Chapter 1 6 



P\;cnogomda of Enewetak Atoll 



C. ALLAN CHILD 



Department of Invertebrate Zoology (Crustacea) 



National Museum of Natural Histori; 



Smithsonian Institution. Washington. D. C. 20560 



INTRODUCTION 



The Pycnogonida (sea spider) fauna of the western 

 and mid-Pacific island chains is almost unknown. Although 

 generally benthic organisms, a few are considered plank- 

 tonic and are associated with hosts in parasitic or commen- 

 sal relationships. Most are known to feed on a wide 

 variety of soft-bodied marine life with coelenterates 

 reported as food more often in the literature than any 

 other group. Pycnogonids should therefore be, and prob- 

 ably are, quite at home on the coral reefs of atolls and vol- 

 canic islands of these Pacific chains. The fact that there is 

 only one paper (Child, 1982) on pycnogonids from the 

 Marshall Islands, and particularly Enewetak Atoll, attests 

 not only to the scarcity of pycnogonids but also to a lack 

 of collections containing pycnogonids from the small and 

 usually remote Pacific islands. 



Pycnogonids are collected fortuitously during the 

 course of benthic samplir.g for other marine organisms. 

 They are almost impossible to see in their regular habitats 

 because of their microscopic size and their cryptic colora- 

 tion, which renders them almost invisible Pycnogonids 

 have been found in the laboratory during the microscopic 

 examination of benthic samples, sometimes long after the 

 collection was made, and it is usually impossible to return 

 to the same collection locality for more samples. That 

 large numbers are found in benthic samples demonstrates 

 the careful collecting and sorting procedure during the field 

 efforts. 



The pycnogonids of the Marshall Islands and Enewetak 

 Atoll predictably will not be as diverse or varied as the 

 fauna of large islands or continental shores. The diversity 

 of benthic habitats on coral atolls is reduced and almost 

 totally excludes the muddy estuaries and weed-encrusted 

 rocky shores of continental masses and large islands. Many 

 groups of organisms are faunistically impoverished on coral 

 atolls. Barnard (1965), in discussing predominantly benthic 

 amphipods, states that "Micronesia primarily offers an epi- 



faunal environment in the shallow sea, with scarce rem- 

 nants of muddy coastal shelves fringing larger islands and 

 continents. Hence the lack of [diverse] shallow sea bot- 

 toms, the diminution of environmental variability and the 

 decrease of food from runoff should be limiting factors." 

 This impoverishment would extend to those groups of pyc- 

 nogonids preying on algal-attached sessile organisms. The 

 lack of large expanses of foliose algae and sea grass beds 

 on atolls, including Enewetak, limits the availability of 

 many kinds of pycnogonid foods that use these plants as 

 substrates. 



The embryology of most pycnogonids, at least for 

 those groups about which we know anything of their life 

 habits, is not conducive to wide island arc dispersal. The 

 eggs and young of many pycnogonids are carried by the 

 male on their ovigers — appendages specifically modified 

 for this purpose — and have no known planktonic dispersal. 

 Their metamorphosis is directly from larva to adult, but in 

 some recorded instances, the eggs are deposited in a host 

 organism, such as a bivalve mollusk, where the young lead 

 a parasitoid existence until they reach a free-living adult 

 stage. Dispersal from island group to island group is prob- 

 lematical, and many pycnogonid species are thought to be 

 endemics, some in quite restricted areas. Many of these 

 endemics may be due to a lack of collecting in adjacent 

 areas. I believe though, that when more is known about 

 pycnogonid distribution, many of these species will remain 

 recorded as endemics. 



Only five pycnogonid species in four genera are known 

 from Enewetak Atoll (Table 1). A sixth, N^mphon 

 micronesicum (family Nymphonidae), is also known from 

 Bikini (Child, 1982). This is a very small faunule when con- 

 sidering the great diversity of this class of arthropods in 

 the oceans of the world. There are approximately 900 

 recognized species of pycnogonids in 76 genera and nine 

 families (a very small faunule when the great diversity of 

 this class of arthropods is considered), and we probably 

 know about one-half or less than half of the world's 

 species. The littoral shores of the world have not been 

 sampled to any great extent, and with a few exceptions, it 

 is in this environment that the greatest diversity, if not the 

 greatest numbers, of pycnogonids live. Further collecting in 

 littoral and sublittoral habitats on Enewetak Atoll and other 



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