A 1989 paper by John Dodge rocked the dinoflagellate community. The much loved and studied dinoflagellate Gonyaulax polyedra, known for its spectacular bioluminescent displays and red tides in southern California and elsewhere, was renamed based on new insights into its morphology and to align the name with that of its spiny cyst, then known as Lingulodinium machaerophorum. So the motile form of the dinoflagellate, originally described by Stein in 1883, was changed from Gonyaulax to Lingulodinium because you can’t have two names for the same organism.
Reluctantly we adopted the new name and used it in our publications. However, Lingulodinium polyedra didn’t seem right, based on the word endings, so Lingulodinium polyedrum became commonly used, even though we cited the Dodge 1989 paper, which called it Lingulodinium polyedra. Out of 220 publications since then, 95% used L. polyedrum. Now it’s time to change again. Based on the Latin derivation, polyedrum is incorrect.
According to Brown’s “Composition of Scientific Words” (p. 715):
Gr. hedra, f. seat, chair, base, plane, side; hedrion, n. dim.; hedraios, sitting,
sessile, sedentary, stable, steadfast; hedranon, n. abode, dwelling, seat;
ephedra, f. a sitting by, siege; ephedros, sitting upon; kathedra, f. seat of a
bishop, abode, fundament, rump: hedrocele, tetrahedral, dodecahedron, octahedrite, cathedral, Sanhedrin, chair, chaise, Edrioaster[Hedrioaster] saratogensis (a cystoid), Ephedra distachys (a joint-fir), Gonyaulax polyhedra (a flagellate).
So, polyedra, according to this authoritative source, is a noun, which means the name is Lingulodinium polyedra, as nouns are non-declinable when used in apposition.
This is how it is explained in the taxonomic resource AlgaeBase:
The epithet “polyedra” is a noun in apposition and is non-declinable, so use of the epithet “polyedrum”, supposedly to agree with the gender of the genus name, is incorrect. ICN Art 23.5 [Melbourne Code]: “The specific epithet, when adjectival in form and not used as a noun, agrees grammatically with the generic name; when it is a noun in apposition or a genitive noun, it retains its own gender and termination irrespective of the gender of the generic name. Epithets not conforming to this rule are to be corrected (see Art. 32.2).” – (6 Jan 2018) – M.D. Guiry
The correct name, Lingulodinium polyedra, now appears in AlgaeBase and the World Registry of Marine Species (WoRMS). In a recent publication we referred to it as Lingulodinium polyedra (F. Stein) J. D. Dodge 1989 (formerly Gonyaulax polyedra; by many authors Lingulodinium polyedrum).
Thanks to Michael Guiry for assistance in resolving this issue.