Here you see the scanned images of male and female stickleback made at a resolution 3200 pixels per inch. External morphology of stickleback is associated with their way of living. First, stickleback share their time between open waters and coastal habitats, and second, behavior of males and females is very much different during the reproduction period (watch the video).
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Streamlined body shape helps stickleback to swim fast, which is very important for fish spending most of their time in water column. Stickleback body shape, however, is not so streamlined as, for instance, herring, sardines or anchovy which spend in the open water their entire life. These species have also larger forked tail which in most fishes is a main moving organ. In stickleback, pectoral fins also are very important for moving why they are so large. Other important elements of the stickleback body are armor structures, such as three strong dorsal spines, a pair of ventral spines, and lateral plates. They cause some difficulties while swimming, but provide effective protection from predators. Thus, in stickleback body morphology we see a trade offs between living in coastal and pelagic habits, and between quick swimming and protection against predators.
Sexual dimorphism of stickleback represents another interesting aspect of their morphology. The White Sea stickleback manifest, in general, typical for the threespine stickleback patterns, although in fish from other parts of the distribution range patterns can be somewhat different. In the proposed images we can only describe the body shape, although coloring is also very important – red breasts and blue eyes in males. These features of colour, however, are poorly expressed on the scanned images. Several aspects of morphological sexual dimorphism should be considered.
First, females exceed males by size, in average, they are about 12% heavier. This, along with numerical prevalence of females, allows to the population maximize energy for reproduction.
Second, males have relatively larger anterior body, eyes and mouth. These organs are important for effective protection of their nesting territory and offspring, and avoiding predators. Mouth, for instance, is actively used not only to get food, but also to fight with other males during spawning season.
Third, females, have larger posterior body where they carry eggs. In addition, one specific armor structure, pelvic girdle, which is situated behind the pelvic spines, protects eggs from below, is larger in females.
Forth, other armor structures such as back spines (originated from the first dorsal fin), which gave the name to the species, and pelvic spines are larger in males.
And fifths, caudal peduncle, which is important for swimming, is larger in females because they spend in open waters more time than males, which protect their territory and nests in the bottom.
Therefore, differences in external morphology between males and females is quite well may be explained by differences in their way of living.