MARINE FLORA AND FAUNA OF THE NORTHEASTERN 

 UNITED STATES. Pycnogonida 



LAWRENCE R. McCLOSKEY 1 



ABSTRACT 



The manual includes an introduction on the general biology, an illustrated key, an an- 

 notated systematic list, a selected bibliography, and an index to the Pycnogonida along the 

 coast of the United States from Maine to New Jersey out to 100 m. 



INTRODUCTION 



The Pycnogonida, or sea spiders, are one of 

 the most curious groups of marine invertebrates. 

 Their peculiar anatomy, a montage of arthropod 

 characters, reveals no clear affinities or homol- 

 ogies with other apparently related arthropods 

 (Fig. 1) . Generally, the first pair of pycnogonid 

 appendages are formed into pincerlike cheli- 

 fores (sometimes called chelicerae or man- 

 dibles) , and the second pair develop as sensory 

 palps (or pedipalps). In addition to these two 

 pairs of appendages, used mostly in feeding, the 

 first segment of the body usually has two pairs 

 of legs. The first pair (when present) are called 

 ovigers, and, in the male, are used to hold and 

 carry the clusters of eggs. (However, in one 

 family, the Colossendeidae, the "ovigers" are 

 used only as grooming appendages.) The sec- 

 ond pair on the first body segment are walking 

 legs — the first of four pairs used for locomotion. 

 The legs often comprise the bulk of the animal's 

 body mass, and in keeping with unconventional- 

 ity, have nine segments, rather than the eight 

 usually found in arthropods. An extra somite 

 (that is, body segment) , with legs, appears in 

 some species, resulting in a total of 10 legs. 

 Two species are known which have 2 extra so- 

 mites and 12 legs. The appendages are never 



1 Systematics-Ecology Program, Marine Biological 

 Laboratory, Woods Hole, Mass.; present address: De- 

 partment of Biology, Walla Walla College, College Place, 

 WA 99324. 



biramous. The abdomen is quite rudimentary 

 and possesses only the anal opening. 



Though superficially spiderlike, pycnogonids 

 do not have the typical arachnid body arrange- 

 ment of a cephalothorax (or prosoma) with six 

 legs and a large abdomen (or opisthosoma) . 

 They differ clearly from the classes Merostomata 

 and Arachnida in having no respiratory or ex- 

 cretory organs, in having the mouth located at 

 the end of a proboscis, and in having multiple 

 gonopores which open on the legs. They show 

 affinity to the chelicerates in the arrangement 

 of the brain which has a protocerebrum and a 

 tritocerebrum, without a deuterocerebrum. 

 Hence, they have been placed in a separate class, 

 Pycnogonida, under the subphylum Chelicerata. 



Within the range covered by this manual 

 (Maine to New Jersey), five species of pycno- 

 gonids may be readily found in nearshore 

 waters: Tanystylum orbiculare, Callipallene 

 brevirostris, Phoxichilidium femoratum, Ano- 

 plodactylus lentus, and Pycnogonum Uttorale. It 

 is these species which have received the most 

 attention. Thomas Hunt Morgan's treatise 

 (1891) on the embryology of pycnogonids, treats 

 Tanystylum orbiculare (p. 4-8, 36-48), Callipal- 

 lene brevirostris (p. 8-22 under Pallene empusa), 

 and Anoplodactylus lentus (p. 4-8 under Phox- 

 ichilidium maxillare) . The habits of Anoplo- 

 dactylus lentus have been examined by Cole 

 (1901, p. 195-206; 1906, p. 740-741), its blood 

 was studied by Dawson (1934, p. 62-68), and its 

 muscles by Jordan (1916). Phoxichilidium 



