Patterns of Geographic Variation in the Egg Morphology of the Fundulid Fish, Fundulus heteroclitus

Abstract

Both the number of oil droplets within the egg and the external morphology of the chorion of Fundulus heteroclitus were found to vary extensively among populations along the US east coast. The number of oil droplets per egg varied clinally from a mean of 12.7 at Mount Desert Island, ME to 172.3 at Jacksonville, FL for individuals maintained in the laboratory under identical conditions. The pattern of variation in the external morphology of the chorion, as established by scanning electron microscopy, was that of a step cline. The chorion of eggs from Mount Desert Island, ME to Newark Bay, NJ was characterized by a surface sculptured with chorionic papillae approximately 0.5 A in diameter. The chorionic filaments were smooth and conically modified at the base. Over the same area, the filament diameters ranged from 0.8-2.2 A and the filament density ranged from 0-5 per 10,000 2' of chorion surface area. In striking contrast, the chorion surface of eggs from Sandy Hook, NJ to Jacksonville, FL lacked papillae, but had granular, pebble-surfaced filaments attached directly to the chorion with no conical modifications at the base. Filament diameters ranged from 0.7-0.8 A and filament densities ranged from 150-220 per 10,000 2' among these southern populations. Samples from a location intermediate between Newark Bay and Sandy Hook, NJ had an intermediate filament density of 82 per 10,000 A 2. This pattern of geographic variation in oil droplet number and chorion morphology was repeated within Chesapeake Bay and Delaware Bay. Within Chesapeake Bay, oil droplets varied clinally from 31.5 per egg at an upper bay locality to 102.0 per egg at a lower bay locality. Within Delaware Bay, oil droplets varied from 46.9 per egg at an upper bay locality to 102.7 per egg at a lower bay locality. The chorionic filament diameter and density at the most southern Chesapeake Bay locality was 0.6 A and 200-210 per 10,000 A 2 while the filament diameter and density from the most northern locality was 1.9-2.4 A and 0-2 per 10,000 12. Within Delaware Bay, the filament diameter and density ranged from 0.7 A and 190 per 10,000 A2 at the lower bay locality to 1.9 ju and 0-2 per 10,000 A2 at the upper bay locality. On the basis of these characters, upper bay eggs were indistinguishable from eggs of coastal populations north of Newark Bay, NJ while lower bay eggs were indistinguishable from eggs of coastal populations south of Sandy Hook, NJ. We suggest that the present distribution of F. heteroclitus populations of different egg types may reflect the consequences of secondary intergradation, with zones of intergradafion at northern New Jersey and within Chesapeake and Delaware bays.