Mesostephanus appendiculatus (Ciurea)

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 : 467

Mesostephanus appendiculatus (Ciurea)

(1. Meap; Figs. 1 , 4–7 )

Diagnosis: Parthenitae. Colony comprised of very active sporocysts, densely concentrated in snail mantle (in enlarged perirectal sinus). Sporocysts translucent white; usually over 1200 µm long, very elongate (length:width up to ~15:1), of relatively consistent width but with tapering anterior and bluntly rounded posterior; body wall with muscular, transverse, crest-shaped annulations, developing cercariae visible through translucent sporocyst wall.

Cercaria . Body translucent colorless; non-oculate; with oral sucker modified as “anterior organ” and no ventral sucker; with typical cyathocotylid excretory system, wherein main collecting ducts have lateral and medial branches all connecting anteriorly, the anterior blind ducts diverge (point antero-laterally); gut branches at 1/4–1/3 body length, caeca each with 3–4 pronounced, sinuous undulations; body ~ 220 µm long, ~1/2 the length of tail stem; tail forked, with dorso-ventral fins on furcae.

Cercaria behavior: Fresh, emerged cercariae remain in water column, swim intermittently with short bursts followed by longer periods of resting and slow sinking; frequently swim in response to vibration.

Similar species: Meap cercariae are readily distinguished from Smcy [2] by their larger size, more anterior position of gut branching, pronounced caeca undulations, and (usually) diverging anterior blind excretory ducts. If the host snail has been dissected, the sporocyst distribution, activity, and annulations provide the easiest way to distinguish Meap from Smcy.

Remarks: Martin (1961) documented the life cycle. He described the sporocysts and cercariae from naturally infected C. californica (although he reported the sporocysts as residing in the visceral mass, which is not consistent with our repeated observations (n> 1000) that they use the perirectal sinus in the mantle). Martin also experimentally infected second intermediate host fishes with metacercariae, using those to infect chicks to get adults. He described the adult and identified it as Mesostephanus appendiculatus . Because M. appendiculatus was originally identified from Romania, it seems likely that it represents a globally distributed species complex.

This species likely corresponds to “ Cercaria cerithidia 22” of Hunter (1942), the “Furcocercous Cercaria ” of Maxon and Pequegnat (1949), and the “large strigeid” of Martin (1955).

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 4–7. Mesostephanus appendiculatus (Meap). 4, Overview of a colony in a freshly deshelled, infected horn snail in sea water, showing the way the active sporocysts “spill out” of their infection site in the mantle upon host dissection. Note the host snail’s non-functional, orange ovaries. Scale bar = 1 cm. 5, Sporocyst, live, with developing cercariae, under slight coverslip pressure. Scale bar = 100 µm. 6, Cercaria, live. Scale bar = 100 µm. 7, Cercaria body, live, under coverslip pressure to better reveal key traits. Scale bar = 100 µm.