Austrobilharzia, Johnston, 1917

Hechinger, Ryan F., 2019, Guide to the trematodes (Platyhelminthes) that infect the California horn snail (Cerithideopsis californica: Potamididae: Gastropoda) as first intermediate host, Zootaxa 4711 (3), pp. 459-494 : 468-469

Austrobilharzia sp.

(3. Aust; Figs. 1 , 12–14 )

Diagnosis: Parthenitae. Colony comprised of inactive sporocysts, often loosely distributed in snail gonad, digestive gland, and basal visceral mass regions (often in mixed-species infections). Sporocysts nearly opaque white; up to over 1000 µm long, elongate (length:width ~5:1), sausage-shaped.

Cercaria . Body translucent colorless; oculate; with oral sucker (modified as “anterior organ”) and ventral suck- er; body ~ 194 µm long, ~equal in length to tail; tail forked, with no fins.

Cercaria behavior: Fresh, emerged cercariae remain in water column, swim in relatively long bursts by undulating tail (forming a distinctive, elongate, figure 8).

Similar species: The cyathocotylids (Meap [1] and Smcy [2]) are the only other fork-tailed cercariae. Aust is most readily distinguished from them by having pigmented eyespots. Aust’s inactive sporocysts are readily distinguished from those of Smcy by their much shorter body lengths.

Remarks: The sporocysts and cercariae have not been formally described. We have initiated work to determine how this species is related to other Austrobilharzia species (Brant & Hechinger, unpublished).

This species corresponds to the “schistosome cercaria” of Martin (1955).

This species is frequently observed in mixed-species infections, where it appears to be invading and killing the other species (this is why we have moved it to the top of the interspecific dominancy hierarchy that characterizes the species in this guild [ Hechinger 2010; Huspeni 2000]). These observations are consistent with what Walker (1979) documented for A. terrigalensis (Johnston) in Velacumantus australis (Quoy & Gaimard) . Research is warranted to understand how Aust interacts with heterospecifics.

Nadakal (1960a) presents information on the cercaria eye-spot pigments of this species.

Like all schistosomatids, Aust lacks a second intermediate host and directly infects its final host (birds).

Figures

FIGURE 1. General characteristics of the parthenitae and cercariae of the trematodes infecting Cerithideopsis californica as first intermediate host. Species numbers and codes follow Table 1 and species accounts. Cercariae are all to scale, with additional magnified views of six small species (indicated by dashed lines). Note the oral stylets (presented in right lateral view) for Pruc and Smmi. Parthenitae are not to scale. Scale bars consistently indicate 100 µm.

FIGURES 12–14. Austrobilharzia sp. (Aust). 12, Close-up of an infection (white sporocysts) that has presumably invaded an already established Acanthoparyphium spinulosum (Acsp) colony (orange rediae) in a freshly deshelled, horn snail in sea water. The Aust sporocysts have infiltrated the snail gonadal, digestive gland, and green basal visceral mass regions. Scale bar = 0.5 cm. Base photo credit: Todd Huspeni. 13, Sporocyst, live, with developing cercariae, squashed with heavy coverslip pressure. Note the much smaller heterophyid cercaria that was in this mixed-infection. Ocular micrometer unit = 10 µm. Scale bar = 100 µm. 14, Cercaria, live, under slight coverslip pressure. Scale bar = 100 µm.