Comparison of functional anatomy in squid and local hydrographic and biological pressures from two different habitats -- Hawaii Sthenoteuthis oualaniensis and Monterey Bay Doryteuthis opalescens.

Background/Question/Methods

Selective pressures on animals will determine some of their anatomical features, and understanding how selective pressures act on specific structural features requires an understanding of local enviromental and biological stimuli. The chromatophore system in squids provides a novel example. In general, the density and size of chromatophores varies according to habitat and niche (Messenger 2001). Chromatophores provide predator avoidance through camouflage, and species that inhabit shallow, coastal environments tend to have higher chromatophore densities than oceanic species, presumably conferring an increased ability to mimic complex benthic features. Chromatophores are also used for intra-specific communication in both coastal and oceanic environments, but little is known about patterns of activity in oceanic species. Comparing chromatophore size, density and activity patterns with video and photographic equipment in coastal vs. oceanic squid species could help us understand how some anatomical features in squid have been determined by specific conditions of a particular habitat.  Two species will be compared: the coastal Doryteuthis opalescens and the oceanic Sthenoteuthis oualaniensis.

Results/Conclusions

Chromatophore measurements were made on both ventral and dorsal sides of individual squid. D. opalescens (ML = 109.22 ± 0.5 mm) had a slightly higher chromatophore density on the dorsal side ( 49.5 chromatophore/mm²) and generally larger chromatophores when fully expanded (1.03 mm diameter) than did S. oualaniensis (ML =114.3 ± 0.5 mm; density =  48.6 chromatophore/mm²  and diameter = 0.978). In comparison S. oualaniensis has a higher chromatophore density on the ventral side (22.1 chromatophore/mm² vs. 15.5 chromatophore/mm²⁾ and larger size (0.904 mm vs. 0.83 mm). None of these were statistically significant, but the general trend of higher dorsal density in D. opalescens would be consistent with an enhanced need for camouflage when viewed against a benthic background. Similarly, the higher ventral density in S. oualaniensis suggests a need for camouflage when viewed from below in an open-ocean environment. In contrast to the small differences chromatophore densities, activity patterns in the two species were radically different. D. opalescens tended to display long-lasting patterns over parts of the fins and mantle, whereas S. oualaniensis activated chromatophores over the entire body in a synchronized, repetitive flashing display. The functional value of this flashing behavior has not been established, but it probably serves a communication role rather than camouflage.