"On the 17th we caught an Octopus,
or Devil fish in the bathing place. This fish is the most horrible thing
to look at in the world! He looks something between a Dragon, Star Fish,
and the Devil. A large Octopus is capable of holding a man under the water
until he is suffocated..."
While I personally have a much greater fondness for cephalopods (squids and octopuses) than Joseph Makin apparently did, his fear may not be completely misplaced. The Gulf of California is home to one of the largest squids on the planet, the "jumbo squid" Dosidicus gigas. (You may recognize the mug shot—D. gigas is also a resident of Monterey Bay during El Nino years such as this one.) It reaches total lengths of nearly 2 meters and can weigh as
much as 50 kilograms. Although one report
exists of a jumbo squid attacking a diver (see "Mugged by a
Squid" in Mark Norman’s Cephalopods: A World Guide), most
sources tell me that these squid are generally not aggressive toward
humans. However, the dive shops in La Paz take tourists on squid dives at
night using shark cages—Squid food for thought. Fortunately we don't
have to worry about it during our daytime SCUBA dives. Dosidicus
gigas migrates down to more than 300 meters depth during the day. This
migration to depth is my main interest on this cruise.
Oxygen levels are astonishingly low at
mid-depths in the Gulf of California. In fact, between 300 and 800 meters,
oxygen is nearly undetectable by the oxygen sensor on the submersible.
Animals require oxygen to harvest the chemical energy in the food that
they eat. Large, active animals require more oxygen than small, slow
animals. So how do large, active animals, such as these squids, manage to
survive in this oxygen minimum layer?
To answer this question, we've been fishing
for jumbo squid at night off the back deck using a large jig that looks
like a glow-in-the-dark, medieval, torture device. So far, Jeff
Drazen has the record squid at 12 kilograms (but the competition is not
over)! I'm measuring the oxygen binding ability of the squid's blood,
which contains a copper-based protein that turns blue when it binds oxygen
but is clear without oxygen. I can exploit this color change to gauge how
much oxygen the blood is carrying.
The majority of animals here migrate out of
the low oxygen layer at night. This is convenient for Jeff Drazen and Bruce
Robison, who are able to catch a great diversity of fishes in fairly
shallow water by trawling at night.
Squids are not the only aggressive creatures we've encountered. Karen Osborn reports that the density of spiders on the bottom during today's dive was so high that she witnessed frequent interactions, often aggressive, between them. Additionally, today's dive caught radiolarians, pteropod molluscs, siphonophores, jellies, and a strange predatory doliolid that is being described as a new species by MBARI scientists. Tomorrow's dive promises to be exciting. Jeff Drazen is planning to put the carcass of his award-winning squid on the bottom to see what large fishes might be attracted.