Parorchis, Nicoll, 1907

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 : 476-478

Parorchis sp.

(9. Pasp; Figs. 1 , 35–40 )

Diagnosis: Parthenitae. Colony comprised of active rediae, densely concentrated in snail gonad region with dispersion into basal visceral mass. Rediae translucent white to colorless, often with prominent pigmented gut; when filled with cercariae, rediae appear opaque white; ~ 1000–2000 µm long, oblong to elongate (length:width up to ~6:1), often tapers in width gradually toward anterior and posterior ends, with posterior appendages that are often not pronounced.

Cercaria . Body opaque white; non-oculate; with oral and ventral sucker; with indistinct row of collar spines; with body spines covering much of tegument; with long esophagus that bifurcates just anterior to ventral sucker; body ~ 425 µm long, ~equal in length to tail; tail with distal gland (tip appears invaginated).

Cercaria behavior: Fresh, emerged cercariae remain in water column, lengthen body and swim by rapidly ventrally folding body with tail extended (often in a somewhat jerky fashion), and will often encyst on dissection dish or in pipette during transfer.

Similar species: Pasp is most readily distinguished from the only other philophthalmid (Clmi [8]) by having the long esophagus anterior to gut branching. Pasp also has tegumental spines, but these are not always readily observable. Like Clmi, Pasp is easily separated from the himasthlids by having a distal tail gland.

Remarks: This species corresponds to “ Cercaria cerithidia 2” of Hunter (1942). Martin (1972) included this species in his key, asserting it was Parorchis acanthus , which had been reported from European and East Coast North American birds, with first intermediate host infections in neogastropod snails in eastern North America (e.g., Lebour 1914, Stunkard and Cable 1932). The use of taxonomically disparate first intermediate hosts suggests that P. acanthus represents a wide-spread species complex. So too, does careful consideration of adult morphologies ( Dronen & Blend 2008), which suggests that there actually may be no P. acanthus in North America. We have two cryptic species in California horn snails, as indicated by analysis of mitochondrial CO1 sequences [ Huspeni 2000, unpublished thesis]). The cercariae of one cryptic species appears to be larger than the other ( Huspeni 2000), but we have not yet confirmed how to distinguish them in practice. It may be that one species has more prominent body spines and an esophagus that branches just anterior to the ventral sucker, while the other has less prominent spines with a shorter esophagus, branching ~midway from pharynx to ventral sucker (unpublished observations). Given all the above, it seems best to abandon referring to this species as P. acanthus , versus Parorchis sp.

Mature, ripe colonies comprise ~26% the soft-tissue weight of an infected snail (summer-time estimate derived from information in [ Hechinger et al. 2009]).

This species has a caste of soldier rediae (noted in Hechinger et al. (2011b) and carefully documented in Garcia- Vedrenne et al. [2016]).

Nadakal (1960b) presents information on the pigments of the rediae and cercariae of this species.

As part of one of the first studies documenting the syncytial nature of trematode integuments, Bils and Martin (1966) examined the fine structure and development of the tegument for the rediae and cercariae of this species.

Fingerut et al. (2003a) presents information on the relationship between cercaria emergence and temperature for this species.

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 35–40. Parorchis sp. (Pasp). 35, Overview of a colony in a freshly deshelled, infected horn snail in sea water. The arrow indicates the colony, which is localized in the gonadal region. Scale bar = 1 cm. Base photo credit: Todd Huspeni. 36, Reproductive redia, live, with developing cercariae, under coverslip pressure. The arrow indicates an attacking soldier redia of Himasthla sp. B from an in vitro experiment (Hechinger et al., 2010). Scale bar = 200 µm. 37, Soldier redia, live. Scale bar = 200 µm. Base photo credit: Andrew Turner. 38, Cercaria, live, under coverslip pressure. Scale bar = 200 µm. 39, Close up of cercaria anterior with particularly obvious tegumental body spines. Scale bar = 100 µm. 40, Developing cercariae, live, under heavy coverslip pressure to make the long esophagi visible. Scale bar = 200 µm.