The fish gill, like any gas exchang

The fish gill, like any gas exchanger, is modified
to: (1) maximize the surface area available for
diffusion of O2 and CO2; (2) minimize the diffusion
distance between the external medium and the
blood; and (3) maximize the perfusion of the tissue.
The gill evolved from the surface epithelium of the
branchial basket of protovertebrates, which was
used in filter feeding, and probably appeared about
550 million years ago in the Pteraspid agnathans
(Gilbert, ’93). Evolutionary modification of a surface
epithelium to facilitate gas exchange is not
without physiological cost: in an aquatic environment
it exacerbates any diffusional movements of
solutes or water into or out of an organism that is
not iso-osmotic to the medium. Ancestors to the vertebrates
were iso-osmotic/ionic to their marine
habitats, but, for reasons that are still debated
(e.g., Griffith, ’87; Evans, ’93), the vertebrates (except
the hagfishes) are not. So aquatic gas exchange
presents osmoregulatory problems to
fishes. Although osmoregulation in fishes is mediated
by a suite of structures including the gastrointestinal
epithelium and kidney, the gill is the
major site of ion movements to balance diffusional
gains or losses. The marine elasmobranchs have
evolved a rectal gland that provides for ion extrusion
(see Shuttleworth, this volume), but there also
is evidence that gill ionic extrusion mechanisms exist
(see below). This review examines current models
for these transport steps in the gill epithelium
in fishes and shows how they are pivotal in acidbase
regulation and nitrogen excretion. For recent
reviews of these subjects, see Perry (’97); Claiborne
(’98); Karnaky (’98); Marshall and Bryson (’98);
Walsh (’98). For the purpose of this review, we
will focus on teleost fishes, with some references
to the elasmobranchs, and do not include gill
transport steps for Ca2+ regulation, which have
been reviewed recently by Flik and his coworkers
(’95, ’96). For a discussion of what little is known
about gill function in the agnatha (hagfishes and
lampreys), see reviews by Evans (’93) and Karnaky
(’98).