Pycnogonids are dioecious. Females can be distinguished from males by the poorly developed ovigerous legs or by their complete absence. Also, females with eggs have very enlarged femurs. The gonad, either testis or ovary, is single, U-shaped and located in the trunk above the intestine. The open end of the U is directed forward and from each arm lateral branches extend far into the legs. In both sexes, the reproductive openings are located on the ventral side of the coxa of different pairs of legs. The specific legs and the number of legs possessing gonopores vary in different species and are not necessarily the same in both sexes. The eggs on reaching maturity migrate into the femurs of the legs containing gonopores.
In most pycnogonids in which egg laying has been observed, the male fertilises the eggs as the female emits them, and the male then gathers them into his ovigerous legs. Glands on the femurs of the male provide cement for forming the eggs into an adhesive spherical mass. The eggs are brooded by the male and the egg masses are held around the middle segments of the ovigerous legs. Bristles on the inner side of the segments aid in retaining the eggs. The male is not limited to one mating during a season and may carry several egg clusters.
The eggs are brooded by the male until they hatch. In most pycnogonids, a larva called a protonymphon hatches from the egg. The larva has only three pairs of appendages, representing the chelifores, palps and ovigerous legs, and each appendage is only three-segmented. A short proboscis is present, but the trunk segments are still lacking. The larva either remains on the ovigerous legs of the male or more frequently becomes an ectoparasite or endoparasite on hydroids or corals. In either case the protonymphon larva is followed by a four-legged and then a six-legged form each with a further pair of posterior buds. When the adult form is reached some appendages may be lost.
Source: Barnes, 1968 & King, 1974.