The Collaborative International Dictionary of English v.0.48:

Sea spider \Sea" spi"der\ (Zool.)
   (a) Any maioid crab; a spider crab. See Maioid, and Spider
       crab, under Spider.
   (b) Any pycnogonid.
       [1913 Webster]

WordNet (r) 3.0 (2006):

sea spider
    n 1: any of various small spiderlike marine arthropods having
         small thin bodies and long slender legs [syn: sea spider,
         pycnogonid]