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Bizarre 'gunslinger' octopus caught for first time

Loren Coleman lcolema1 at maine.rr.com
Mon Feb 10 10:10:09 EST 2003

Australian Broadcasting Corporation

Bizarre 'gunslinger' octopus caught for first time
Wednesday, 5 February  2003

The small male keeps his eighth 'reproductive' arm in the big white pouch at
the front (pic: David Paul, University of Melbourne)

For the first time, the tiny male of a bizarre species of octopus, which
carries a slew of stinging weapons and is 100 times smaller than its
females, has been captured alive by Australian scientists off the Great
Barrier Reef.
The find was made by a group of scientists from the universities of
Melbourne, Tasmania and Leeds, during a 'blackwater hang' - a night dive
suspended in deep water. The discovery is described in the latest issue of
the New Zealand Journal of Marine and Freshwater Research.
"You always hope you might bump into something like this," said Dr Mark
Norman, an honorary fellow at the department of zoology at the University of
Melbourne. "The chance is infinitesimally small."
Dead males have been found in trawls and plankton nets before, but this is
the first time a live one has been seen. Known as the blanket octopus, or
Tremoctopus violaceus Chiaie, it spends its entire life cycle in the open
ocean, so either sex is rarely encountered.
"We know so little about the sea," Norman told ABC Science Online. "These
are one of those almost mythical creatures of the sea. For us, it's as
exciting as swimming with a giant squid."
While other octopus species also have size differences between the sexes -
known as sexual size-dimorphism - the blanket octopus is the most extreme
example known. The male specimen discovered is 2.4 cm long, weighing just
0.25 kg. Mature females have been measured at 2 m in length, weighing about
10 kg.This mature female blanket octopus is 100 times larger than the male
(Pic: Koji Nakamura, Japan Underwater Films)

The male seems to have adapted to his small size, having a comparatively
large eye which could help in locating females. Being small may also mean
that they take less time to mature.
But for the male, sex is a one-off and probably fatal affair. They allocate
one of their arms to reproduction, keeping it in a pouch in the centre of
their tentacles. When they mate, the pouch ruptures, sperm is injected into
the tip of the modified arm, which is then severed and passed to the female.
Thereafter, the male then almost certainly dies, while his detached arm
remains in the female's mantle cavity until it is used to fertilise her
eggs. Females are often found with several arms in their cavity, indicating
competition amongst males.
The scientists have never found a dead male with a new arm, suggesting that
they die after mating. Other octopuses are known to be able to drop off
their arms as decoys to predators, in the same way that some lizards lose
their tails. Arms tend to grow back in six to eight weeks.
But the blanket octopus' kamikaze approach to fertilisation - shedding an
arm that keeps living when the rest of the animal dies - is only found in a
couple of other octopuses, said Norman. "It looks like the arms do live for
a long time [inside] the females."
The small size of the male may be an adaptation for self protection. Male
and immature female blanket octopuses arm themselves with weapons snatched
from competitors, taking stinging tentacles from the Portuguese man-of-war
jellyfish and holding them in the suckers of two pairs of their upper arms -
like a gunslinger with a pair of revolvers.
When threatened, they pull their arms over their bodies and expose the
stinging tentacles.
But this only works if the octopus is small and would probably not be enough
to protect a large mature female: no female longer than 7 cm has been
observed to carry the tentacles. Scientists speculate that the bigger
suckers in the mature animal may not be able to hold the tentacles without
stinging themselves.

Danny Kingsley - ABC Science Online


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