No. 13 - Hermaphroditic Salps
Our nets catch a wide variety of creatures. This summer the
fastest growing, the most widespread, and the most abundant of
the planktonic animals is the Salpa thompsoni, or just salps to
their friends. Salps are an open ocean tunicate, an animal
composed of a gelatinous tube that sucks water in one end and
propels it out the other after passing it over a mucus membrane
that vacuums it clean of all edible material. Everything that is
caught is moved over to a small sphere containing the digestive
and reproductive organs. That's basically its life plan: it
squirts, consumes and reproduces. Salp sex, however, is a little
unusual. In the summer, solitary salps bud off hundreds of
clones in long chains without the need of a partner. The buds
grow rapidly in size as they Hoover the sea in vast clouds.
They're also hermaphroditic -- an individual is both female and
male -- and this generation reproduces sexually, either with
itself or by filtering sperm broadcast from one of its neighbors.
Each bud will harbor a single embryo which will then over-winter
and begin budding the following spring.
When conditions are right the salp population can increase 1000
times from one year to the next, and then decline just as
rapidly. This happens perhaps two years out of five and we refer
to these as salp years. During a salp year the number of other
species common in the plankton is reduced by two-thirds, the
water is clear as sea algae is consumed as fast as it can grow,
and krill can't find enough to eat to sustain their reproduction.
This is one of those years. Earlier this month the buds were no
larger than 1/2"; now the biggest are several inches and new ones
are produced each day. They dominate the catches everywhere we
sample and as they grow their weight becomes ponderous. Because
they're mostly water, however, they aren't very attractive as
food for the penguins, seals and whales that normally feed on
krill.
We humans don't appreciate salps either, which have the
consistency of a large, clear goober -- with a dark brown ball
imbedded at one end. They clog the nets and expand a typical
catch from a quart in volume to several gallons. They triple the
work of finding the other animals in the sample and they
themselves must be sorted, sized, counted and disposed of. After
dealing with this growing menace, station after station, the
biologists are a little whacked out. The population is still
growing and they're worried that next month's survey could bury
them.
-Roger