Octopus (3 page)
Authors: Roland C. Anderson
Under this variable skin, an octopus, like other mollusks, has several ganglia, collections of nerve cells that help to regulate activity, spaced around their body. In the octopus, the ganglia are centralized in the brain, which is a large one for invertebrates. Besides these nerve centers, a huge part of the octopus's brain is devoted to controlling the skin display system, and two areas are devoted to storing learned information. We tend to think that nerve cells belong in the brain, but in the octopus three-fifths of the nerve cells are in the arms. They are probably needed there to control the very complicated movement that octopus arms and suckers can make.
The blood of octopuses and other mollusks is blue because of their oxygen-carrying pigment, hemocyanin. Hemocyanin isn't as efficient as our hemoglobin at carrying oxygen. For the slow-moving standard mollusks, that doesn't matter much, but since the octopus's physiology is faster, its circulatory system is modified. The blood is circulated in a closed system, a series of arteries going out to the body and then veins bringing the blood back to the heart, like in humans. Still, this system isn't adequate for the octopus, so the animal has evolved two accessory hearts at the base of the gills that push blood out to get it to the oxygen faster.
Mollusks are in the phylum Mollusca. Since the head-foot group is so different from the snails and clams, they are defined as the class Cephalopoda. Among the cephalopods, there's one really different group, the few species of Nautilus, which are remnants of a huge group that dominated the seas in ancient times; they are slow, live in deep water, and have a scavenging lifestyle. The rest of the cephalopods are the subclass Coleoidea, comprising four orders: true squid, or Teuthoidea; cuttlefish, or Sepioidea and Sepiolida; one deep-sea species of vampire squid, or Vampyromorpha; and the octopuses, or Octopoda. Animals in these four orders have the same basic body plan, though the addition of two elastic tentacles defines the squid as decapods (or ten-footed).
Octopuses face ordinary challenges in unusual ways. Their amazing mobility lets them go almost anywhere they want on the sea bottom to find a hidden crab or snail. The changeable skin allows them to conceal themselves against a wide range of backgrounds or to dazzle a potential mate. They can walk, swim, or pull themselves along the bottom with extraordinary grace, and they can solve problems that other mollusks might not
even comprehend. All of these characteristics make octopuses fascinating to study.
In the following chapters, we take you through the life of an octopus. We start with the octopus egg, in
chapter 1
. Then we describe the issues that matter to an octopus as it goes through the different stages of its existence, and we conclude with mating and the end of life. In the final chapter, we discuss other cephalopods related to octopuses that help us understand the octopus. Then we offer a guide to obtaining and keeping an octopus in an aquarium.
In this book, as authors we refer to each other as “we” or by our first names. Other research findings are cited by first and last author names, and those studies as well as ours are included in the reference list at the back of the book. In the text, you'll find that sometimes the squid is used as the basis of discussion when no good octopus example is known, because squid are similar in structure and physiology.
By the way, the plural of octopus isn't octopi, because the word is Greekâoctopous to be exactânot Latin. The Greek plural would be octopodes, but we call them octopuses.
1
In the Egg
O
ctopuses are oviparous: they lay eggs like chickens do. Some animals are viviparous, like humans: offspring grow inside the mature female, who then gives birth to fully developed young. Other animals are ovoviviparous, like snakes: the female essentially lays eggs inside herself, the eggs hatch inside, and she gives birth to live young. Almost all octopuses lay eggs outside their body and the embryos develop inside the eggs. The process of producing viable eggs and ensuring that the young successfully hatch out is the most important task of a female octopus. This task comes at the end of her life, and she totally devotes herself to it.
The female octopus has two ovaries, where the eggs are produced. As in many other invertebrates or fish, the octopus's ovaries are cream-colored and granular. The ovaries of mature females take up much of the space inside the body: the egg masses may be 25 to 30 percent of the female's weight in some species, like the red octopus (Octopus rubescens), and up to 40 percent in other species. If humans had this high a gonad index, a 120-lb. (55 kg) woman would have a 48-lb. (22-kg) baby!
During mating, the male inserts a spermatophore, or sperm packet, by passing it along its arm to the oviduct of the female. There, the spermatophore turns inside out, the sperm are pushed to the end of the packet nearest the oviduct, and the end of the spermatophore bursts open, releasing the sperm. The sperm are stored in the wall of the oviduct in a special organ, the spermatheca, until they are needed to fertilize eggs. When eggs travel down the oviduct before they are laid, they pass through the spermatheca and get fertilized. We know that live sperm can be stored in the spermatheca for a long time, because, for example, a female giant Pacific octopus was shipped from Tacoma, Washington, to an aquarium in New York City. She was kept alone for seven months and then laid fertile eggs.
Before the female octopus can lay her eggs, she must find a suitable place for them. Most shallow-water octopuses, like common octopuses and
Hawaiian day octopuses, lay their eggs in a den. The den might be in a crevice or cave in a rock wall, in an excavation under a rock, or under human-made underwater structures such as concrete blocks, pilings, or shipwrecks. Red octopuses have laid eggs in beer bottles, aluminum soft drink cans, and even in an old shoe. Smaller species of octopuses, such as the Caribbean pygmy octopus, may lay their eggs inside clamshells or even a beer can. And several octopus species carry their eggs with them, not laying them in dens at all. The female argonaut, a pelagic octopus, makes a delicate and minimally coiled shell up to 18 in. (46 cm) across in which to lay her eggs.
If undisturbed, octopus females almost never leave their clutch of eggs. For better protection, a number of shallow-water female octopuses even block up their den openings with rocks, shells, or other material, and don't move the obstructions until the eggs are about to hatch. Hidden dens make it difficult for scientists to find nesting female octopuses and study this important part of their life cycle; Jim Cosgrove (1993), because he has such intimate knowledge of their habitat, has successfully found the dens of nesting giant Pacific octopuses.
Like a chicken's egg, what we call an octopus egg is a complete package, with cushioning material, a yolk for nutrition, and a shell for protection. As a fertilized octopus egg passes down the oviduct getting ready to be laid, it goes through the nidamental gland, where it is coated with a nutritious and protective, clear jelly. It then goes through the oviducal gland, which forms a protective sheath, the chorion, around the egg and its jelly. In some octopus species, the egg is then ready to be laid and guarded by the female. Octopus eggs are typically oval or teardrop shaped, and most are tiny, the size of a grain of rice.
In most octopus species, the chorion is drawn out on one end of the egg to form a stalk. In some species, the female lays large eggs and uses this stalk to attach each egg singly to the ceiling of her den. In other species where the female lays small eggs, a long strand or string is produced by the oviducal gland, and strings of many eggs, up to several hundred, are delicately woven together by the suckers on the female's arms near the mouth to form a festoon of small eggs. Each festoon is then attached individually to the ceiling of the den, forming a cluster. In some species, such as the common octopus or the giant Pacific octopus, there may be hundreds of festoons with tens of thousands of eggs. The female may take more than a month to deposit all these eggs in the den.
The Mysterious Argonaut
Argonauts (
Argonauta argo
) are an enigmatic group of octopuses that are highly specialized. Females begin to build a coiled paper-thin shell like a cornucopia when they are young, and they live in it and lay their eggs in it. They secrete the shell with greatly modified paddle-shaped arms. These arms cover the shell, even when they are swimming. The shell doesn't provide any flotation and these octopuses are good swimmers, living in the open ocean in tropical and subtropical zones, swimming constantly with their water jets to keep them from sinking.
The argonaut was named after the crewmen of the ancient Greek mythological hero Jason, who made epic journeys aboard the ship
Argo
, because argonauts were found “wandering” in all the tropical oceans. The coiled argonaut shell looks somewhat like a nautilus shell, but argonauts are different from chambered nautiluses (
Nautilus
spp.): the shell is part of the body of nautiluses, but female argonauts can leave their shell and then reenter it.
Very little is known about argonauts. We know the eggs are fertilized somehow by pygmy males about an inch long (2.5 cm), which have never been seen in the wild. The males somehow deposit a detached arm containing sperm into a female, and she then uses the sperm to fertilize the eggs. We know little about what argonauts eat, although ones in aquariums have eaten shrimp and small fish. No one has kept them in aquariums longer than about three weeks. Young coming from the tiny eggs, just
of an inch (2 mm), have never been raised. There is also some indication that argonauts are social. Chains of six to eight argonauts have been photographed, each female holding onto another's shell. We don't know if this was a chance occurrence or deliberate.
We also know little about species of argonauts. They have been identified by characteristics of their shells, largely from those that have washed up on shore, not the actual animal. There appear to be six or eight species with several more undescribed. Norman (2000) tells of a mass stranding he investigated that took place in Australia. Thousand of females with eggs in their shells washed up on the beach, and most of the females and their eggs died. Such mass strandings of argonauts are not uncommon; they may occur from weather or oceanographic conditions.
âRoland C. Anderson
In general, octopuses lay either tens of thousands of tiny eggs or approximately 100 larger eggs, depending on the species. The eggs of the common octopus are about
in. (3 mm) long, while those of some deep-sea species are more than 1½ in. (40 mm) in length. Each strategy works, or else the species would die off.
Scientists have described the different reproductive strategies of animals in terms of their ecology and life style as a continuum on a scale from r to K. The rate of population increase is the r factor at one end of the scale, and K stands for the carrying capacity of the environment at the other end. Animals using the r strategy mature early at a small size, have a short life span, produce a large number of young with no parental care, and die shortly after reproducing. Animals using the K strategy mature later in life, live longer, produce fewer young with parental care, and can reproduce more than once. Different octopus species fall different places on the râK line. The r strategy is used by those species producing planktonic young, and those producing benthic young are farther toward the K end of the scale. The deep-sea spoon-arm octopus is one with the highest K value; it produces just a few young and has a relatively long life span of four years or more.