x, 97 p. : ill. A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / In all species of pycnogonids (sea spiders) males care exclusively for the offspring, making this group essential for studies on sex roles, sexual selection, and the evolution of parental investment. Unfortunately, little is known about pycnogonid mating patterns, larval development, or the costs associated with parental care. The mating habits of both male and female Achelia simplissima were studied experimentally and reveal that both sexes routinely mate multiple times and have multiple mates. Parental males experience higher frequencies of predator attacks and epibionts and a lower rate of movement as compared with nonparental males. However, parental males are harder to dislodge than nonparental males and suffer no change in feeding frequency as a result of parental care. The external morphology of the first larval stage of Achelia simplissima was described using SEM photos and compared with other larval pycnogonids. Morphological characteristics suggest a "parasitic" mode of postembryonic development. / Committee in Charge: Alan Shanks, Chair; Svetlana Maslakova; Craig Young