«(Shell pictures taken from Burch and Tottenham 1980) Family Physidae The family Physidae has a holarctic distribution with extensions into Central ...»
Amy R. Wethington
(Shell pictures taken from Burch and Tottenham 1980)
The family Physidae has a holarctic distribution with extensions into Central and South America.
Physidae have the following features which separate them from other pulmonate families: high
spired sinestral shell, radula with teeth in v-shaped rows, no lateral teeth, and no hemoglobin or
pseudobranchia. They are hermaphrodites with the ability to self fertilize and can be found in a wide variety of freshwater habitats worldwide. As a group, they display a wide variety of life history parameters which are affected by: climate, type of habitat, and whether predators or parasites are present. Members of this group can have between 1 and 3 generations per year and in some cases, they can reproduce continually (e.g. warm springs). Dillon (2000) provides an excellent ecological overview of this group. This family is also generally reproduction selected and can handle harsh environments, including environments that have been polluted. This family can be divided into six groups: Aplexa group (Aplexa), marmorata group (Stenophysa), fontinalis group (a), acuta group (c), gyrina group (b) and pomilia group (bc). These six groups are phylogenetically different, have distinctive penial morphology, and to some extent have distinctive shell morphology. Shell morphology has an environmental component in addition to a genetic component so using shell alone to distinguish members of this group can lead to mistakes in identification (DeWitt et al 1999, DeWitt et al 2000, Burnside 1998, Britton in press).
The classification within this family has been heavily debated. The most commonly used classification for this family has been that of the Burch guides (Burch and Tottenham 1980, Burch 1982, 1988, and Burch and Jung 1992) and adopted by Turgeon et al (1998). The classification is based on George Te’s (1978) dissertation (see also Te 1975, 1980). The letters or word above the nodes reflects the penial morphology groups that Te (1978) used in his dissertation. There are 36 species along with 43 subspecies and morphs listed in the Burch guides mentioned above.
c Sub-Genus Alampetista Costatella bc Costatella Genus Sub-Genus Physella bc Petrophysa Sub-Genus Sub-Family b Physella Physinae Family Physidae Genus a
Figure 1. Te (1978) classification for Physidae.
Taylor (2003) has recently suggested an alternate classification scheme that divides each subfamily into one to seven tribes with a total number of 23-24 genera and around 80 species.
The diagram below shows Taylor’s genera along with the penial group Taylor assigned to each.
Taylor (2003) did not separate the bc from the b penial group. Taylor (2003) feels that the area of greatest diversity for this group is Central and South America instead of North America as suggested by Te (and the Burch guides).
Figure 2. Taylor (2003) classification for Physidae Burch and Jung (1992) recognized that more work is needed including molecular genetics to better resolve Physidae taxonomy.
The two classification schemes above are based solely on morphology and both are weighted with regards to penial characters. Recent genetic work has been done on Physidae in both the Lydeard and Dillon laboratories. There is a strong correlation between penial morphology, reproductive isolation and a molecular phylogeny developed for the group (Dillon et al 2002, Wethington & Guralnick in press, Dillon & Wethington in press, Wethington 2004). But there are some Te and Taylor groups improperly placed or assigned, including: Petrophysa, bc penial group, and Stenophysa. Our sampling was primarily of North America and we only recognized 13 species (which includes Physa fontinalis and Physa marmorata, not of North America). We suggest that there are two genera in this family, Physa and Aplexa (Wethington 2004). The penial groups may be ranked at the subgenera level.
The penial morphology is Aplexa type with no preputial gland and a one-part glandular sheath.
The habitat is generally temporary wood pools or ditches, sometimes Aplexa can be found in extremely shallow streams. The animals can be Aplexa elongata found mostly floating at the surface in pairs when their habitat is (Say 1821) inundated with water during Spring to early summer. Their distribution is circumboreal and they are mostly found at high latitudes.
marmorata group Shell is shiny, somewhat narrow, and ovoid-fusiform. Shell length can be as long as 16 mm. Aperture is large and is about 0.75 that of the shell length. Mantle reflects broadly on both sides. Foot generally has a median stripe in the last section that narrows to a triangular tip.
The penial morphology consists of no preputial gland and a one-part muscular penial sheath.
This group has a huge amount of genetic variation both within species and within populations. This variation along with a lack of reproductive isolation between nominal species has lead to many species within this group becoming synonymized under the name Physa acuta, including P. heterostropha, P. integra, P.
cubensis, and P. virgata (Dillon et al, 2002; Wethington 2004).
Basal populations of this group uncovered in mitochondrial based phylogenies have also been found to be at least partially reproductively isolated from geographically nearby populations in the one case that has been closely studied (Dillon, personal communication).
acuta group: Physa zionis Shell is a rounded oval shape. Spire is very short and rounded.
Aperture is almost as long as the shell itself and very broad. The animal is highly pigmented, practically a solid black. The foot of the animal is very large and broad apparently adapted to provide a sufficient suction for the animal to climb the vertical cliffs where it is primarily found (where the rock seeps). Their foot is also helpful in keeping the animal from being swept off the canyon wall when the Virgin River floods.
Physa zionis Penial morphology is of the type “c”: preputial gland present Pilsbry 1926 along with a one-part muscular penial sheath.
Physa zionis is probably the most misunderstood physid. Highly endemic to its type locale: along the Narrows trail in Zion National Park. It can be found proximately close to P. gyrina, but the latter is restricted to horizontal, stream-based habitat. Not much is known of living specimens, just that they are quite prolific, are extremely small as adults (5 mm is a whopper), egg masses range in size from 1 to about 4 embryos, they are definitely not P. gyrina, and they are a basal member of the acuta group.
General Description Illustration (shell) Penial (10 mm between lines) Morpholog y gyrina group Shell shape is oval to round. Spire is raised with rounded or somewhat flattened sutures to being short with a prominent shoulder. Mature sizes can range from being extremely small (around 4 to 5 mm) to extremely large (as large as 24 mm).
Shell color is variable from pale yellow, to dark with red stripes, to black. The shell shape for Physa gyrina is skinny with a prominent spire. Physa ancillaria has a globose shell with a more flat spire, generally thicker, and sometimes with prominent shoulder.
Penial morphology is of the type “b”: preputial gland present along with a two-part penial sheath (one part glandular, one Physa gyrina part muscular). The two sheaths are roughly equal in size.
Say1821 Physa gyrina is a specialist. It can survive under harsh conditions as long as the overall environment is stable and rich. Physa gyrina is found in rivers, streams, intermittent ponds, and large lakes all over North America and has been introduced at least in Europe. Physa ancillaria is more restricted to the more Northern latitudes of North America and can be found in large rivers and large lakes. There may be an underlying physiological difference between these two species which affects how well the animals get oxygen and thermo-regulate based on the difference in culture ability (Dillon & Wethington in press, Wethington 2004).
Physa gyrina and P. ancillaria are genetically very similar.
Physa ancillaria Mitochondrial DNA can separate them into the two groups, Say 1825 but allozymes cannot. The two species can interbreed at least to some extent and a recent allozyme study showed that geography is more important than shell shape with regards to genetic distance between populations of both forms. It is possible that this may be one amazingly diverse and phenotypically plastic species, P. gyrina.
pomilia group Shell is oval shaped with a pointed, but rounded spire.
Sutures are prominent and whorls are convex as in P. acuta. It has been commonly referred to as a subspecies of P.
heterostropha (now synonymized with P. acuta), but it is genetically and biologically distinct.
Penial morphology is of the type “bc”: preputial gland present along with a two-part penial sheath (as in P. gyrina), but unlike P. gyrina, the muscular sheath is much larger than the glandular sheath.
This species is found more often in large rivers, but can also be found in ponds. Individuals can reproduce at an unusually Physa pomilia small size, around 4 mm or less. It is less common than small size, around 4 mm or less. It is less common than Conrad 1834 members of the acuta or gyrina group. It has a rather disjunct and patchy distribution in the United States (Alabama, South Carolina, Kansas, Connecticut, and California (Physa costata) populations are known).
-NotesFigure 4. This figure is taken from Paraense and Pointier 2003. It depicts the internal reproductive anatomy of Physa acuta in detail.
Dissection Instructions for use in identifying physids.
You will need a dissecting microscope, a petri dish, a pair of iris scissors, two forceps, and either pond water or ethanol (or favorite preservative solution).
Step one: Separate the shell from the snail. You can do this by carefully crushing the shell with the two pair of forceps and then pulling the shell fragments off to one side.
Step two: Cut away the mantle and pull off to either side.
Step three: Use the scissors to make a cut between the eyes and posteriorly. Try to cut the mesentery only. You should be able to see the heart and the penial organ will be on the snail’s left side. The penial organ will be folded up and wrapped onto itself. I usually take the penial apparatus out and stretch it to better see the different parts. Be careful to include as much of the preputium as possible in order to diagnose the presence or absence of a preputial gland.
Figure 5: Diagram for dissecting the penial organ from a physid along with a diagram showing the penial organ for the six categories of physids. PREP = Preputium. G PREP = Preputial gland.
SAR = Sarcobelum. SPM = Muscular penial sheath. SPG = Glandular penial sheath.
Table 2. Currently recognized species (names as in Burch guides + some species described since then), G-rank, and
tentative distribution of the North American Physidae:
Britton, D. K. in review. Environmental and genetically induced shell shape variation in the freshwater pond snail Physa virgata. American Malacological Bulletin.
Burch, J. B. 1982. North American freshwater snails: identification keys, generic synonymy, supplemental notes, glossary, references, index. Walkerana 1(4):1-365.
--------. 1988. North American freshwater snails: introduction, systematics, nomenclature, identification, morphology, habitats, distribution. Walkerana 2(6):1-80.
Burch, J. B. and J. Tottenham. 1980. North American freshwater snails: species list, ranges, and illustrations. Walkerana 1(3):1-215.
Burch, J. B. and Y. Yung. 1992. Freshwater snails of the University of Michigan Biological Station Area.
Burnside, C. 1998. Ecophenotypic variation in shell morphology within the freshwater pond snail, genus, Physella (Pulmonata: Basommatophora) and its taxonomic implications. Ph. D. Dissertation, University of Texas, Arlington. 206 pp.
DeWitt, T.J., A. Sih, J.A. Hucko. 1999. Trait compensation and cospecialization: size, shape, and antipredator behaviour. Animal Behaviour 58: 397-407.
DeWitt, T.J., B.W. Robinson, and D.S. Wilson. 2000. Functional diversity among predators of a freshwater snail imposes an adaptive trade-off for shell morphology. Evolutionary Ecology Research 2: 129Dillon, R. T., Jr. 2000. The Ecology of Freshwater Molluscs. Cambridge University Press.
Dillon, R. T., Jr. and A. R. Wethington. In press. No-choice mating experiments among six nominal taxa of the subgenus Physella (Basommatophora: Physidae). Heldia.
Dillon, R. T., Jr. and A. R. Wethington. In preparation a. No reproductive isolation between the freshwater pulmonate snails Physa acuta and Physa virgata.
Dillon, R. T., Jr. and A. R. Wethington. In preparation b. Michigan physids, Te revisited.
Dillon, R. T., Jr., A. R. Wethington, J. M. Rhett, and T. P. Smith. 2002. Populations of the European freshwater pulmonate Physa acuta are not reproductively isolated from American Physa heterostropha or Physa integra. Invertebrate Biology 121(3):226-234.
Paraense, W. L and J-P Pointier. 2003. Physa acuta Draparnaud, 1805 (Gastropoda: Physidae): A Study of Topotypic Specimens. Mem. Inst. Oswaldo Cruz, Rio de Janeiro, Vol 98(4): 513-517.
Taylor, D. W. 2003. Introduction to Physidae (Gastropoda: Hygrophila) biogeography, classification, morphology. Revista de Biologia Tropical Vol 51 Supplemental, 1: 1-287.
Te, G. 1975. Michigan Physidae, with systematic notes on Physella and Physodon (Basommatophora:
Pulmonata). Malacological Review 8(1-2):7-30.
--------. 1978. The systematics of the family Physidae (Basommatophora: Pulmonata). Ph. D. Dissertation.
University of Michigan.
--------. 1980. Arch. Molluskenk., 110(4/6), pp. 179-184.
Turgeon, D. D., J. F. Quinn, Jr., A. E. Bogan, E. V. Coan, F. G. Hochberg, W. G. Lyons, P. M. Mikkelsen, R. J. Neves, C. F. E. Roper, G. Rosenberg, B. Roth, A. Scheltema, F. G.