Gammarus

Thoma, Brent P. & Heard, Richard W., 2009, A new species of the genus Gammarus (Crustacea: Amphipoda: Gammaridae) from the low salinity habitats in the northern Gulf of Mexico, Zootaxa 2015, pp. 42-54 : 53

Key to the species of Gammarus known to occur in the waters of the northern Gulf of Mexico (modified from LeCroy 2000).

1. Pleon 1–3 having posterodorsal margin entire, without dorsal processes; eye large, extending almost to dorsal margin of head; telson lobes of male not elongate, tip with spines interspersed with long setae, setae longer than spines. ......................................................................................................................................................... Gammarus tigrinus

- Pleon 1–3 (occasionally only 1–2), posterodorsal margin with median posteriorly directed process; eye medium, not extending almost to dorsal margin of head; telson lobes of male elongate, tip with spines only, lacking terminal setae, or with spines interspersed with short setae, setae not longer than spines.......................................................... 2

2. Pleon 1–3 (occasionally only 1–2) with dorsal processes enlarged, angled dorsally away from following pleonite, having at most a few apical setae; 2nd antenna having setae with lengths not exceeding length of following flagellar article; telson lacking medial spines in distal half, terminal setae, when present, never exceeding the length of terminal spines ................................................................................................................................ Gammarus lecroyae n.sp.

- Pleon 1–3 (occasionally only 1–2) with dorsal processes small, following contour of following pleonite, typically with posterior margin having several minute setae; 2nd antenna having setae with lengths greatly exceeding length of following flagellar article; telson with one or more medial spines in distal half, terminal setae typically exceeding length of terminal spines ............................................................................................................ Gammarus mucronatus