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Abstract BROODING IN SOUTHERN SEAS: ADAPTATION OR CIRCUMSTANCE? John S. Pearse During the past century, much attention was focused on how polar
marine animals adapt to what was perceived as an exceptionally harsh
environment. How marine invertebrates breed exemplified this attention.
Because the Challenger Expedition in the latter part of
the 19th century had discovered species of Antarctic echinoderms
that carried embryos on their bodies and lacked dispersive larvae,
it was accepted that this mode of reproduction (brooding) was characteristic
of polar invertebrates. Further work in Greenland reinforced this
view and brooding was widely accepted as an adaptation to conditions
inhospitable for pelagic larvae. The supposed predominance of brooding
in polar seas became a favored topic of theoretical ecologists.
During the second half of the 20th century, however, more and more
examples of polar species with pelagic larvae came to light. Many
are non-feeding and rich in yolk. However, many of the most conspicuous
and abundant species have pelagic feeding larvae. Thus, neither
lack of larval food nor other supposed harsh conditions are important
constraints on mode of reproduction. Brooding species, which are
found worldwide, are not particularly abundant in the Arctic. But
they are unusually abundant in the Antarctic, and many, if not most
brooding species in the Antarctic are members of species-rich clades.
It appears to be the unique history and conditions of the Southern
Seas, especially the Antarctic Circumpolar Current, that have favored
speciation in brooders over the past 30 million years.
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