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First published online November 5, 2004
Journal of Experimental Biology 207, ii (2004)
Copyright © 2004 The Company of Biologists Limited
doi: 10.1242/jeb.01354
Inside JEB |
SQUID'S EYE FOR SCALE
kathryn{at}biologists.com
Lou Zeidberg admits that he's been mesmerised by cephalopods ever since he took biology at high school. So when he joined William Hamner's lab at the University of California at Los Angeles he was keen to work with cephalopods, and his luck was in; having been fascinated by marine invertebrate biology all of his career, Hamner was diversifying into squid. Zeidberg snapped up the opportunity, and began trawling through the squid literature, but it soon became apparent that although adult squid were well studied, virtually nothing was known about the animal's hatchling and juvenile life stages. So when Zeidberg heard that Bruce Robison of the Monterey Bay Aquarium Research Institute had collected digital movies of the elusive juveniles swimming at night in the shallow coastal waters off the Northern California coast, he realised that he may have a unique opportunity to find out more about the mysteries of their life style.
But Zeidberg's first question, when he saw the movies, was how could Robison be sure that these squid were juvenile? After all there were no convenient markers on the movies to scale the squid; they could just be large adults a long way off. Although Robison assured him that they `looked like juvenile squid' Zeidberg decided to identify key juvenile characteristics that could be used to discriminate the young from their elders. Fortunately, Zeidberg had had an earlier stroke of luck. He was offered over 150 juvenile squid that had been collected over a five-year period in small-scale trawls on ocean floor of the continental shelf off Los Angeles. Zeidberg wondered whether he could measure these youngsters' dimensions to quantify their defining characteristics, and use this information to determine the free-swimming squids' sizes from the film alone (p. 4195).
Unfortunately, most squid body parts are highly compressible; not reliable indicators of the animal's life stage. So Zeidberg decided to focus on the squids' only incompressible tissues; the eye and pen structure, which sets the cephalopod's mantle length. Having calculated correlations between the squid's mantle length and some of the squid's more compressible appendages, he then calculated a regression between a dimensionless ratio (the squid's eye diameter to its mantle length) and the dependable mantle length. If he could measure the same dimensionless ratio from the film images, then he could use the regression to estimate the squid's size. Using only the film image and calculated regression, he'd know the size of the squid caught in the camera's lens! Zeidberg had found a way to size the animals from film alone, and by measuring the position of the squid on consecutive film images, he would know the distance that the squid travelled during the time between frames, giving him their speed too.
Zeidberg is optimistic that this new technique could prove powerful for sizing other creatures captured on film, but for the time being, he's returning to his first passion, which is watching animals for a living.
References
Zeidberg, L. D. (2004). Can allometry
measurements from in situ video recordings determine the size and
swimming speeds of juvenile and adult squid Loligo opalescens
(Cephalopoda: Myopsida)? J. Exp. Biol.
207,4195
-4203.
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