The
epithelium sits on a typical basal lamina composed
of an electron-dense and a thinner electron lucent
layer. The thickness of the epithelium and basal
lamina is often less in fishes trying to minimize
their blood to water diffusion distances in order to
optimize gas exchange (e.g., active pelagic teleost
FISH GILL MORPHOLOGY 197
T. albacares, 0.55 mm, versus sedentary benthic
teleost Solea variegata, 2.80 mm, mean total blood–
water distance; see Hughes, ’84). Mucous cells and
MRCs are not as commonly encountered in the
lamellar epithelium, although there are exceptional
cases.