My original post was kindly corrected by Dr. Bonnie Bain, who pointed out that the secretions- mainly emitted by cement glands on the femurs- the eggs were wrapped in were probably not mucus, but no one has investigated the material in depth. Dr. Bain’s paper has a lot of other interesting details about the mating of the sea spiders (pycnogonids), including some fascinating descriptions of courting behavior, and aggression between females for mates.
Bain, B.A. and Govedich, F.R., 2004. Courtship and mating behavior in the Pycnogonida (Chelicerata: Class Pycnogonida): a summary. Invertebrate reproduction & development, 46(1), pp.63-79.
Other references: Woods, Anne. REPRODUCTION & LIFE CYCLE OF SEA SPIDERS. Pets on Mom.com. https://animals.mom.com/spiders-reproduce-live-birth-eggs-7404.html Accessed 11/20/2020.
Image reference: Bernard Picton, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
This is a series of little illustrated factoids about animal reproduction, posted daily. I’d love to hear your suggestions/ feedback. I also post these on my insta.