Suggested Common Name: Seapills
Number of subordinate taxa: 690 species in 3 families worldwide (but see notes), with 105 species in 3 families in our area.
Etymology: after Sphaeroma Bosc, 1802, see that account for more information. The common name is a combination of "sea" and "pillbug", refering to the conglobulation abilites of most members of this superfamily.
Taxonomic History: Sphaeromatidea Latreille, 1825
Description: (modified from Bruce & Poore, 2003) Body frequently conglobulating. Maxilla 1 endopod as long as exopod with 3-4 setae. Pereonites 2-7 dorsal coxal plates not separated by suture (except in Ancinidae). Pereopod 7 ventral coxal plates not meeting at middle. Pleon with 1 free segment (pleonite 1), 3 segments to fused to each other (pleonites 2-4) and pleonite 5 fused to telson, or with more pleonites fused. Pleopod 5 with scale patches (except in Tecticipitidae and extralimital Paravireia). Uropod exopod free or absent; endopod fused to protopod.
Type taxon: Sphaeromatidae Latreille, 1825
Notes: This charasmatic group of isopods can immediately be identified by their compact (often conglobulating) stature and the fusion of the uropod endopod to the protopod. Most species occur in shallower waters and many species accessible to naturalists are brightly colored, making them one of the most commonly found marine isopod groups. The three families are fairly easy to distinguish from each, with most commonly-encounted species occuring in Sphaeromatidae. Along with the 3 families, there is a incertae sedis genus, Paravireia, which has a disjunct range with 2 species around New Zealand and 1 species in the Canary Islands. It can be distinguished from all other Sphaeromatoids (and most other isopods) by its complete lack of uropods.
The overall biogeography of Sphaeromatoidea has never been fully discussed. All modern genera (besides Paravireia) are arranged into 3 families: Ancinidae, Tecticiptidae and Sphaeromatidae. The biogeography of Ancinidae is different between its two constituent genera, with Ancinus being endemic to the tropical and temperate Americas, while all but 1 species of Bathycopea are endemic to the north Pacific (B. typhlops appears to be subcosmopolitan in deeper waters). The monogeneric family Tecticeptidae is entirely endemic to the north Pacific, with 1 species occurring along the west coast of North America and the other 10 species occurring in an area between the Sea of Japan and the Bering Sea. Sphaeromatidae is a cosmopolitan family, with representatives present in every ocean except the Arctic Ocean, with a distinct concentration in diversity in the central Indopacific. Recent genetic analyses (see Wetzer, Bruce & Pérez-Losada, 2018) have revealed that the north Pacific-endemic Gnorimosphaeroma is the most basal genus in the family, with most other species being organized into 2 subfamilies (Sphaeromatinae and Dynameninae), both with a clear southern hemisphere centers of diversity. From this, a potential center of origin for Sphaeromatoidea can be reconstructed in the North Pacific, with Ancinus potentially representing an early radiation south and through the Central American Seaway, while another line representing Sphaeromatinae + Dynameninae (all sphaeromatid genera besides Gnorimosphaeroma) reaching Australasia, where they rapidly diversified and spread across the planet through both the Tethyan Seaway and the Southern Ocean. More research, mainly in taxon discovery and genetic phylogeny, is needed to fully figure out the biogeography of Sphaeromatoidea, as the placement of many genera remains undetermined.
Based on the morphology of Seroloidea, it appears that subchelate pereopod 1 is an plesiomorphic trait in Sphaeromatoidea. This would indicate that the ambulatory state of pereopod 1 in Sphaeromatidae is a derived trait in relation to the rest of the superfamily. Alternatively, the subchelate pereopod 1 could have evolved in parallel in Seroloidea and Ancinidae + Tecticiptidae, with Sphaeromatidae branching off before the evolution of a subchelate pereopod 1 in Sphaeromatoidea, however, this path seems less likely than the first one.
Subordinate taxa: Ancinidae, Sphaeromatidae, Tecticipitidae
Key to families
1 a. Head fused to pereonite 1, often with suture separating two; pleopod 1 uniramous or biramous; uropods uniramous; maxilla 1 medial lobe with 1 simple seta; maxilla 2 medial lobe reduced; maxilliped endite short-rectangular, lacking apical spines --> Ancinidae
b. Head free; pleopod 1 biramous, rarely uniramous; uropods biramous (with articulating exopod) or rarely uniramous; maxilla 1 medial lobe with 3-4 spines; maxilla 2 medial lobe with plumose setae; maxilliped endite elongate with plumose apical spines --> 5
2 (1) a. Pereopod 1 prehensile, ♂ pereopod 2 prehensile (ambulatory in ♀); head anterior margin extended, hiding antennae bases; mandible incisor flat, blade-shaped, with 1 single posterolateral tooth, lacinia straight or distally finely serrate with single small lateral tooth, spine row absent, molar long, blade-like; branchial folds only present on pleopod 5, not on pleopod 4; coldwater Pacific --> Tecticeptidae
b. Pereopods 1-2 ambulatory; head anterior margin various; mandible incisors and lacinia multicuspid, teeth large, spine row present, molar flat, grinding or crushing, or a chitinized lobe; branchial folds present on at least the endopods of both pleopods 4-5 or entirely absent; widespread --> Sphaeromatidae
Brandt, A., & Poore, G. C. (2003). Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships. Invertebrate Systematics, 17(6):893-923.
Bruce, N. L. (1993). Two new genera of marine isopod crustaceans (Flabellifera: Sphaeromatidae) from southern Australia, with a reappraisal of the Sphaeromatidae. Invertebrate Systematics, 7(1):151-171.
Wetzer, R., Bruce, N. L., & Pérez-Losada, M. (2018). Relationships of the Sphaeromatidae genera (Peracarida: Isopoda) inferred from 18S rDNA and 16S rDNA genes. Arthropod Systematics & Phylogeny, 76(1):1-30.
Published: Jan 1, 2023
Updated: Mar 3, 2023