Breeding of the sea spider Phoxichildium femoratum (Rathke, 1799): functional anatomy perspective

Sea spiders (Pycnogonida) are one of two arthropod taxa that exhibit external fertilization, making them interesting subjects for the reconstruction of plesiomorphic arthropod breeding features. However, data on pycnogonid breeding are limited, particularly their fertilisation was not observed at all. Most knowledge on of their breeding is mere assumptions based on occasional observations. In this work, we observed and photo- and video- recorded the breeding behaviour (courtship, oviposition, fertilization, and eggmass formation) of Phoxichilidium femoratum (Rathke, 1799). Additionally, we studied eggmasses for the presence of sperm and additional cement. The male initiates mating by climbing onto the female’s back and courts her until oviposition. During oviposition, the male gonopores are in proximity to the eggmass, and sperm is injected into the swollen vitelline envelope surrounding the laid eggs. Once oviposition is complete, the male leaves the female hooking the eggmass with one of his ovigers. No additional shaping of the eggmass by the male was recorded, nor was there any fastening of the eggs with additional cement. The eggs in the eggmass are secured solely by swollen vitelline envelopes. Thus, fertilization in a mucous eggmass is undoubtfully demonstrated for the species (and likely characteristic of most sea spiders). Our observation questions the cementing function of male femoral glands, widely accepted in review literature. Also, basing on comparison of P. femoratum with other sea spiders we suggest a hypothesis on eggmasses evolution.