LATERAL PLATE MORPH VARIATION IN CALIFORNIA POPULATIONS OF THE THREESPINE STICKLEBACK, <i>GASTEROSTEUS ACULEATUS</i>

Abstract

Marine and anadromous populations of Gasterosteus aculeatus tend to be morphologically uniform, whereas freshwater populations inhabit diverse environments and exhibit great morphological variability (Bell, 1976a, 1984; Wootton, 1976). Although several structures of G. aculeatus vary geographically, the lateral plates have been studied most intensively. They are modified scales that are arranged in a single row along either side of the body. Lateral plate variation includes the lateral plate trimorphism (Hagen and Gilbertson, 1972, 1973), and plate number variation within each of the morphs (Bell, 1984). The complete morph has a continuous row of plates running from the nape onto the caudle peduncle, the partial morph has an abdominal plate row separated from a plate row on the caudal peduncle by an unplated area, and the low morph has only abdominal plates. A genetic basis for lateral plate morph variation has been established (Muinzing, 1959; Hagen and Gilbertson, 1973), and the pattern of geographical variation initially was attributed to divergence in glacial refugia and postglacial secondary intergradation (Miinzing, 1963; Miller and Hubbs, 1969). This explanation has been criticized (Hagen and McPhail, 1970; Bell, 1976a), and Hagen and Moodie ( 1982) suggested an alternative model for the global distribution of lateral plate morphs. They showed that in regions with cold winters and extreme annual temperature ranges, the complete morph dominates and the low morph is virtually absent. In regions with relatively warm winters and consequent low annual temperature flux the low morph is monomorphic. All three morphs occur in intermediate climates. Lateral plate morph frequencies vary markedly over short distances among freshwater populations in regions that harbor all three morphs. Although the functional significance of lateral plate morphs is not understood, the complete morph appears to be associated with more rapidly flowing water than the low morph (Hagen, 1967; Bell, 1982). In this paper we describe spatial variation of lateral plate morphs at two scales. We describe the distribution of the plate morphs among coastal drainages throughout most of the length of California, which includes the transition from low morph monomorphism to trimorphism. Within the cooler northern half of the state, where all three morphs occur, we analyze the distribution of the lateral plate morphs in relation to environmental variation.