'Tis the season for Messages from The East. And in recent years I have taken some inspiration from research conducted on Semisulcospira, a genus of pleurocerid snails widespread throughout Japan, Korea, Taiwan and eastern China. As a potential intermediate host for the lung fluke Paragonimus westermani, Semisulcospira has become one of the better-studied of all the freshwater prosobranchs.
In many of the details of its general biology, life history, and habitat, Semisulcospira reminds me a great deal of our North American Goniobasis or Elimia. Scores of species have been described (30 from Japan alone, according to Davis, note 1), almost entirely on the basis of shell morphology. The most striking difference between Goniobasis and Semisulcospira is that the latter is a brooder - holding eggs in a modified pallial oviduct until hatch. But although ovoviviparity is often associated with parthenogenesis in freshwater gastropods, I'm not aware of any evidence of asexual reproduction in Semisulcospira (2).
Ovoviviparous reproduction does, however, facilitate laboratory rearing studies. And in 1998 and 2000, Misako Urabe (of Nara Womens' University) published a pair of papers describing a set of experiments directly exploring the heritability of shell morphology in Semisulcospira that deserve to be more widely known.
Urabe worked with Semisulcospira reiniana, an extremely variable species co-occurring with (and sometimes indistinguishable from) the more widely distributed S. libertina in the Kamo River of Kyoto. The plate at left (from Davis, see Note 1) shows both S. libertina (figs 1-4) and S. reiniana (figs 5 & 6) from several Japanese populations (scale bar in mm).
In the 1998 work, Urabe (3) used mother/offspring regression to estimate the heritability of the three shell shape parameters of Raup (4): S (the roundness of the generating curve), W (the rate of whorl expansion), and T (the rate of whorl translation). In the wild, snails collected from more rapid currents tended to have larger W and smaller T, that is, larger body whorls and lower spires (5). But lab experiments showed the heritabilities of all three shell shape parameters to be nonsignificant - no different from zero.
In the 2000 work, Urabe (6) reared juveniles to 6.8 mm under standard conditions, marked sibships with waterproof tags, and then divided them into two aquaria - one with a sand substrate and the other with pebbles. After one year, snails were scored by their "rib intensity," a 0/1 coding system for shell sculpture developed by the author. Urabe showed both that mothers with stronger shell ribbing tended to produce higher percentages of ribbed offspring, but that snails grown to maturity in the tank with the sand substrate tended to form ribbed shells more frequently than those grown on pebbles.
Quite a few researchers have directly demonstrated shell phenotypic plasticity using pulmonate snails as models. Amy Krist published a nice experiment showing that crayfish predation can affect aperture shape in Goniobasis livescens a couple years ago (7). But the experiments of Urabe are the best demonstrations of phenotypic plasticity in freshwater prosobranchs of which I am aware.
So to what extent might we expect Urabe's conclusions for Semisulcospira to generalize to the pleurocerids of North America? The evidence is all around us that they do. I've seen populations of Goniobasis catenaria inhabiting Piedmont streams in North Carolina, for example, that intergrade seamlessly from strongly carinate shells to shells almost entirely smooth, apparently in correlation with the substrate (8). And I dedicated my entire post in February 2007 to a phenomenon I called "Goodrichian Taxon Shift," the broadening and thickening of shell morphology demonstrated by populations of North American pleurocerids as rivers widen, deepen, and slow. Which brings us to our "Message from The East."
In recent years, modern research programs have led to substantial taxonomic revisions in the Asian Pleuroceridae. At least 17 of the 30 specific nomina of Semisulcospira described from Japan on the basis of shell character had been synonymized under S. libertina by 1972, according to Davis (9). The clear implication of Urabe's work, that such trends will continue, has recently been borne out, as we shall see. And might the long-benighted pleurocerid fauna of North America also be overdue for a taxonomic revision along similar lines? Again, said the Zen Master, we will see.
Notes
(1) Davis, G. M. 1969. A taxonomic study of some species of Semisulcospira in Japan (Mesogastropoda: Pleuroceridae). Malacologia 7: 211-294.
(2) Males seem to occur in high frequency in all populations.
(3) Urabe, M. 1998. Contribution of genetic and environmental factors to shell shape variation in the lotic snail Semisulcospira reiniana (Prosobranchia: Pleuroceridae) J. Moll. Stud. 64: 329-343.
(4) Raup, D. M. 1966. Geometric analysis of shell coiling: general problems. J. Paleontol. 40: 1178-1190.
(5) To be quite precise - snails active in fast currents at night tended to show the shell shape relationship best. Apparently Semisulcospira populations are nocturnal, much like the Melanoides in my goldfish bowl. Urabe suggested that, since shell growth only occurs when the mantle edge is flush with the aperture, "it is only the environment that a snail experiences when it is active at night-time that affects shell formation." Interesting!
(6) Urabe, M. 2000. Phenotypic modulation by the substratum of shell sculpture in Semisulcospira reiniana (Prrosobranchia: Pleuroceridae). J. Moll. Stud. 66: 53-59.
(7) Krist, A. C. 2002. Crayfish induce a defensive shell shape in a freshwater snail. Invert. Biol. 121: 235-242.
(8) The smooth form is given the subspecific name "dislocata," while the strongly carinate form is G. catenaria s.s.
(9) Davis, G. M. 1972. Geographic variation in Semisulcospira libertina (Mesogastropoda: Pleuroceridae). Proc. Malac. Soc. Lond. 40: 5-32.