Comments: Complex patterns of variation make taxonomic treatment difficult. This stickleback may be considered a species complex with many unique and reproductively isolated populations, subspecies or species.
Populations exist that are strictly marine, anadromous, and freshwater resident. The marine and anadromous forms have given rise to diverse resident phenotypes. Subspecies have been recognized in the past, but current scientific discussion of this species complex recognizes multiple distinct species within evolutionary radiations; current genetic research is underway to determine relationships between evolutionary groups and species before names can be assigned (Hatfield 2001a, 2001b). Lateral plate morphs of resident freshwater forms are recognized as lows (i.e., lateral plates on anterior parts of the fish only), partials (i.e., lateral plates on anterior and posterior ends of the fish with a gap between), and completes (i.e., lateral plates in a continuous row anterior to posterior). In a rare form, plates are entirely absent; G. a. williamsoni is an endangered plateless form exhibiting reduction in pelvic structure, and only occurs in drainages in southern California. Lows from the Pacific coast of North America have been called G. a. microcephalus, Pacific coast completes are G. a. aculeatus, and Penczak (1964, in Wootton 1976) designated lows from Iceland as G. a. islandicus. A plateless form occurring in Shay Creek, San Bernardino County, California, has been identified as G. a. santaeannae (or santa-annae) but is currently recognized as synonymous with G. a. williamsoni (Ross 1973, Moyle et al. 1989).
Studies of allozyme variation (Haglund et al. 1992) and mitochondrial DNA sequences (Orti et al. 1994) in Asian, North American, and European populations recognized two primary clades: (1) European, North American, and some Japanese samples, which could be divided into an (1a) Atlantic basin clade comprising the eastern North American and European populations, and a (1b) basal Pacific basin assemblage comprising western North American and some Japanese populations; and (2) a divergent group of Japanese populations. The divergent Japanese clade deserves further study and possible taxonomic recognition.
Sympatric species pairs bearing "limnetic" and "benthic" life histories and distinct morphologies have evolved in several British Columbian lake systems (Thompson et al 1997, Hatfield 2001, Hatfield and Ptolemy 2001). Some populations of these are endangered or already extinct (Wood 2003).
Several low-lying lakes and streams in the Cook Inlet area contain rare and evolutionarily divergent populations of G. aculeatus including three populations polymorphic for lateral plate morphs, several populations polymorphic for pelvic armor morphs, one lake containing 2 freshwater morphs of the species (a benthic and a limnetic feeder), and one lake containing both anadromous and resident freshwater forms of the species (von Hippel, pers. comm.). Bell and Orti (1994) viewed divergent populations in freshwater habitats around Cook Inlet as parts of an endemic radiation warranting special consideration for conservation as a unit.