Brook Floater

Alasmidonta varicosa

DIAGNOSTICS 5

SHELL

  • size: small to mid-sized; up to 70 mm
  • thickness: moderately thin
  • shape: trapezoid to elliptical with truncated dorso-posterior margin; straight to slightly concave ventral margin; posterior ridge rounded and broad
  • width: inflated, more so in posterior end
  • surface: smooth except short grooves along posterior slope; severe umbonal dissolution common
  • beaks: few, coarse single or double-looped ridges (prematurely faded away early at young age)
  • color/markings: yellowish to greenish or brown, with abundant dark green rays, irregular, small spots sometimes present
  • sexual dimorphism: none
  • pseudocardinal teeth: rudimentary, flattened; one in each valve (left smaller); interdental projection weak or absent
  • lateral teeth: absent
  • nacre: bluish white with pinkish suffusions

Soft parts: foot peach or orangish

Similar species/lookalikes: Most similar with related A. marginata. Posterior ridge is sharper in A. marginata, and posterior ridge more sharply angled with that feature, as well as ending in a sharper posterior point at shell margin. Posterior slope ribbing is also more pronounced in A. marginata, but that feature can be variable, therefore less pronounced in some individuals. In A. varicosa periostracumis iis usually devoid of any dark spots peppering the shells, while in A. marginata that feature is the rule. with very occasional specimens being found entirely spotless. Hinge/dentition features is very similar in both species.
Lastly, according to currently known distrubution data, these two species respective ranges do not even nearly overlap, as A. varicosa is restricted to Eastern New Brunswick and Nova Scotia, and A. marginata's Northeasternmost range ends in Southeastern Quebec.
Young live specimens of Strophitus undulatus from high gradient, soft water habitats can share a very similar appearance to equally immature A. varicosa. The best features to differentiate is such case is the fine ribbing/ridges of the posterior slope of A. varicosa, a feature unknown in S. undulatus.

DISTRIBUTION/WATERHEDS 6

NEW BRUNSWICK
Gulf of St-Lawrence dr.:

  • Miramichi R. < Southwest Miramichi R.
    _ Kouchibouguacis R.

  • Bouctouche R.
  • Shediac R.
  • Scoudouc R.

Bay of Fundy dr.:

  • St-Croix R.
  • Petticodiac R. < Little R.

NOVA SCOTIA
Gulf of St-Lawrence dr.:

  • Mattatall Lake Br.
  • Dewar R. < Wallace R.

Bay of Fundy dr.:

  • Annapolis R.
  • Shubenacadia R. < Gays R.

Atlantic dr.:

  • Lahave R.
  • St-Mary's R. < East R., Loohaber L.
  • Salmon R.

Sources and Credits

  1. (c) Smithsonian Environmental Research Center, some rights reserved (CC BY), https://www.flickr.com/photos/serc_biodiversity/38568290512/
  2. Daderot, no known copyright restrictions (public domain), http://commons.wikimedia.org/wiki/File:Alasmidonta_varicosa_-_Springfield_Science_Museum_-_Springfield,_MA_-_DSC03461.JPG
  3. (c) Evan Raskin, some rights reserved (CC BY-NC), https://www.inaturalist.org/photos/9654562
  4. (c) Dwayne Sabine, some rights reserved (CC BY-NC), https://www.inaturalist.org/photos/48779316
  5. Adapted by Philippe Blais from a work by (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Alasmidonta_varicosa
  6. (c) Philippe Blais, some rights reserved (CC BY-SA)

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