Status of Exosphaeroma amplicauda (Stimpson, 1857), E. aphrodita (Boone, 1923) and description of three new species (Crustacea, Isopoda, Sphaeromatidae) from the north-eastern Pacific

Abstract Exosphaeroma amplicauda (Stimpson, 1857) from the west coast of North America is reviewed and redescribed and revealed to be a group of closely related species. A neotype is designated and the species redescribed based on the neotype and topotypic specimens. Exosphaeroma amplicauda is known only from the coast of California, at Marin, Sonoma and San Mateo Counties. Exosphaeroma aphrodita (Boone, 1923), type locality La Jolla, California and previously considered nomen dubium is taken out of synonymy and re-validated. A further three species: Exosphaeroma paydenae sp. n., Exosphaeroma russellhansoni sp. n., and Exosphaeroma pentcheffi sp. n. are described herein. Sphaeroma octonctum Richardson, 1899 is placed into junior synonymy with Exosphaeroma amplicauda. A key to the Pacific West Coast Exosphaeroma is provided.


Introduction
The Sphaeromatidae is a large family, currently with 99 accepted genera (WoRMS, World Register of Marine Species, Bruce and Schotte 2013) and nearly 700 species. The phylogenetic relationships of the Sphaeromatidea were reviewed by Wetzer et al. 2013, but no family-wide treatment since the time of Hansen (1905) and the much later key of Harrison and Ellis (1991) are available. The most recent comprehensive treatment for the United States is Richardson's (1905) monograph, which to a degree was updated by Kensley and Schotte (1989). The number of described species and genera of North American Sphaeromatidae have slowly increased over the 20 th century but many species remain poorly known and attributed to inappropriate genera. At last count, marine and freshwater sphaeromatids in North America included 21 genera with a total of 67 species (seven species inquirenda, incertae sedis or both).
The North American western coast lies within the East Pacific biogeographic zone, and the Sphaeromatidae are represented by 37 species in 11 genera, six of these regarded as species inquirenda and incertae sedis (see Appendix 1). While some western coast United States species have been described in detail (e.g. Bruce and Wetzer 2004;Carvacho and Haasmann 1984;Espinosa-Pérez and Hendrickx 2002;Hendrickx and Espinosa-Pérez 1998;Wetzer and Bruce 2007) many others remain poorly described, and unrecognizable by modern standards (see Appendix 1).
One such poorly-known North American species is Exosphaeroma amplicauda (Stimpson, 1857). The original description of Exosphaeroma amplicauda is brief with a single postage-stamp sized (1.5×2.0 cm) figure of the dorsum taken from specimens "found adhering to fragments of star-fishes picked up on the beach of Tomales Bay by Mr. Samuels, 6.4 mm long and deposited at the Smithsonian" (Stimpson 1857). Stimpson (1858) later provided a paragraph-long description without additional details.
The species was redescribed by Kussakin (1979) based on material collected from Amchitka Island, Alaska, some 2000 kilometers north of the type locality. Differences between Kussakin's (1979) description and fresh material of what appeared to be E. amplicauda from California, including the type location, prompted a re-evaluation of the species. Reviewing morphological and molecular data, we realize that there is a 'species flock' of five morphologically similar species on the western coast of North America. Such 'species flocks' have been reported for other sphaeromatid genera (e.g. Paracassidina -see Bruce 1994; Oxinasphaera -see Bruce 1997) and other families (e.g. Cymothoidae, see Bruce 1986;Cirolanidae, see Bruce 2004; Aegidae, see Bruce , 2009; Serolidae, see Poore 1987), but this is the first such example in the East Pacific.
We redescribe Exosphaeroma amplicauda from the type locality Tomales Bay (central California coast) and E. aphrodita from San Diego, and describe three new closely related species: E. paydenae sp. n., Aleutians; E. russellhansoni sp. n., Puget Sound, E. pentcheffi sp. n., Palos Verdes Peninsula.

Material and methods
Descriptions are based on the male holotype, female allotype, and topotypic paratypes. Specimens examined have been assigned a USNM or LACM catalog/type numbers. Numbers preceded by "RW" are field and station numbers. Species descriptions were prepared using DELTA (Dallwitz et al. 1997). Setal terminology broadly follows Watling (1989).
Specimens prepared for SEM were cleaned for 10-20 seconds in a Branson 1200 ultrasonic cleaner in a weak solution of Branson GP jewelry soap and distilled water. Specimens were then dehydrated with 100% ethanol. Specimens were placed in solutions of pure ethanol and distilled water in the ratios 2:1, 1:1, 1:2, and finally into 100% ethanol (20 minutes per treatment). Once dehydrated and in 100% ethanol, hexamethyldisilzane (HMDS) was used to replace the ethanol in the specimens. Specimens were transferred through ethanol and HMDS solutions in the following ratios 2:1, 1:1, 1:2 and finally into 100% HMDS (20 minutes per treatment). Specimens were transferred from the final 100% HMDS to fresh HMDS and allowed to evapo-rate overnight. Specimens were mounted on carbon conductive tabs and coated with gold/palladium using an Emitech K550x sputter coater (Quorum Technologies, LTD, Kent, UK) and imaged using a Hitachi S-3000N variable pressure SEM (Hitachi, Troy, MI) at the LACM.
Drawings were made with the aid of a camera lucida and illustrations were electronically "inked" with Adobe Illustrator CS6. Whole body illustrations were made with a Wild M5D stereo dissecting scope. Appendages were illustrated by dissecting off the appendage and placing them in glycerol on a depression slide and then imaged using a Nikon Labophot-2 compound scope. Specimens were measured by tracing their dorsal surface along their longitudinal axis with the aid of a camera lucida. A scale bar in the same plane as the specimens allowed calculation of total body length. All lengths reported were mesured in this fashion and may slightly overestimate total body length because pereonites and pleonites are expanded in this position. The lengths given in the "Material Examined" are of the largest specimen of each species and sex. Not all specimens were measured. If a length is provided and multiple specimens were present in a lot, the length refers to largest specimen. In all species mature males appear larger than females, but body lengths for mature adults are similar. Males in all species have much broader uropods than females, which contributes to this illusion. Large sexually mature males tend to be rare compared to females and subadults. Gravid females are rare. Smaller non-gravid individuals cannot be sexed. Females of the different species are virtually indistinguishable and cannot be confidently assigned to a species without an accompanying male. It appears that the largest males guard harems. No individual male-female mate guarding was observed (as occurs in Exosphaeroma inornata Dow, 1958 which also occurs on the Pacific west coast). All species described herein occur in aggregates either under rocks or amongst dead barnacle tests.
We provide dorsal and lateral line drawings of all males for each species. We also provide dorsal and lateral SEMs of both males and females of each species.

Key to the north-eastern Pacific species of Exosphaeroma of the North American West Coast
This key is based on adult ♂ characters. Also note that weak pereon tubercles are visible only with SEM and not necessarily evident with light microscopy -e.g., compare   (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), E. 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); E. antikraussi Barnard, 1940, E. kraussi Tattersall, 1913 planum Barnard, 1914 and E. varicolor Barnard, 1914 (all South Africa); and E. 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 E. inornata (known from Puget Sound, Washington to central-southern Baja California Norte, Mexico). E. inornata differs from the "amplicauda group" in that E. inornata lacks marked sexual dimorphism. Males mate guard individual females with males clasping and holding females until mating. E. inornata can roll up into perfect balls, and their bodies are unornamented. This distinguishes them clearly from the "amplicauda" clade (E. amplicauda, E. aphrodita, and the three new species described here).
The type specimens of E. 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.
E. russellhansoni sp. n. is characterized by: pereonite 5-6 each without ornamentation, pereonite 7 with weak median process ( Figures 9A, B; 23A, D). Appendix masculina distal end curving mesially, apex weakly hooked mesially, length 11.4 basal width ( Figure 12B). E. amplicauda is strongly sexually dimorphic; females lack dorsal tubercles on pereonites 1-7. Overall for all species in this 'species flock' the males have a larger pleotelson and uropods. Weak pereon tubercles are visible only with SEM and not necessarily evident with light microscopy. Tubercles visible with light microscopy are figured in the line drawings (compare Figures 1 and 21).
We searched all probable museum collections for Stimpson's type specimens, but to no avail (see Acknowledgements). It is highly likely that the type specimens are lost. The original and subsequent description (Stimpson 1857(Stimpson , 1858 do not allow for definitive identification of the species. There are five morphologically similar species in the northeast Pacific. A neotype is here designated to stabilize the use of the name Exosphaeroma amplicauda (Stimpson, 1857) and conserve Stimpson's concept for the species.
We borrowed the types of Sphaeroma octonctum Richardson, 1899 (USNM Cat. No. 22574); Richardson (1899) noted that there were five specimens from the type locality, Monterey Bay). We received only four specimens-three had been previously dissected, some with pleopods removed, and only one specimen was entire. None of these specimens are adult males, and these specimens are indistinguishable from female Exosphaeroma amplicauda from Tomales Bay. We place Sphaeroma octonctum into junior synonymy with Exosphaeroma amplicauda.  Antennula peduncle article 1 length 1.5 width, anterior medial margin with palm setae absent; article 2 length 1.1 width, inferior distal margin with palm setae absent; article 3 length 2.6 width; flagellum with 9 articles ( Figure 6B). Antenna reaching posterior margin of pereonite 3, peduncle article 1 with fine, simple setae on superior margin; flagellum with 11 articles ( Figure 6A).
Color. Without chromatophores. Preserved specimen pale cream. Remarks. Exosphaeroma paydenae sp. n., unlike other Exosphaeroma sp. in this 'species flock', lacks strong sexual dimorphism. Males have overall larger pleotelson and uropods than females. E. paydenae sp. n. is morphologically most similar to Exosphaeroma russellhansoni sp. n. E. paydenae sp. n. can be identified by: pereonites 1-7 without tubercles; pleon with one anterior weak tubercle on either side of longitudinal axis, one posterior weak tubercle on either side of longitudinal axis; pleotelson dorsal surface without ornamentation ( Figures 5A, B; 22A, B).
Exosphaeroma russellhansoni sp. n., in contrast to E. paydenae has only one weak tubercle on either side of longitudinal axis of its pleon; pleotelson dorsum, with 2 small anterior tubercles (Figures 9A, B; 23A, D). Weak pereon tubercles are visible only with SEM and not necessarily evident with light microscopy. Tubercles visible with light microscopy are figured in the line drawings (compare Figures 5 and 22). Kussakin (1979) provided new figures for what he considered to be specimens of E. amplicauda from Alaska. In his description he wrote "one sample (three specimens) from Alaska was examined from the collections of the Zoological Institute, Academy of Sciences of the USSR." We here recognize the Alaska specimens as E. paydenae sp. n., which does not overlap in occurrence with species from further south, all described herein.
Etymology. This species is named to honor LACM Trustee and long supporter of science at the Natural History Museum of Los Angeles County, Joan Payden. She is thanked for her gracious philanthropy which in part supported ARW as an undergraduate student researcher. ARW's research experience describing and redescribing the Exosphaeroma along our coast piqued his interest in marine isopods and launched his career in Crustacea at the LACM. Antennula peduncle article 1 length 1.2 width, anterior medial margin with 1 palm seta; article 2 length 1.2 width, inferior distal margin with 3 palm setae; article 3 length 2.9 width; flagellum with 9 articles ( Figure 10B). Antenna reaching medium margin of pereonite 2, peduncle article 1 with numerous fine simple setae on anterior posterior margin; flagellum with 13 articles ( Figure 10A).
E. amplicauda is distinguished by: pereonites 5 and 6 with one weak median tubercle, and one weak lateral tubercle; pereonite 7 with weak median process and paired lateral tubercles (Figures 1A, B; 21A, D). Appendix masculina distal end curving mesially, straightening at distal tip, length 15.4 basal width ( Figure 4B). E. russellhansoni sp. n. is strongly sexually dimorphic; females lacking dorsal tubercles on pereonites 1-7. Weak pereon tubercles are visible only with SEM, but not necessarily evident with light microscopy, and therefore are omitted from line drawings (compare Figures  9 and 23).
Distribution. Washington, Puget Sound and San Juan Island.
Etymology. Named to honor Russell Kenneth Hanson, ARW's only maternal uncle who has shaped the person Adam is today by so graciously sharing with Adam his insatiable curiosity, life-long pursuit of perfection and tireless work ethic.  Description of male. Body length 1.8 width; pereonites 5-6 each with 7 longitudinal rows of strong tubercles, pereonite 7 with strong median process with 3 lateral tubercles ( Figures 13A, B; 24A, D). Pleon with 1 medium tubercle on posterior margin, on either side of longitudinal axis (Figures 13A, B; 24A, D). Pleotelson length 0.85 width, dorsal surface with 3 strong medial tubercles on either side of the longitudinal axis, with 1 strong medial tubercle between the longitudinal axis and lateral margin, pleotelson covered with numerous, additional, small tubercles; ventrolateral ridge extending posteriorly 0.80 of total length, with long setae (Figures 13A, B; 24A, C, D).
Description of female. Body length 2.3 width; pereonites 2-6 each with 7 longitudinal rows of strong tubercles, pereonite 7 distomesial margin convex with strong median process, and 3 lateral tubercles ( Figure 24E, F). Pleon with 1 posterior strong tubercle on either side of longitudinal axis ( Figure 24E, F). Pleotelson length 0.61 width, dorsal surface with 3 strong medial tubercles on either side of the longitudinal axis, with 1 strong medial tubercle between the longitudinal axis and lateral margin, pleotelson covered with numerous, additional, small tubercles ( Figure 24E, F). Uropod exopod proximolateral margin rolled; endopod length 3.6 width, extends past exopod, dorsal surface covered with numerous small tubercles, mesial margin without setae ( Figure 24E, F).
Size. Largest ♂ 6.8 mm, largest ♀ 4.6 mm. Colour. No chromatophores: preserved specimen pale buff, whitish. Remarks. Exosphaeroma pentcheffi sp. n. unlike the other Exosphaeroma species in this 'species flock' lacks strong sexual dimorphism and is unique in that females shares the same dorsal ornamentation as males; males differ from females in having slightly stronger tubercles, longer pleotelson and longer uropods. Females of E. pentcheffi sp. n. are the only females of this 'species flock' that can reliably be identified at the species level. E. pentcheffi sp. n. males can be identified by: pereonites 5 and 6 having 7 longitudinal rows of strong tubercles, pereonite 7 with a strong median process with 3 lateral tubercles; pleotelson dorsum with 3 strong medial tubercles on either side of the longitudinal axis, with 1 strong medial tubercle between the longitudinal axis and lateral margin, pleotelson covered with numerous, additional, small tubercles ( Figures  13A, B; 24A, D, E, F). Weak pereon tubercles are visible only with SEM and not necessarily evident with light microscopy. Tubercles visible with light microscopy are figured in the line drawings (compare Figures 13 and 24).
Distribution. California, Los Angeles County, Palos Verdes Peninsula.
Size. Largest ♂ 8.3 mm, largest ♀ 8.7 mm. Color. No chromatophores: preserved specimen pale buff, whitish. Remarks. Exosphaeroma aphrodita can best be identified by: pereonite 5 without ornamentation, pereonite 6 with one lateral weak tubercle, pereonite 7 with weak median process, and paired weak lateral tubercles; pleotelson dorsum with one anterior median strong tubercle and two weak medial tubercles. E. aphrodita is strongly sexually dimorphic; females lack dorsal tubercles on the pereonites. Weak pereon tubercles are visible only with SEM and not necessarily evident with light microscopy. Tubercles visible with light microscopy are figured in the line drawings (compare Figures 17 and 25). Exosphaeroma aphrodita, considered nomen dubium by Brusca et al. (2007), is here revalidated. Pearl Lee Boone described this species and several other isopods and tanaids without providing figures (Boone 1923, pp. 147-156). The original description states that "the type and additional material were collected at La Jolla, California and are in the collections of the United States National Museum." We examined all of the USNM material available. We conclude that the species is valid.
Distribution. California, San Diego-La Jolla.    Systematics grant DEB-0129317, and Giar-Ann Kung is thanked for her help with the operation of the NHM SEM (funded by DBI-0216506). Gary Poore taught ARW the fine art of electronic inking. Phyllis Sun with her digital expertise helped keep drawing progress moving forward, and Jean Pongsai is thanked for her help preparing the final figures. Our gratitude goes to volunteer par excellence, Jim Cline, for his decade long dedication to isopods, help with sample sorting, and his invaluable contributions to the isopod image gallery (http://isopods.nhm.org/images) which these authors and colleagues around the world have come to rely on.