Exosphaeroma Stebbing, 1900

Wall, Adam R., Bruce, Niel L. & Wetzer, Regina, 2015, Status of Exosphaeromaamplicauda (Stimpson, 1857), E. aphrodita (Boone, 1923) and description of three new species (Crustacea, Isopoda, Sphaeromatidae) from the north-eastern Pacific, ZooKeys 504, pp. 11-58 : 14-15

Taxon classification Animalia Isopoda Sphaeromatidae

Exosphaeroma Stebbing, 1900

Exosphaeroma Stebbing, 1900: 553, Restricted synonymy. - Bruce 2003: 327.

Type species.

Sphaeroma gigas Leach, 1818; by original designation ( Stebbing 1900).

Remarks.

A diagnosis and comprehensive synonymy was provided by Bruce (2003). The genus occurs in shallow water in all the world oceans and is one of the few sphaeromatid genera extending to southern reaches of the Southern Ocean. Greatest diversity is found in the Southern Hemisphere. The genus has groups of morphologically similar species, including those species close to the type species, and a group of species with a broad rim to the pleotelson ventral margin, while some species have broad uropods and a posteriorly produced pleotelson apex. At present, the relationships between these different species groups remains unassessed.

Exosphaeroma amplicauda (Stimpson, 1857), Exosphaeroma aphrodita and the three new species described herein form a distinct group within the genus Exosphaeroma . This group of species is characterised by a posteriorly produced and somewhat posteriorly depressed pleotelson, with an acute apex, flattened ventrolateral margins, and the posterior margin overriding a shallow exit channel; the uropods are distally wide and the exopod is distally broadly falcate. The dorsum varies from smooth to nodular. Typically mature males of the " amplicauda group" have a large pleotelson and enlarged posterior coxal plates and cannot completely roll up or fold. Some similar species are known from the Southern Hemisphere, including Exosphaeroma alveola Bruce, 2003 (southeastern Australia); Exosphaeroma antikraussi Barnard, 1940, Exosphaeroma kraussi Tattersall, 1913, Exosphaeroma planum Barnard, 1914 and Exosphaeroma varicolor Barnard, 1914 (all South Africa); and Exosphaeroma montis (Hurley & Jansen, 1977) (New Zealand). All other North American Exosphaeroma have an evenly rounded pleotelson, with a narrow ventral margin, and uropods that are not posteriorly wide.

Other Exosphaeroma occurring between Alaska and the Mexican border that are morphologically not closely related to the Pacific west coast species include Exosphaeroma inornata (known from Puget Sound, Washington to central-southern Baja California Norte, Mexico). Exosphaeroma inornata differs from the " amplicauda group" in that Exosphaeroma inornata lacks marked sexual dimorphism. Males mate guard individual females with males clasping and holding females until mating. Exosphaeroma inornata can roll up into perfect balls, and their bodies are unornamented. This distinguishes them clearly from the " amplicauda " clade ( Exosphaeroma amplicauda , Exosphaeroma aphrodita , and the three new species described here).

The type specimens of Exosphaeroma rhomburum (USNM 22573) were borrowed and consist of two specimens from Monterey Bay, neither specimen is an adult male. Richardson’s (1899b: 835) original species description only figures the pleotelson, and she did not note whether the description was based on a male or female. We were not able to further evaluate the status of this species.

In collections from the type locality at Tomales Bay in 2009 for Exosphaeroma amplicauda , we found "family groups" with all life stages (gravid and non-gravid females, subadult males, juveniles, and adult males). These family groups consisted of ca. 10-30 individuals, but in which adult, fully mature males are rare, leading us to conclude that males in these species guard harems rather than guard individual females. For every 10 individuals, we found one, sometimes two, large adult males. We found no evidence for multiple male morphs in these collections [e.g., alpha, beta, gamma males in Paracerceis sculpta as described by Shuster (1989)].