Octopus (7 page)

Hawaii, on the other hand, sits in the mid Pacific, isolated in the center of a mid-oceanic gyre of ocean currents that circle these islands. As a consequence, plankton including young octopuses tend to stay in the area, and currents from other islands rarely reach Hawaii. According to the theories of biogeography—essentially why an animal lives at which places—Hawaii should then have a high percentage of endemic animals (those only found there), including octopuses, and it does. Because of the currents circling the islands, planktonic offspring from several octopus species remain there rather than getting carried elsewhere or lost.

Living in the plankton exposes the octopus paralarvae to a great chance of getting eaten because there's no place to hide, and they do get eaten. Out of the 50,000 eggs laid by a common octopus, it only takes one male and one female hatchling to survive in order to reproduce and maintain the species. And, in fact, if more survived, the ocean would be awash with octopuses.

Predators in the plankton are varied in their methods of catching and eating prey. Jellyfish trail out their long tentacles that are studded with lethal stinging cells. Schools of millions of fish, like herring, snap up billions of planktonic animals each day. Large whales consume plankton like a vacuum cleaner sucks up dust. To get an idea of how menacing these planktivores are, picture a paralarval octopus, just ¼ in. (0.6 cm) long, as a potential meal for a typical jellyfish, 3 ft. (1 m) across with tentacles up to 60 ft. (18 m) long. Such large animals as jellyfish, whales, or whale sharks eat tremendous numbers of tiny planktonic animals just to survive, including the tiny paralarval octopus.

But the large planktivores are not the main predators of plankton animals. That distinction belongs to arrow worms, which are not worms at all but are members of their own phylum, Chaetognatha. The second most abundant animal group in the sea, they are about 1 in. (2.5 cm) long and have paired fins and a tail for swimming. They have eyes and cilia for detecting their prey, mostly tiny crustaceans called copepods, which they grasp with spiny mouthparts resembling the jaws of the alien of movie fame. To an octopus paralarva, an arrow worm or a ctenophore would appear to be the size of a great white shark (see plate 8). We actually know little about how octopus paralarvae live in the tough world of the plankton and almost nothing of their ecology there. (For insights, you may wish to read Roger Villanueva and Mark Norman's substantial 2008 review.) What little we know comes from a few precious observations in the wild, as well as from field collections obtained with a plankton net and from raising a few species in the lab.

Scientists collect plankton, including paralarval octopuses, by pulling a fine-meshed net through the water. There have been many modifications to the old-fashioned hoop-type plankton net utilized by naturalist Charles Darwin on the Beagle in the 1830s. Devices on nets can now record the depth they collect from, characteristics of the water such as temperature and oxygen, and even the volume of water strained by the net, allowing calculation of the original concentration of the plankton collected. Unfortunately,
as some scientists have learned in their hunt for planktonic giant squid, nets towed through the water are impossibly harsh on planktonic creatures, especially tiny gelatinous octopus paralarvae. Most paralarvae die from the rough contact with the net, the change in pressure or temperature as they are brought up, being mashed together with thousands of other organisms at the bottom of the net or even just from being lifted out of the water. When placed in a shipboard aquarium, some may live for a few minutes, giving us a tantalizing glimpse of their natural behavior. Paralarval octopuses have been attracted to lights hung overboard from a boat at night and then dipped with special nets, which is the most effective, least harmful method of collecting these tiny fragile creatures.

Such collections of preserved paralarval octopuses, either fresh or in museums, give us clues as to what they look like and what they eat from our microscopic examination of their stomach contents. But most behavioral observations come from rearing a few species in the lab. To date, paralarval examples of only a few species have been reared, including the common octopus, the giant Pacific octopus, and the Caribbean pygmy octopus. They are so tiny and their physiology is so specialized that the task is very difficult. Little Californian two-spot octopuses, which hatch out as benthic young, were used by David Sinn and colleagues (2001) to trace the development of personality, and are a good model of early development of cephalopod behavior.

From examination of many preserved specimens, scientists have outlined the specific adaptations that paralarval octopuses have for living in the plankton. These are possession of statocysts (balance organs, necessary for maintaining position and posture relative to up and down), a “lateral line” along the body that gives them touch sensitivity, large eyes, jet-propelled locomotion, an ink gland, and very fast growth. The few chromatophores they have at this stage are usually contracted, making the paralarval octopus nearly transparent. The large bulbous eyes are used to find prey and see predators. The prehensile arms of a paralarval octopus (see plate 9) are much shorter than those of adult animals, but they are still used to seize prey and are used as rudders and keels when swimming. When octopuses hatch, they have about a day's supply of food in the remains of their nutritious egg sacs, but after absorbing this nutrition, the newly hatched paralarvae need to eat quickly. Like adult octopuses, they already have a working ink gland stored in an ink sac and can poof ink in the faces of planktonic predators. And their growth rate is phenomenal.
For baby octopuses, the time before settling—leaving the plankton and drifting to the bottom of the ocean—must be weighed against the time to disperse. There are advantages and disadvantages to settling early and settling late.

Keeping paralarval octopuses alive in the lab is a difficult task, but it allows scientists to observe the behavior of hatchlings, if even briefly. Among the major challenges to raising newly hatched octopuses in captivity, the first is simulating the open-ocean environment. Paralarval octopuses are not used to swimming into walls, like the vertical sides of aquarium tanks. Scientists at the National Resource Center for Cephalopods, in Galveston, Texas, and at the Seattle Aquarium have used a coating of a tiny checkerboard pattern on the tank sides to help tiny octopuses see the walls and avoid jetting into them. Tanks have also been designed with circular walls and upwelling circular currents to keep the drifting paralarvae from hitting the walls or sinking to the bottom. New or recirculated water must be drained from the tank without sucking in the paralarvae, so tank drains have to be screened thoroughly.

Another problem of raising paralarvae is providing them with adequate food. In the wild, they eat tiny copepods, shrimp and crab larvae, and larval fishes. These organisms must move (be alive) in order to attract the attention of the paralarvae. At the Seattle Aquarium, staff worker Susan Snyder fed her paralarval giant Pacific octopuses tiny bits of shrimp meat but had to keep the pieces moving for the tiny animals to eat them, and she had to be meticulous in keeping uneaten food off the bottom to prevent fouling the tank. Providing live food for paralarvae is a distinct challenge. Villanueva and Norman (2008) fed common octopus paralarvae the young of several crab species. Paralarvae are also susceptible to disease, primarily a bacterial one presenting as white patches on the arms and body, which can be treated by chloramphenicol. Disease may be a reflection of the water quality, especially if you use seawater from an urban harbor.

Another problem in raising paralarvae is their cannibalism. Octopuses have no recognition of their own species, and will eat smaller individuals of their own species (conspecifics), whether parlarvae or adults. Each female produces hundreds of thousands of paralarvae, and in the ocean, dispersion by the surface currents usually keeps them from encountering one another. But in the lab, animals are usually reared together and are relatively crowded, giving them the opportunity to eat each other (see Julio Iglesias-Garcia et al., 2007).

Although few species of paralarval octopuses have been raised in the lab, scientists have made a number of valuable observations on newly hatched paralarvae octopuses of species that may live for up to a week or so before dying, probably from starvation. When one aquarium worker attempted to keep giant Pacific octopus paralarvae alive, he fed them bits of krill, which sat on the surface film of the water a few minutes before they sank. The young octopuses turned upside down and actively searched out the food particles floating on the surface, which is known as neustonic feeding.

When Villanueva gave them crab zooea larvae in 1994, he watched the octopuses go through the positioning, orientation, and jet attack sequence that John Messenger described in 1977 for cuttlefish. He also observed that the paralarvae grasped and probed the food for several seconds before moving it along the arms to the mouth, suggesting that they contacted the food with the chemosensory cells in the suckers to establish whether it was edible before ingesting it. He also found that the hatchlings were selective: they would eat krill more often than brine shrimp or larval fish, and survived much longer when fed that food than when fed the other foods.

Richard Ambrose (1981) observed the behavior of Verrill's two-spot octopus hatchlings both in the field and in the lab. He found that they swam backward except when catching prey, using their water jets as a primary mode of movement much as squid do, and that they kept a near-constant upward tilt of 45 degrees, with arms trailing down. At hatching and afterward, they exhibited a strong positive phototaxis—swimming toward the light—as do most octopus paralarvae. This behavior is likely because the paralarvae need to swim upward to keep from sinking down from the rich surface layers of the ocean. When swimming toward a prey item, such as a copepod, the paralarva used a forward swimming movement in a straight line toward the prey item, which it then grasped with its arms. These paralarvae also ate brine shrimp, but they only lived six days, probably because of a nutritional deficiency.

As they grow in the plankton, the paralarvae quickly change in shape as well as size. We usually describe their growth as a percentage of weight gain per day. In the paralarval stage, octopus growth is about 5 percent per day. To put this in perspective, at this rate a 5-lb. (2.3-kg) human baby would put on ¼ lb. (0.1 kg) the first day, and at the end of the first month would weigh 22 lb. (10 kg). This very fast growth rate is rarely achieved by any other organism.

The change in form is necessary because the paralarvae are going through a big transition in living style as well—they are soon going to settle to the bottom and begin the life style of an adult octopus. The rest of the body grows faster than the eyes, which therefore shrink in proportion to the body, as in human children. Although still used for sighting prey, predators, and conspecifics, the eyes will no longer have to be able to see underneath the body when the adult octopus is sitting on the substrate, or sea bottom.

The arms grow longer. Common octopus hatchlings have arms 37 percent as long as the mantle, but at settlement, the arms are 91 percent of the mantle length and are still short of the up to 400 percent of mantle length at adulthood. Benthic adult octopuses use their much longer arms in different ways from the paralarvae. The arms are used for probing under rocks, throwing arms in parachutelike webovers in prey capture, for defense, and in mating.

In a quick transformation just before settling, the maturing paralarvae grow more chromatophores and suckers on their arms, compared to hatchlings having only a couple of working chromatophores and a couple of suckers per arm. Adult octopuses live in a much more complex and varied environment than the paralarvae do, and they have to develop skin patterns and papillae for camouflaging, and suckers for manipulating the environment and catching prey. Sigurd von Boletzky (1987b) suggested a further transition of the hatchlings, their brain size. The brachial lobe, the brain area that controls the arms, enlarges, and there may be changes in the memory area of the brain, the vertical lobe, which helps the adult octopus to adapt to changing environments, to capture various prey animals, and to escape from different predators.

Another change from the planktonic to the benthic mode of life takes place in the octopus's cardiopulmonary system. Since planktonic paralarvae swim constantly, using their water jets as a squid does, they must get oxygen out of the water at the same time as they are swimming. Therefore, the water jet is used for locomotion as well as for respiration. Benthic octopus adults are rather poor swimmers: human scuba divers can frequently keep up with or even pass them. Martin Wells and his students (1978) showed one reason for their poor swimming: an adult octopus goes into temporary cardiac arrest and oxygen debt while swimming—the three hearts literally stop beating. After a major swim, the octopus needs to rest a while and “catch its breath.” In addition, the pigment in the blood of the
octopus is the oxygen-binding hemocyanin, which is less efficient than our hemoglobin. But to date, no one has measured and reported the cardiorespiratory efficiency of paralarval octopuses.

We scientists know little about the important transition from a pelagic to benthic life for octopuses. It's a big ocean out there, the paralarvae are still tiny, and only when someone finds them by accident at the right time do we gain any insight into the process. Maybe as they get bigger and heavier, they sink. Maybe as the brain develops, different areas mature and dictate bottom-seeking behavior rather than planktonic behavior.

The paralarvae that are ready to settle and take up a benthic life have to be changed both physically and mentally from their life in the plankton. Species of octopus paralarvae have been studied in groups. Red octopus paralarvae have been observed from an ROV (remotely operated underwater vehicle) at a depth of 500 ft. (150 m), hovering in mid water off Los Angeles in the Catalina Channel. Since the observed paralarvae were far below the plankton layer, they had probably achieved the right size for settlement and were on their way to the bottom, gradually drifting down en masse. Many adults were seen on the bottom at this location, and thousands of these paralarvae were seen together in a loose group. Based on the results of rearing experiments in the laboratory, the paralarval size may be the most important factor in determining whether an octopus is ready to settle or not. It is possible that these red octopuses were the correct size for settling but that they were in water that was deeper than normal and therefore were taking a long time to get down to the bottom.