Giant Floater
Pyganodon grandis
Diagnostics 4
SHELL
- size: mid-sized to very large. maximum 160 mm, most under 100 mm
- thickness: thin
- shape: HIGHLY VARIABLE. oval to elongate, sometimes triangular. weak posterior ridge, straight to slightly curved ventrally. small wing on posterior slope very common.
- width: inflated to very inflated. very elongated, older individuals can acquire a near-cylindrical cross section
- surface: smooth to rough, rarely shiny.
- beaks: inflated, close to middle of hinge, usually extending well above hinge line. sculptures relatively diminutive in relation to overall size of shell: very variable: 3-5 fine double loop ridges, uneven in height, often forming peaks or nodules on either side of the inter-loop notch, itself usually sharply angled U or V (most noticeable from an axial view of the shell hinge line). this notch is also often very reduced/absent, commonly causing bar gaps. best defined bars usually of clearly uneven spacing as well.
- color/markings: tan-yellow to pale green in young, to dull brown to dark brown in older adults. faint green rays common in young.
- sexual dimorphism: none
- pseudocardinal teeth: none
- lateral teeth: none. hinge line often rather thick
- nacre: silvery-white, with shades of cream or salmon in cavity
SOFT PARTS: foot white to cream
Similar species/lookalikes: this already very variable species' shell size, shape, inflation and thickness are also all very much influenced by variation of water quality, hardness, substrate and flow rates. Typical-shaped individuals, with a compact, inflated, 'pot-bellied" ventral margin and almost median beaks are relatively easy to identify, as well as large live adults or shells by their size alone. but smaller specimens with a more elongate shape, especially very young ones, can easily be confused with Eastern floater, paper pondshell, cylindrical floater. and even creeper, when beak sculptures are too degraded to be of use. In such live specimens without defined sculptures, the straight, not upturned posterior ridge can be a helpful feature., but isn't entirely reliable. When sculptures are not degraded, paper pondshell and cylindrical papershell can readily be differentiated through that feature as well as their thinner shells. Live creepers have much wider and heavier beak sculptures and are much darker colored as adults, the size at which they could most easily resemble small to mid-sized giant floaters. Most problematic in differentiating from Eastern floater as in this taxon's beak sculptures are also quite variable, and even more so when this feature cannot be used. when present though, this feature is most helpful, keeping in mind the "uneven-uneven'' nature of P. grandis' sculptures, meaning main bars being uneven in height as well as spacing, as opposed to P. cataracta's "even-even'' nature regarding these same traits. further complicating field IDs, hybridization between these two taxons is known and documented, leading to an even more confusing spectrum of intermediates.
Sources and Credits
- (c) Philippe Blais, some rights reserved (CC BY-NC-ND), uploaded by Philippe Blais
- (c) Nancye Saunders Drukker, some rights reserved (CC BY-NC-ND), uploaded by Nancye Saunders Drukker
- (c) Sam Kieschnick, some rights reserved (CC BY), uploaded by Sam Kieschnick
- Adapted by Philippe Blais from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Pyganodon_grandis
