This is a photo submitted by one Peter to Avril Bourquin’s Man and Mollusc web page. The freshwater snail was purchased complete with living hitchhikers from a pet store in New York (collection site unknown). Vigorous discussion on the CONCH-L mailing list identified the snail as Vitta usnea (Röding, 1798), a member of the family Neritidae familiar to aquarists and shell-collectors as the olive nerite.
V. usnea is often found in flowing freshwater with tidal influence, but is quite able to tolerate low-salinity conditions and in at least some cases occurs far upstream. Say (1822, who christened this species Theodoxus reclivatus), is quoted as writing the following: “I found this species in great plenty, inhabiting St. John’s river in East Florida, from its mouth to Fort Picolata, a distance of one hundred miles, where the water was potable.”
There appears to be some controversy about larval development — Andrews (1935) reported direct development into benthic miniature adults, bypassing the typical veliger stage, but “the reported maximum diameter of the egg capsules (~1 mm) is too small to hold 40 metamorphosed juveniles,” as he reported, “of any neritid species” (Kano, 2006). What’s more, Holthuis (1995) reported V. usnea to have what Kano (2006) terms a “type A” opercular nucleus — an operculum morphology typically associated with floating, planktotrophic veliger larvae. (The operculum is the calcareous lid that seals the aperture of the shell when the snail’s body is withdrawn.) The geographic distribution of this species (Florida to Colombia, including Trinidad and the Greater Antilles), for my part, makes me inclined to rule against Andrews.
Larval development, at any rate, appears to require saline conditions — although captive specimens will strew the aquarium glass with hard, salt-grain-like egg capsules, the larvae uniformly perish in freshwater. This carries the curious implication that V. usnea populations sufficiently upstream from the ocean would be unable to recruit except through upstream migration. I don’t know whether any research on larval tolerance for freshwater in diadromous nerites has yet been done, but it seems to me that there may be physiological/developmental differences between estuarine V. usnea and the upstream populations reported by Say. Some more discussion on that at CONCH-L.
As for the barnacles, they seem to be Balanus improvisus (Darwin, 1854). Darwin himself, according to Southward and Crisp (1987, p. 127) “noted that Balanus improvisus was found in a small stream in the estuary of La Plata, near Monte Video, where at high water specimens apparently were covered by the brackish and occasionally almost fresh waters of the estuaries.” Branscomb (1976) reports that a population in the Chesapeake Bay “appeared unaffected by unusually high freshwater run-off in June which lowered salinities in the bay for 1972.”
So what salinity parameters do these relate to? Southward and Crisp continue (emphasis mine):
Balanus improvisus occurs in the Baltic Sea to salinities 3-5‰ (Segerstrâle 1957, Barnes & Barnes 1962). Specimens collected from British estuarine localities could be acclimated to function in salinities less than 3‰, at which salinities they hyperregulate the body fluids at a constant osmotic pressure corresponding to about 3‰ (Foster 1970, Fyhn 1976). B. improvisus has been recorded on the elytra of water beetles by Tarasov & Zevina (1957), a novel dispersal situation for any barnacle, and there are specimens in the British Museum (Natural History) from the shell of a freshwater crayfish collected in northern Iran (Southward, pers. comm.).
The specimens depicted above, of course, were living in full freshwater, but how long they held out was not reported. I have personally seen the same species-combination, but can’t recall whether the crustaceans were still alive.
Holthuis BV. 1995. Evolution between marine and freshwater habitats: a case study of the gastropod suborder Neritopsina. PhD dissertation, University of Washington.
Kano, Y. (2006). Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods. Journal of Morphology, 267(9), 1120-1136.
Segerstrâle, S.G. 1957. Baltic Sea. In J.W. Hedgpeth (ed.), Treatise on Marine Ecology and Paleoecology 1: 751-800. New York: Geol. Soc. America.
Southward, A. J., & Crisp, D. J. (1987). Barnacle Biology (p. 443).
Tarasov, N.I. & G.B. Zevina 1957. Cirripedia Thoracica of the seas of the USSR. Fauna SSSR 6(1): 1-267 (in Russian).