, z. FISHERIES RESEARCH BOARD OF CANADA Translation Series No. 1011 Characteristic features of the structure of cephalopod molluscs associated with controlled movements By G. V. Zuev Original title: Osobennosti stroeniya golovonogikh mollyuskov, svyazannye s upravleniem dvizheniem. From: Ekologo-morfologicheskie issledovaniya nektonnykh zhivotnykh. Kiev. (Special publication), 1966. Translated by the Translation Bureau (AT) Foreign Languages Division Department of the Secretary of State of Canada Fisheries Research Board of Canada Biological Station, St. John's, Nfld. 1968 The devices for neutralization of the force of gravity are part of a whole array of means of movement of aquatic organisms. Yu. G. Aleev (1963), who studied the devices of fish aimed at the neutralization of the force of gravity, established that all such devices of the fish may be divided into two following groups. 1. The devices of hydrostatic action, i.e. the devices aimed at the reduction of body's specific gravity. The hydrostatic action devices are essentially peculiar to the inhabitants of the pelagic zone and all those fish which spend the major part of their time in the main body of water, out of contact with the substrate. 2. The hydrodynamic action devices, i.e. devices, based on the creation of special supporting forces and facilitating the adoption of the suspended state. The hydrodynamic devices are further divided into passive devices, functioning only during the movement of the fish and the active ones, functioning irrespective of the movement or immobility of the fish (Aleev, 1963). THE HYDROSTATIC ACTION DEVICES The capacity of the organism to be in the state of static balance depends on the equality of the specific gravities (density) of the organism and the medium. If the equality is disturbed, the organism either sinks under the force of the residual weight, or rises. 2 The buoyancy (A) of the organism is understood as meaning /15 the difference between the specific gravity of the organism and the water: Q — Q1 e: à where 9- specific gravity of the organism, Q i - specific gravity of the water. If the specific gravity of the organism is equal to the state of specific gravity of the water (Q.q 1), the organism is in the static balance. In this case, the force (G) of gravity applied to the centre of the gravity of the body, and the opposing force Q of the hydrostatic pressure of the water (resistance force), applied to the geometrical centre of the body volume, i.e. in the centre of hydrostatic pressure, are numerically equal (G = Q). In that case the buoyancy of the body is neutral (à = 0 ). If the specific gravity of the organism is greater than that of the water (Q>.9 1 ), then the value of G exceeds the value of Q. In that case the buoyancy of the organism is negative (à < 0). If the specific gravity of the organism is smaller than that of the water (9 <9 ), then the force Q is greater than G and the organism 1 rises to the surface. In this case the buoyancy of the organism is positive (à> 0). From the point of view of organism energetics the most perfect form are those with the specific gravity near that of water. In other words, these are forms with neutral or near neutral buoyancy. Such organisms do not have to make additional movements in order to remain in the main body of water over considerable periods of time. If the buoyancy is neutral, the energy spent on keeping the balance in water is practically nil, - 3 - The neutral buoyancy of the aquatic animals may be obtained by different means. Gas and fat lighter than tissue, intake of water and other means serve this purpose in the invertebrate plankton forms (Zernov, 1949). In the vertebrates (fish, pinnipeds, whales) the specific gravity of the body is reduced either by means of swimming bladder (some fishes), the air in the lungs, or deposits of fat in different tissues and organs (mammals, fishes). But in the majority of the nektonic animals the specific gravity of the body exceeds the specific gravity of water (Lowndes, 1955; Tomilin, 1957; Aleev, 1963; Andriyashev, 19 )44). The cephalopod molluscs also possess different devices aimed at the reduction of the specific gravity. The pelagic forme of Cephalopoda are known already from the Cambrian and Silurian eras; these are the representatives of Nautiloidea and Ammonoidea groups, the shells of which were filled with gas (Zittel, 193)4 ; Shimanskii, 1962, and others). The siphon, which connects the chambers of the cephalopod's /16 shell, was considered by the majority of the authors (Appelliif, 1893; Abel, 1916) as the organ which conducts the gas into the chambers and regulates its quantity, i.e. as an original hydrostatic apparatus. The recent investigations on the buoyancy of the Sepia officinalis (Denton, 1961; Denton and Gilpin-Brown, 1960, 1961a, 1961b, 1961c; Denton, Gilpin- Brown, Howarth, 1961), which has a massive inner shell (sepion) have experimentally proved that the sepion serves as a hydrostatic apparatus. The shell of the Nautiloidea represents a tube coiled into a flat spiral which may reach about three full coils in the adult specimens. The body cavity is subdivided into chambers by means of a number of septa. In the last one •- the living chamber, occupying about half of the outer coil of the spiral, the body of the animal is situated; the other chambers - 14 are filled with a mixture of gases. Approximately in the centre of each septum is located the septic ostiole with the septic tube. Tà...e outgrowth of the posterior part of the body, the so called siphuncle, passes through these ostioles. The soft part of the siphuncle is represented by the connecting tissue, the intermediate spaces of which are filled with blood. Furthermore, the arterial blood vessel - the siphuncle artery, passes through the connecting tissue. The studies on the hydrostatics of the contemporary Nautiloidea (Bidder, 1962) have shown that the shell °healers contain not only gas but liquid as well and that the amount of the liquid decreases with the age of the animal. The gas, collected in the chambers, is nitrogen with its pressure near atmospheric (at atmospheric pressure). The Nautiloidea are capable of changing their buoyaney. The buoyancy is controlled by means of the siphuncle, which regulates the inflow and the discharge of the gas ("fluid" ? Translator) from the chambers. Recently the opinion was expressed (Joysey, 1961) that all Cephalopoda having shells with air filled cavities may osmotically secrete liquid from their body tissues and suck it back. This liquid plays the role of the ballast. The fact that the amount of liquid decreases with the age of Nautiloidea indicates the possibility of inflow and discharge of the liquid from the chambers. The gas hydrostatic apparatus of Nautiloidea simplifies their sylmming and hovering in water, yet, in turn, excludes the possibility of rapid vertical displacements owing to the great compressibility of the gas. An abrupt change of pressure upsets the conditions of static balance and some time is needed to restore them. The features of the hydrostatics of the Nautiloidea, the general shape of their bodies are such that one -5 may with sufficient precision refer to them as slow-swimming megaloplanktonic/17 animals. The pattern of life of the contemporary representatives of this most ancient order of cephalopods further convinces us that this judgement is correct. The Belemnoidea were essentially predators and, with a smell exception, belonged to the nektonic forms (Abel, 1916; Krymgolets, 1958; Naidin, 1965). It is worth mentioning that the functional significance of the rostrum was long a matter of controversy among scientists. The role of the inner skeleton of the Belemnoidea was studied in detail by O. Abel (Abel, 1916). As is known (Zittel', 1934; Kondakov, 1940; Krymgol'ts, 1958), the inner skeleton of the Belemnoidea consisted of three major parts: the phragmocone, the proostracum and the rostrum. The phragmocone consisted of a more or less long core divided into chambers by closely spaced transverse septa. The dorsal edge of the phragmocone was stretched forward in form of a thin wide lamella, or the proostratum, covering the soft tissues of the body. The rear end of the phragmocone was as if embedded in a lime sheath which extended in the form of a runner, pointed at the end, i.e. the rostrum. The controversy about the functional significance of the inner skeleton of the belemnites could be solved only by mathematical calculations. At the request of O. Abel, such calculations were made by F. Gafferl'. It emerged that the chambers of the phragmocone, if filled with gas or air, created a positive buoyancy, pushing the animal to the surface of the water. So it was proved that the inner skeleton of the belemnites played the role of a hydrostatic apparatus. Apparently, the rigid rostrum was used by the belemnites as a support, a streamlined formation and a counterweight in their three dimensional orientation in the body of water (Zuev, Makhlin, 1965; Naidin, 1965). To the best of the author's knowledge, no one studied the hydrostatics of the Octopoda. Data on specific gravity and buoyancy of octopi are absent from the literature. We have conducted measurements of the specific weight of the octopi Octopus vulgaris, Eledone moschata, Eledone cirrosa, of the Mediterranean Sea and the Octopus sp. from the Red Sea (Zuev, 1963) using the volume-weight method with the formula: ^ P (2) ' where P weight of the octopus in the air (gr.), V - volume of the octopus (cm ). The specifie gravity was determined using live specimens only. As a result of these investigations, it emerged that the specific gravity of all the above-mentioned types of octopi was 1.06, i.e.