Plasticity in sexual size dimorphism and Rensch’s rule in Mediterranean blennies (Blenniidae)

Comparative analyses of sexual size dimorphism (SSD) across species have led to the discovery of Rensch’s rule. This rule states that SSD increases with body size when males are the largest sex, but decreases with increasing size when females are larger. Within-species comparisons of SSD in fish are rare, yet these may be a valuable tool to investigate evolutionary patterns on a fine scale. This study compares SSD among closely related populations of three species of Mediterranean blennies (Blenniidae): Microlipophrys canevae (Vinciguerra, 1880), Parablennius incognitus (Bath 1968), and Aidablennius sphynx (Valenciennes, 1836). SSD varied more among populations than among species and Rensch’s rule was confirmed within two species. It is not likely that the variation among populations in SSD mirrors genetic variation, as many of the populations were in close proximity of one another, with a high potential for genetic exchange. This study complements larger scale analyses of other taxa and demonstrates the fine scale on which evolutionary processes responsible for Rensch’s rule may be operating.

Female Preference for Nests With Eggs Is Based On the Presence of the Eggs Themselves

In many fish species of which males care for eggs in a nest, including Aidablennius sphynx, females prefer to mate with males that already guard eggs. In this paper we present two aquarium experiments with this Mediterranean blenny to determine whether the females actually use the presence of eggs as a cue or depend on male display behaviour signalling the presence of eggs. In experiment 1 the test-female was presented with two nests, one with eggs and one without eggs, and only one male in a closed container between the two nests. This male only served to stimulate the female. In all tests females preferred to lay eggs in the nest already containing eggs. This shows that females of this species can base their choice on the presence of the eggs alone. In experiment 2 the test-female was presented with two males in nests of which only one had eggs. Transparent partitions deprived the female of the opportunity to inspect the nest contents. The male with and the male without eggs did not differ in time spent displaying towards the female, and the female did not show a preference for either male, based on visual cues or odours. After removal of the partitions the female visited both nests. The nest she visited first was equally likely to be the nest without eggs as the nest with eggs. In contrast, the female finally deposited eggs preferentially in the nest with eggs. We conclude that in this experiment females did not base their preference for males with eggs on male behaviour, but on the nest contents after inspection. This does not exclude an influence of male behaviour under other conditions. Furthermore, females generally deposit their eggs adjacent to as many as possible of the eggs already present, suggesting that females can localize and possibly estimate the amount of eggs already present.

Mate Choice in Aidablennius Sphynx (Teleostei, Blenniidae); Females Prefer Nests Containing More Eggs

Criteria for female mate choice were investigated in a natural population of a Mediterranean blenny, Aidablennius sphynx. Removable test tubes in concrete blocks were offered as nests. Each tube was guarded by a male and females laid eggs in the tubes. Nests with larger broods received significantly more spawning females. The numbers of eggs laid increased with brood size from empty nests to intermediate brood sizes, but this tendency is reversed if nests contain very large broods. Nests with and without broods were exchanged experimentally among males. The number of eggs a male received, after an empty tube was replaced by a tube with eggs, was significantly higher. Conversely, the number of eggs received after a tube with eggs was replaced by an empty tube, was significantly lower. Male display did not increase the probability that females spawned. Male size did not correlate with the numbers of eggs received. It is discussed that female preference for large broods might be adaptive because the number of eggs present in the nest is probably a reliable predictor of the survival chances of the eggs.