The brain of Antarctic fish of the perciform suborder Notothenioidea w
as analysed with light- and electronmicroscopical methods. The overall
organization and ultrastructure of the optic tectum is very similar t
o that of fish from temperate climates. However, unusual structures we
re observed in neurons and glial cells, sometimes in high frequencies.
The structures are ovoid or elongated, about 200-600 nm in diameter a
nd surrounded by two layers of membranes in a uniform distance of abou
t 30 nm. The enclosed inter-membrane space is similar to extracellular
space, both in size and in cytochemical calcium precipitation, while
the interior of the structures resembles cytoplasm. These structures a
re sometimes connected to neuronal processes, so that they seem to ori
ginate by a sort of budding process, but most of them are isolated as
can be concluded from thick sections of up to 800 nm thickness, analys
ed with energy-filtering transmission electron microscopy (EFTEM). The
se unusual objects are present in high abundance in members of the whi
te-blooded Antarctic fish family Channichthyidae. These so-called icef
ish lack haemoglobin and exhibit the highest degree of cold adaptation
. The red-blooded notothenoid fish had smaller amounts of these struct
ures and they were observed even in fish from temperate climates (trou
t, carp, cichlid fish). In fish from temperate climates the unusual su
bstructures were more abundant during adaptation to cold water tempera
tures (winter) than to warm conditions (summer). Therefore, the findin
gs may indicate a general phenomenon of cold adaptation with unusual i
nteractions of neurons and glial cells, but the precise function is no
t yet understood.