Egg Size Determines Offspring Size in Neotropical Cichlid Fishes (Teleostei: Cichlidae)

Abstract

Within the family Cichlidae, the size of eggs varies tremendously across species (Breder and Rosen, 1966; Fryer and Iles, 1972; Coleman, 1991). Sizes range from the small eggs (0.9 mm in diameter) of rams (Microgeophagus ramirezi) to the massive eggs (4.5 mm in diameter) of some of the African mouthbrooding cichlids such as Cyphotilapia frontosa (unpubl. data). The hatchlings of the large-egged mouthbrooders are clearly much larger than the hatchlings of the rams and other small-egged species (pers. obs.). However, the relationship between egg size and hatchling size for smaller differences in egg size has not been explored. For example, within the Neotropical cichlids, the range of egg sizes is much smaller: small-egged species such as dimerus have an average egg diameter of 1.3 mm, whereas the largest-egged species, Cichlasoma (Theraps) tuba has an average egg diameter of 2.4 mm (RMC, unpubl. data). Does this relatively small difference in egg size actually translate into a consistent difference in the size of the hatchlings? Furthermore, does the variation in egg size within a single laying account for differences in hatchling size? Understanding the consequences of differential allocation to egg size between eggs in a single clutch and within and among species will help us understand the factors shaping the evolution of egg size. A larger egg may not necessarily translate into a larger hatchling for several reasons. For example, a given measure of egg size may not be a true representation of the energetic and material resources of an egg. This could be the case if larger eggs are simply filled with more water than smaller eggs, and size differences may solely be accounted for by water content. In such a case, the difference in egg size may be insignificant to the resulting hatchling. This is particularly plausible because fish eggs are hydrated and expand to some extent when they leave the female (Kamler, 1992). An ideal measure of egg size may involve a complete biochemical analysis of the contents of an egg, reporting protein, lipid, and carbohydrate content (e.g., Kamler, 1992); however, this is not practical for many studies. Indeed, for situations when the eggs are particularly small and yet must be kept alive, even weighing individual eggs or measuring their volume can be difficult. We investigated the relationship between egg size and hatchling size in 26 species of Neotropical cichlids. We also examined eggs and hatchlings produced by 17 females within one species [convict cichlids, Cichlasoma (Archocentrus) nigrofasciatum], and eggs and hatchlings from within single clutches of two species (convict cichlids and rainbow cichlids, Herotilapia multispinosa) to look at the effects of intraspecific and intraclutch variation in egg size.