Schellitrema, Dronen & Blend & Mohammed & Bannai, 2021

Dronen, Norman O., Blend, Charles K., Mohammed, Essa T. & Bannai, Majid, 2021, Reconsideration of the species assigned to Faustula Poche, 1926 (Digenea: Microphalloidea) with the proposal of five new genera in the Faustulidae Poche, 1926, Zootaxa 5027 (2), pp. 231-253 : 243-244

Schellitrema n. gen.

( Fig. 7 )

Type species: Schellitrema gasterostei ( Schell, 1973) n. comb. Type and only species.

(Syn. Faustula gasterostei Schell, 1973 )

Museum specimens: Holotype USNM 1368020 .

Diagnosis: Body relatively small, somewhat biconic-shaped; anterior extremity broadly rounded, followed by short, narrower isthmus-like neck region; maximum width near midlevel of body; posterior end of body relatively extensive, gradually tapers to relatively rounded point; tegument aspinose. Forebody occupies less than onehalf of body length, no obvious concentric rings present on ventral surface. Oral sucker terminal, bowl-shaped. Ventral sucker pre-equatorial, muscular, somewhat smaller than oral sucker, slightly wider than long. Prepharynx absent. Pharynx elliptical, muscular. Esophagus relatively short. Intestinal bifurcation short distance anterior to ventral sucker. Ceca relatively long, terminate about two-thirds of distance down body from anterior end. Testes 2, multilobed, symmetrical, mainly intercecal, at midlevel of body short distance posterior to ventral sucker. Cirrus sac clavate, median, overlaps ventral sucker dorsally, surpasses it posteriorly by short distance; sac encloses cirrus, moderately long ejaculatory duct with few prostatic glands, short pars prostatica and relatively long, tubular to nearly saccate, unipartite seminal vesicle. Genital pore postbifurcal, immediately anterior to ventral sucker on midline of body. Ovary multilobed, immediately posttesticular on midline of body. Vitellarium in 2 compact clusters, 1 on each side, posterolateral to ventral sucker. Uterus largely posterior to testes, fills most of posttesticular space. Metraterm weakly developed. Excretory vesicle V-shaped, excretory arms extend to level of pharynx. Intestinal parasite of marine teleosts (three-spine stickleback, G. aculeatus Linnaeus [ Gasterosteidae ]; only reported host) off west coast of USA.

Etymology: The genus is named in honor of the late Dr. Stuart Schell (Schelli) in recognition of his many contributions to the study of parasitic helminths and his original description of F. gasterostei (= Schellitrema gasterostei ) within the faustulid-like trematodes (trema).

Remarks: Schellitrema gasterostei has a median genital pore, located immediately anterior to the ventral sucker; a cirrus sac; the ovary immediately posterior to the testes in the hindbody; an unarmed ejaculatory duct and metraterm; testes located immediately posterior to the ventral sucker in the hindbody; and this species is parasitic in fish suggesting placement in the Faustulidae . It is similar to species of Faustula by having a ventral sucker that is near the anterior one-third of body; a median genital pore that opens immediately anterior to the ventral sucker and just posterior to the intestinal bifurcation; a cirrus sac that dorsally overreaches the ventral sucker along the midline of the body and surpasses the ventral sucker posteriorly by a short distance; a posttesticular, distinctly multilobed ovary (composed of some 8 lobes); and a uterus that is posttesticular and largely in the hindbody. Schellitrema gasterostei (Syn. F. gasterostei ) was described from the three-spined stickleback, G. aculeatus (Gasterosteidae) , from Puget Sound, Washington, USA. Unlike species of Faustula , this species has a distinctive, irregularly elliptical, aspinose body with a large terminal oral sucker; a narrower isthmus-like forebody; a broad midlevel region; a tapering, relatively long hindbody (approximately 60% of the body length); a ventral sucker that is smaller than the oral sucker (111–135 vs 149–180); a relatively narrow, clavate cirrus sac; multilobed testes that are in the hindbody and vitelline fields that are in 2 small compact groups, composed of relatively few follicles and mostly mid- to postacetabular being confined to the midlevel of the body or slightly more anteriorly. We agree with Bray (2008) that this species should not be assigned to Faustula . Bray (2008) suggested the possible assignment of this species to Pronoprymna Poche, 1926 ; however, S. gasterostei has an extensively mutilobed ovary rather than the more typical trilobed ovary as described for species of Pronoprymna and the stickleback is an unusual fish host as members of the genus are generally intestinal parasites of clupeiforms. Although it is entirely possible this species does not belong in the Faustulidae , based on the information available we propose Schellitrema to accommodate this unusual species, Schellitrema gasterostei ( Schell, 1973) (see type species description of F. gasterostei [= S. gasterostei ] and fig. 4 of Schell 1973).

Of the 13 previous recognized species assigned to Faustula (WoRMS 2021a) , there are 3 species described in clupeiform fishes from the Ganges River (River Ganga ), India that appear to represent undescribed genera: Gangafaustula makundai n. gen., n. comb., Lingulitrema hilsai n. gen., n. comb. and Varanasifaustula indica n. gen., n. comb. There are a number of characteristics that indicate that these 3 species represent separate genera and should not be retained in Faustula . The most obvious difference is that species of Faustula have a distinctive cirrus apparatus wherein there is a short, usually thick-walled cirrus; a relatively short tubular to chamber-like pars prostatica and a seminal vesicle composed of an elongate, upper narrow canal spiraling throughout the middle onethird of the cirrus sac usually containing minimal amounts of sperm terminating in a wider, more oval, chamber-like sac at the posterior extreme of the cirrus sac containing the bulk of the sperm (see the redescription of F. gangetica by Garner et al. 2019; fig. 3).

Figures

FIGURES 7–8. Diagrammatic illustrations of faustulids (ventral views): 7. Schellitrema gasterostei (Schell, 1973) n. comb. based on Schell (1973); representative of Schellitrema n. gen. 8. Gangifaustula makundai (Agarwal & Verma, 1981) n. comb. based on Agarwal & Verma (1981); representative of Gangafaustula n. gen.