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U.S. NATIONAL MUSEUM BULLETIN 291 



Figure 20. — Bathyteuthis abyssicola and B. bacidifera: a, index of gill length to mantle 

 length; b, index of gill width to mantle length; c, index of gill length to gill width. 



B. hacidifera has larger gills than B. abyssicola, but in one dimen- 

 sion (gill length) the eastern Pacific form of ahyssicola approaches 

 hacidifera. This is not totally unexpected since the Eastern Pacific 

 Equatorial Water Mass is extremely low in oxygen content and certain 

 species may have to adapt to the lowered oxygen concentrations (see 

 following discussion and Ebeling and Weed, 1963; Marshall, 1960; 

 Walters, 1961). The slight increase in gill length in eastern Pacific 

 abyssicola, however, is not accompanied by an increase in number of 

 gill filaments or a more favorable gill-width to gill-length ratio. 

 Meristic characters might be preferred to morphometric features as 

 being more stable and more indicati^•e of differentiation. In this case, 

 however, the meristic and morphometric characters combine to lead 

 to the conclusion that the observed differences in gill size between 

 abyssicola and bacidifera are specific and are not only a phenotypic 

 expression of environmental conditions. Furthermore, the differences 

 within populations of abyssicola are also genetic and are maintained 

 within the geographic boundaries of the populations. 



