Most squid, Sthenoteuthis included, have less than a year to be born, grow, and reproduce before they expire. This translates into an incredibly high metabolism. Young squid in captivity can eat nearly half their body weight five times per day 2.5 times their body weight per day! That would be like me eating 450lbs of food! Adult Sthenoteuthis are easily the most common cephalopods we see on blackwater dives. They can be seen as the large shoals the edges of our lights as we drift through the pelagios on our bi-weekly Black Water Dive.
This week we get to meet the anomaly that is the purpleback flying squid (Sthenoteuthis oualaniensis). They are called “flying squid” because, like Exocoetids (aka flying fish) they can leap and glide across the surface of the water to escape predators that skirt the edges of our lights as we drift through the pelagios. They spend their days hiding in a region between 200-1000 meters deep in the midwater known as the oxygen minimum zone. This is an area where animals with higher metabolic demands cannot sustain their bodily functions due to low levels of available oxygen. So while the low oxygen levels keep the squid safe from their most fearsome predators like tuna, the squid itself requires large concentrations of oxygen as well, thus the conundrum. It turns out that squid kept in tanks containing low-oxygen water can shut their metabolism down to a tenth of the normal rate of consumption. This means that the squid may not only be hiding from predators in the oxygen minimum zone, but it may be using the cold, dark, low-oxygen environment of the OMZ to shut down its metabolism and wait out daily periods of low-foraging opportunity when the squid would otherwise just starve.
Pan-tropical spotted dolphins (Stenella attenuata) often come in to forage on the schools of Sthenoteuthis attracted to our lights. One night, a dolphin pod came in to dine and hit the squid school just under the water’s surface. I watched this shiny object drifting down under the scene and recognized it as a squid’s eyeball drifting into the blackness.
Anyone who has spent any time around Honokohau Harbor has seen a few of these strung up at the fuel pier. Marlin get huge, so it is excusable if you gawked in awe at the 400 pound dead sea monster on display, but the lifeless form is missing what makes them really special. By nearly every measure, living marlin are probably the most impressive fish swimming in our oceans today. For starters, they are world travelers. One animal was tagged off the coast of Delaware and later recovered near Mauritius- a journey of over 9000 miles! At 50 miles per hour, marlin are not only faster than anything most divers have seen underwater, they are faster than most speed boats. And while that 400 pound animal may be more than twice your bodyweight, at 1400 pounds, the largest animal on record was more than 3 times a big as that. Thus, I’d like to use this blackwater blog to celebrate the marlin for the oceanic apex predator that it is. And I’d like to start with a little known species of odontocete called the dwarf sperm whale (Kogia sima).
Dwarf sperm whales have never been photographed underwater. They are notoriously difficult to approach and are known for slipping under the water and descending just out of sight of humans. So imagine my excitement as I found myself in the water with my camera as a pod slowly swam toward me! The boat crew watched in near silence as their fins dipped below the surface, and the animals descended. I may have seen a shadow in the distance, but that was probably my imagination. I just floated hoping the animals would get curious and come back to play. A minute or so later, the boat crew saw me nearly jump out of the water, muffling some sort of alarm through my snorkel. Angry reef fish are quickly calmed by simply swimming away. Even aggressive sharks can be stiff-armed. But what can you do when a 12-foot long marlin charges you in blue water?
Three photos taken in sequence during the encounter.
We have never seen an adult marlin on a blackwater dive, but we have seen quite a few of their young. Marlin breed in the late summer and into the early fall and are capable of spawning as many as 7 million eggs at one go. The resulting young (pictured below) will take 2 years to reach sexual maturity but grow at a rate of more than half an inch per day. Out of that 7 million eggs, maybe one or two will reach sexual maturity and far fewer will reach the legendary benchmark of “grander” (over 1000 pounds).
Post flexion marlin photographed on a blackwater dive in May of this year (2015)
Fortunately for me, the apex predator granted me life as it decided to turn at the last minute (as seen in the sequence of photos above), but I’m afraid we have not been so sympathetic to the fish. Blue marlin have declined by more than 30% of their population in just the last 14 years alone and fishing pressure for this species is increasing. No blue marlin fisheries are considered to be sustainably managed. Thus, like other banner-species of ocean conservation such as oceanic sharks and even bluefin tuna, marlin are in a lot of trouble. Sure, they provide a lot of meat, but because they are apex predators, pollutants such as mercury bioaccumulate in their tissues, so they have some of the highest levels of mercury (>.5 parts per trillion) of any marine fish!
When we speak of Gasterosteiformes in Hawaii, we are generally referring to the order of pipefishes and seahorses. Offshore, that usually refers solely to the pelagic seahorse, Hippocampus fisheri. We see seahorses infrequently in the late summer and fall, usually at a rate of one seahorse for every ten dives or so. Imagine Bryce and my surprise, one night, when we came across a member of this family that we didn’t even realize was possible in Hawaii.
Meet the Hawaiian seamoth (Eurypegasus papilio). Sea moths are related to seahorses, pipefish, sea dragons, and sticklebacks in that they all possess bony external plates. Seamoths shed their plates, and in some cases they will go through a set of plates every 5 days. As an adult, this strange little fish will settle in water between 80 and 115 meters deep from the Big Island to Kure. The underslung mouth implies that the Hawaiian sea moth feeds on benthic animals, and the snout is probably used to help extricate small crustaceans from their holes in the sea floor. For now, the young is limited to the epipelagic zone, attracted to our lights just like, well, a moth.
Young animals stay near the surface until maturity, but large ones like the squid pictured above, stay down deep and only come to the surface at night. Adult animals are so rare, it is likely that none have been seen alive and only a few exist in museum collections. A quiet hush fell over the blackwater divers when I picked up the large specimen for shipment to a cephalopod expert as we all realized that at our feet lay one of the rare night beasties that could show up on our favorite dive. Once again, we are reminded of just how many animals we have yet to see down there.
Salps are the most common organism in the epipelagic environment and thus, far and away the most common animals we see on a blackwater. Unlike most of the blackwater inhabitants, salps, aka tunicates or sea squirts, share our Phylum chordata. That means that at some point in their lives they have a notochord.
I will be the first to admit that all of this taxonomy isn’t very interesting, so lets change the subject to sea squirt sex. The individual oozoid stage is asexual and reproduces by selfing or making a whole bunch of genetic copies of itself in the form of a chain that can either be circular (cyclosalpa) or in a long string (salpa). These chains are composed of a bunch of individuals known as blastozooids which are all attached by a complex network of “plaques” or information sharing connections that allow the salps to swim in a coordinated, synchronized fashion. The blastozooid, or colonial, phase of a salp is the sexual one. To simplify this, if you lived life like a salp, your kids, the result of sex with your spouse (or the mailman-we don’t judge at KHD!), would spawn a bunch of identical twins (reproduction through parthenogenesis) that would then go off to find someone else to reproduce with.
Are you creeped out yet? No? Because it gets weirder. The blastozooid, or sexual form of a salp, undergoes sequential hermaphroditism. So they are females that produce female gametes (eggs) when they first mature, but eventually change sex into males as they get older. While this process seems very complicated, it is also very efficient. Salps can reproduce almost as fast as some bacteria! That means that when conditions are just right (lots of phytoplankton food) they can bloom and inflict severe repercussions on the local plankton productivity.
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Cyclosalpa blastozooid colonies-These are many salp zooids in their sexual phase.
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The empty cavities of salp zooids are habitat for a whole host of organisms. Look for crustaceans like this phronima, argonauta octopodes and even fish inhabiting the interior of salps.