After treatment for 10 h with 0.004% colchicine in ®ltered sea
water, gill tissue was dissected and immersed in distilled water for
30 min. This material was ®xed in three changes of 3:1 ethanol and
acetic acid for at least 30 min each. The freshly ®xed gills were then
transferred into 1:1 ethanol and acetic acid. Thirty minutes later
they were cut into 0.5 cm pieces, smeared on clean microscope
slides, and air-dried. For conventional karyotype studies, slides
were stained with 10% Giemsa solution (Sigma Chemical Co.) for
20 min, rinsed in tap-water and allowed to dry. Suitable mitotic
metaphases were photographed with a high numerical aperture,
100 ´ objective, on an Olympus BH-2 microscope using Kodak
Technical Pan Film. Chromosome arm lengths were measured in
11 well-spread metaphase plates with a calibrated ocular micrometer.