SOMERTON: LIFE HISTORY OF DEEP-SEA KING CRAB 



To demonstrate that walking legs are more 

 elongated for crab species inhabiting greater 

 depths, two leg dimensions, merus length and 

 propodus length, were compared between deep 

 and shallow species (Table 1). Two trends in 



Table l.— Merus height (MH) and propodus length (PL) of 

 the right first walking leg, expressed as a fraction of merus 

 length (ML), are shown for one adult male specimen of three 

 species in the family Lithodidae and three species in the genus 

 Chionoecetes. Under each taxonomic section, species are ar- 

 ranged according to depth , with the shallowest species at the top, 

 except for C. hairdi and C. opilio, which occur at similar depths. 



relative leg dimensions are evident. First, the legs 

 become thinner with depth; that is, merus height 

 to merus length decreases. Second, the distal 

 portion of the legs becomes relatively longer with 

 depth; that is, propodus length to merus length 

 increases. 



The selective advantage of long slender legs for 

 deepwater crabs is not obvious. Barnes (1974) 

 suggested that: "Many abyssal crabs have long 

 slender legs for crawling about over soft bottoms." 

 Presumably, this means that the propodi and the 

 dactyli are placed flat on the substrate and used, 

 like snowshoes, to spread the body weight more 

 evenly. However, underwater photographs of both 

 L. couesi (Figure 8) and Geryon quinquedens (see 

 Wigley et al. 1975, fig. 4) show individuals walk- 

 ing over apparently soft bottoms on the tips of the 

 dactyli. An alternative explanation, suggested by 

 Childress (1971b), is that reduced musculature in 



Figure 8. — Photograph on the Patton Seamount ( 500 m) of Lithodes couesi (lower) and Chionoecetes tanneri (upper) walking across a 



soft bottom on the tips of their dactyli (photograph taken by P. Raymore). 



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