Spike

Eurynia dilatata

DIAGNOSTICS 4

SHELL

  • size: average to large, maximum 135 mm. most under 90 mm.
  • thickness: moderately thick to thick.
  • shape: elongate with tapered and pointed posterior. rounded anterior. posterior ridge sharpest near beaks, straight. posterior slope narrow, with margin more or less symmetrical with ventral margin in younger specimens, resulting in a 'long drop' general shape. older adults with a straighter ventral margin and more long-triangular shape, resulting from shell growth being more pronounced in posterior-ventral than anterior-ventral. In these older specimens the ventral margin growth lines thus form an acute angle between the more recent one and middle-shell ones. odd individuals can be much more compact/triangular, or more elongate than typicals
  • width: moderately compressed to slightly inflated. widest in anterior third or quarter.
  • surface: smooth, matte, satin-like. fine, broken radial ridges occasionally present anterior to umbo
  • beaks: lightly inflated, flat, anterior, and slightly elevated above hinge line, which usually dips anterior to beak. sculptures: 4-5 heavy single loops more pronounced and strongly curving towards hinge in posterior. often dissolved beyond recognition
  • color/markings: usually tan, greenish-brown to dark brown in older animals, rarely marroon, with faint green rays in posterior common in young
  • sexual dimorphism: none
  • pseudocardinal teeth: strong, low, slightly wider than adjacent thick hinge plate
  • lateral teeth: strong and thick, protruding well below hinge line into cavity. Lower left valve tooth much thicker and shorter than top one
  • nacre: very variable. uniformly dark purple in most specimens, but can be pink, gold, salmon, peach or white, or combinations of these. Entirely white, peach are least common

SOFT PARTS: foot white

Similar species/lookalikes: small specimens of Black sandshell are more elongate and are widest closer to middle of shell than adult Spikes, which are usually widest in anterior third. Ligumia recta's base periostracum color is usually darker at any age than the spike, and its nacre is more violet than dark purple, and when present that color is restricted to the cavity, rarely extending beyond the palial line onto the lip. Spikes are much smaller than black sandshells on average, and their shells much thicker when comparing similar-sized specimens of L. recta. It can also be mistaken for very elongate Eastern Elliptios , but in that taxon the posterior slope is usually more pronounced/wider, and ventral margin growth lines remain relatively parallel throughout life, not angled as in mature Spikes. Compared with E. complanata, the beaks are usually more anterior, and the hinge plate is usually much thicker, but those two features are very variable in that last taxon, so not very reliable. nacre colour in E. complanata is also an unreliable distinguishing feature when dealing with a non-purple nacred E. dilatata, as it can also display the same wide palette of aberrant colors, albeit less commonly. lateral teeth in E. complanata and L. recta are of roughly equal length, and do not protrude downward into cavity. Only kidneyshell have the same lateral tooth morphology, but that taxon never has colored nacre, and is more compact in general shape. It also usually sport wide, broken green rays on periostracum.
Combination of elongate shape, colored nacre and SHORTER LOWER LEFT-VALVE LATERAL TOOTH is diagnostic.

IMPORTANT NOTE: the taxonomic status of this species could eventually be reassigned to another genus within the tribe Pleurobemini, according to recent genetic studies.

DISTRIBUTION/WATERSHEDS 4

ONTARIO
-- Saugeen R.

  • Ausable R.
  • Sydenham R.
  • Thames R.
  • Grand R.

QUEBEC

  • St-Lawrence (Montreal area)
  • Richelieu R.

Sources and Credits

  1. (c) Philippe Blais, some rights reserved (CC BY-NC-ND), uploaded by Philippe Blais
  2. (c) Matthew Ireland, all rights reserved, uploaded by Matthew Ireland
  3. (c) Rob Curtis, some rights reserved (CC BY-NC-SA), uploaded by Rob Curtis
  4. (c) Philippe Blais, some rights reserved (CC BY-SA)

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