Guide to the trematodes (Platyhelminthes) that infect the California horn snail (Cerithideopsis californica: Potamididae: Gastropoda) as first intermediate host Author Hechinger, Ryan F. text Zootaxa 2019 2019-12-17 4711 3 459 494 journal article 24567 10.11646/zootaxa.4711.3.3 21de8a76-6a04-45d4-8e81-5fb524718c58 1175-5326 3586554 85D81C2D-0B66-4C0D-B708-AAF1DAD6018B Acanthotrema hancocki (Martin) (10. Acha; Figs. 1 , 41–44 ) Diagnosis: Parthenitae. Colony comprised of active rediae, densely concentrated in snail gonad region. Rediae translucent white, grey, weak yellow, or colorless; ~ 500–1000 µm long, elongate (length:width ~4:1 to 10:1), sausage-shaped. Cercaria . Body mostly translucent colorless; oculate; with oral sucker and no ventral sucker; with seven pairs of penetration glands, the bodies of which lie in a relatively compact cluster, anterior to the genital primordium and excretory bladder; body ~ 175 µm long, much shorter than tail (<1/2 length); tail with dorso-ventral fins (originating in middle third of tail length, extending around tail tip) and lateral fins (originating basally, next to cercaria body, and inserting in middle third of tail length). Cercaria behavior: Fresh, emerged cercariae remain in water column, swim intermittently in short bursts, with periods of resting and slow sinking. Similar species: Acha is most reliably and readily distinguished from Euca [11] by the position of the penetration gland bodies, which are readily observable with flattened cercariae at 100x on a compound scope (and even sometimes at the dissection scope). Although Acha does have wider lateral tail fins than Euca on average, there appears to be overlap; so, tail fin width is not a consistently reliable distinguishing trait. Martin (1972) used the flame cell grouping to distinguish Acha from Euca (groups of 3 versus 2, respectively), but the flame cells are difficult to see, requiring leaving specimens on a slide for a while and 1000x magnification. Remarks: Martin (1950b) documented the life cycle and described this species (as Parastictodora hancocki ). He described the mother sporocyst, rediae and cercariae from natural infections, and metacercariae and adults from experimentally infected second intermediate and final hosts. I suspect that cercariae of Acanthotrema hancocki were accidentally pooled with Euhaplorchis californiensis to comprise Maxon and Pequegnat’s (1949) Pleurolophocercous I and pooled with Phocitremoides ovale cercariae to comprise their Pleurolophocercous II. This species has also been referred to as Stictodora hancocki in ecological and evolutionary papers, but, since Lafuente et al . (2000) , the appropriate genus has been Acanthotrema . Mature, ripe colonies comprise ~18% the soft-tissue weight of an infected snail (summer-time estimate derived from information in [ Hechinger et al . 2009 ]). Acha infection causes (stolen) snail bodies to grow over 1.5x faster than uninfected snails ( Hechinger 2010 ). This species has a caste of soldier rediae ( Garcia-Vedrenne et al . 2017 ). Using Acha (reported as Euca, see Euca remarks) from Bolinas Lagoon (central California ), Koprivnikar et al . (2010) performed laboratory experiments examining the effects of salinity, temperature, and pH on cercaria survivorship and activity.