although set lower) and pointing posteriorly --> Robustura syrticus
b. ♂ pleopod 2 endopod lateral lamella not as above, fused to margin of apical lobed, proximoapical lobe shorter or directed anteriorly, longer or shorter than distoapical lobe --> 7

7 (6) a. ♂ pleopod 2 endopod apical lobe entire (not divided into sublobes), extremely elongate and reflexed medially, seperating from straight posteriorly-directed medial lobe by half the total length of the endopod, lateral lobe not conspicously constricted medially --> Robustura simplex
b. ♂ pleopod 2 endopod apical lobe divided into two lobes (proximoapical lobe inconspicous in R. nunezi), not elongate and dividing from medial lobe deep in the apical half of the endopod, lateral lobe conspicously constricted medially --> 8

8 (7) a. ♂ pleopod 2 endopod medial lobe seperated from apical lobe by a fairly deep conspicous notch, appearing to jut out laterally from the apical lobe, proximoapical lobe longer than distoapical lobe, seperated by a notch and rounded until the proximally-pointed acute tip --> Robustura mirabilis
b. ♂ pleopod 2 endopod medial lobe seperated from apical lobe by a very shallow convex notch, making the tip appear to be somewhat entire with a concave margin, proximoapical lobe shorter than distoapical lobe, appearing as a small acute tooth basal to the distoapical lobe --> Robustura nunezi

Sources

Baldari, F., & Argano, R. (1984). Description of a new species of Microcerberus from the South China Sea and a proposal for a revised classification of the Microcerberoidea (Isopoda). Crustaceana, 46(2):113-126.

Chappuis, P. A., & Deboutteville, C. D. (1956). Études sur la faune interstitielle de Iles Bahamas récoltée par Madame Renaud-Debyse. I Copépodes et Isopodes. Vie et Milieu, 7(3):373-396.

Coineau, N. & Botosaneanu, L. (1973). Isopodes interstitiels de Cuba. Resultats des Expeditions Biospeologiques Cubano-Roumaines a Cuba. Academy of the Socialist Republic of Roumania, 191-222.

Wägele, J. W., Voelz, N. J., & Vaun McArthur, J. (1995). Older than the Atlantic Ocean: discovery of a freshwater Microcerberus (Isopoda) in North America and erection of Coxicerberus, new genus. Journal of Crustacean Biology, 15(4):733-745.

Published: Feb 15, 2023