Male sea spiders often mate with more than one female. During courtship, male pycnogonids use their ovigerous legs to induce the females to release eggs. Once the female begins to lay eggs, the male fertilizes them as the female holds them on her ovigerous legs. In some genera, the eggs are relatively large and the female only releases 4-5. In other genera, the eggs are small and the female releases more than 100 eggs. Depending on the leg type, after fertilization, the male will either gather the eggs one by one onto his legs or hooks his ovigerous legs into the egg mass and gathers most of the eggs into a single mass onto his legs.
Male sea spiders carry cemented egg clutches gathered from females until they hatch; the actual amount of time the male carries the clutches depends on species and geographic location. Upon hatching, larvae have only three pairs of legs and develop an additional pair (or pairs) upon subsequent moltings. The most common larval type is a free-swimming naulpius-like larva. However, in some species, the males continue to provide care of the larvae until after several moltings. In this case, the females produce eggs with a large amount of yolk and the young continue to live off of the yolk while attached to the male's ovigerous legs.
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