Octopus by Roland C. Anderson Read Free Book Online Page B

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    Other sea creatures use jelly and air as flotation substances. The mesoglea, or jelly layer, of jellyfish helps them maintain their buoyancy. We have been recently finding many new deep-water species of jellyfish, which, even though they are light, have to swim to keep from sinking to the bottom. Some jellyfish use air for flotation, such as the Portuguese men-of-war, as do some cephalopods (such as cuttlefish and nautiluses), fish, and marine mammals. Cuttlefish have an internal shell of porous air-retaining calcium, a cuttlebone, and nautiluses have a coiled shell with air-filled chambers.
    Another trick to increase buoyancy is to decrease the density of one’s tissues. The few mid-water cephalopods, those that live just above the deep bottom, are usually gelatinous, with tissues close to the density of the deep water they live in. The flapjack devilfish (Opisthoteuthis californiana) and other octopuses like it are sometimes called jellyfish octopuses, because their flabby, gelatinous bodies remind researchers of jellyfish.
    Many of the planktonic animals, including young octopuses, swim to maintain their level in the ocean. The methods of swimming are as diverse as the planktonic organisms themselves. Some general methods include beating of the hairlike cilia, thrashing with one or several whiplike flagella, paddling with fins or feet, pulsing, oscillation or undulations of the whole body, or the jet propulsion used by paralarval octopuses. Hatchling octopuses, like squid, may use jet propulsion to raise them in the water column, and then take a long glide to rest before pumping again.
    It is hard for us to be able to imagine a baby octopus living in the plankton, constantly avoiding sinking into the abyss, watching for gigantic predators that would scoop it up, and deciding which way to swim. We humans are used to living in two dimensions, not looking overhead much, looking forward from our head, and pivoting to see behind us. Our eyes are binocular—both eyes are directed forward with the fields overlapping. Many predators have this arrangement, because it is the best method for tracking mobile prey by sight. But living in the plankton is a three-dimensional experience. Animals in the plankton in open water, such as octopus paralarvae, need to look up and down, left and right, forward and backward to avoid getting eaten by hungry predators. So these animals have big, wide-angle eyes that are adapted to seeing in all directions.
    For the most part, living in the plankton means living in a miniature world. Most planktonic organisms are tiny: a hatchling common octopus is just over 0.1 in. (3 mm) long. Within a drop of ocean water, there is a tiny world, with plants, grazers, and predators. Young octopuses have good vision and some swimming ability, so they can jet after tiny crustacean larvae and capture them with their stubby arms. They also try to avoid getting eaten by sinking quickly when a predator looms. Many do get eaten, but the species usually is not extinguished because there are so many hatchlings. Many of the predators are tiny themselves.
    Since living in plankton means being carried by the ocean’s currents, paralarval octopuses go wherever the currents carry them, and such dispersal is good because it can lead to a wide species range. Yet, paralarvae from octopuses in shallow waters carried by currents out into the mid ocean will probably die. The mid-oceanic islands of Hawaii and Bermuda provide interesting parallels and contrasts for planktonic dispersal of octopuses. Bermuda sits in the mid Atlantic, but it is swept by an arm of the Gulf Stream, the largest river on earth. Although Bermuda is at the same latitude as South Carolina, it is kept subtropical by warm currents that sweep by equatorial South America and through the Caribbean before hitting it. After flowing past Bermuda, the Gulf Stream flows to the cold North Atlantic.
    Bermuda is colonized by planktonic paralarvae and