The convergent evolution of cephalopods and fish has often been discussed on an anatomical basis, but recent advances in the knowledge of cephalopod life cycles, physiology, and biochemistry suggest that there are constraints on cephalopods that prevent them from competing directly with fish. These advances are reviewed against the background of detailed information on fish from the perspective that the basic inefficiency of the jet-propulsion system has required bioenergetic, physiological, and biochemical adaptations in squid which maximize their metabolic rates. Such "high-energy" adaptations are suggested to have resulted in the short life history and semelparous reproductive patterns that seem to characterize these coleoid cephalopods. Conversely, the physiology and biochemistry offish give them distinct advantages for long lives and iteroparity.

The constraints on cephalopods: why squid aren't fish

The study of cephalopod populations currently lacks the means to define populations adequately and to resolve basic systematic confusions Quantitative data are usually only available from indirect sources such as commercial fisheries and from estimates of consumption by higher predators Despite these methodological difficulties it is clear that cephalopods comprise a major component of biomass globally, throughout all fully marine habitats Life-cycle characteristics common to the coleoids - early and/or semelparous breeding, rapid growth, short lifespan, little overlap of generations, vulnerability to predation and environmental variables - result in wide inter-annual fluctuations in abundance Most of the pelagic forms also undertake large- or meso-scale migrations which, coupled to shifting patterns of oceanographic variables, contribute to the unpredictability of distribution and density associated with many cephalopod species Temporal and spatial patterns of breeding, seasonality, growth, recruitment and mortality are clearly evident in most of the better-studied species But exceptions to pattern (eg variable growth rates, extended breeding, complex recruitment) also seem to be important intrinsic characteristics Levels of genetic variation in cephalopods are relatively low, and their population dynamics appear to be influenced principally by phenotypic plasticity in response to environmental variability In such universally short-lived species the maintenance of this diversity balances the risks of mortality factors combining at any one time to cause periodic local extinction The extent and scale of the interactions between cephalopod populations and other trophic levels suggests that major ecological perturbations such as environmental shifts, or imposed effects such as commercial fishing, whether directed at cephalopods or other species, are likely to have an impact on their populations As short-lived species with high turnover of generations, plastic growth and reproductive characteristics, high mobility and catholic predatory habits, they are always poised to respond to changed balances in their environment Studies on cephalopod populations have expanded considerably in numbers and scope in the last 25 years, driven by increased interest in and recognition of their roles in the marine ecology, as well as their increasing value as globally exploited resources Despite these recent advances, the information and concepts arising from their study is only slowly entering mainstream biological thought and becoming accommodated in broad-scale models of the marine ecosystem

Cephalopod populations: definition and dynamics

When using cephalopods as experimental animals, a number of factors, including morality, quality of information derived from experiments, and public perception, drives the motivation to consider welfare issues. Refinement of methods and techniques is a major step in ensuring protection of cephalopod welfare in both laboratory and field studies. To this end, existing literature that provides details of methods used in the collection, handling, maintenance, and culture of a range of cephalopods is a useful starting point when refining and justifying decisions about animal welfare. This review collates recent literature in which authors have used cephalopods as experimental animals, revealing the extent of use and diversity of cephalopod species and techniques. It also highlights several major issues when considering cephalopod welfare; how little is known about disease in cephalopods and its relationship to senescence and also how to define objective endpoints when animals are stressed or dying as a result of the experiment.

Ethical and welfare considerations when using cephalopods as experimental animals

Over 1200 squids were captured by night lighting, trawling, or seining in the northern Gulf of Mexico for laboratory maintenance. Two types of recirculating sea water systems were designed and evaluated: a 2 m circular tank (1500 liter capacity) and a 10 m long raceway (10,000 liters). Mean laboratory survival was: Loligo plei (12 to 252 mm mantle length, ML) 11 days, maximum 84 days; Loligo pealei (109 to 285 mm ML) 28 days, maximum 71 days; Lolliguncula brevis (27 to 99 mm ML) 19 days, maximum 125 days. Smaller squids showed significantly poorer survival than larger ones. All squids fed well on a variety of live estuarine fishes and shrimps. Growth rates depended upon stage of maturity. The highest rates were Loligo plei 59 mm/month (23.8 g/mo), Loligo pealei 77 mm/mo (67.3 g/mo), and Lolliguncula brevis 31 mm/mo (17.2 g/mo). General aspects of behavior and body patterning were species-specific and were useful indices of the squids' condition. Key factors for laboratory survival were (1) prevention of skin damage, (2) tank systems with sufficiently large horizontal dimensions, (3) high quality water, (4) ample food supply, (5) no crowding, (6) maintaining squids of similar size to reduce aggression and cannibalism, and (7) segregating sexes to reduce aggression associated with courtship, mating, and egg laying.

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Survival, growth, and behavior of the loliginid squids loligo plei, loligo pealei, and lolliguncula brevis (mollusca: cephalopoda) in closed sea water systems.

1. 1. Measurements of oxygen consumption (Voo2, ml h−1) and mantle cavity pressure (P, kPa) of squid, Illex illecebrosus , of various masses (M, kg), swimming at various speeds (U, ms−1) in a tunnel respirometer yielded two relationships:
 1. (a) Voo2 = 245M0.73 e1.59U
 2. (b) Voo2 = 555M0.75 P0.77

2. 2. Both jet frequency and peak pressures increase with increasing speed. Patterns varied considerably between individuals, but total area under the pressure curve (P) was well correlated with oxygen consumption and speed.

3. 3. A differential pressure transducer linked to an ultrasonic transmitter carrying average pressure data encoded on impulse frequency was designed to be carried inside the mantle cavity. It was tested in both the swim-tunnel and in video-taped free-swimming animals.

4. 4. The relationships above held over a range of speeds from 0 to 1.4 ms−1, the maximum speed observed, indicating that such transducers could provide direct estimates of both the metabolic rates and activities of these pelagic carnivores in nature; their use to test hypotheses about several energy-saving strategies is discussed.

5. 5. Average thrusts, calculated from pressure data, are consistent with rigid body drag predictions and with overall locomotor efficiencies of 5 %, which are also consistent with the relative costs of transport and efficiency estimates for fish.

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Monitoring the Metabolic Rate and Activity of Free-Swimming squid With Telemetered Jet Pressure

1. Female specimens of Illex illecebrosus of 20 to 28 cm mantle lengths lived, fed and grew in a 15 m diameter pool for periods up to 82 days; deaths during the first week were associated with skin damage during capture and those after the eighth week with precocious sexual maturation.2. Under the conditions described, squid survived repeated capture and urethane anesthesia, as well as tattooing and surgical procedures. Techniques for handling are described.3. Although the cause of early maturation is not certain, squid entering the pool were exposed to increased photoperiods, and a relationship is hypothesized between this stimulus and the natural November spawning migration.4. The rapid sexual maturation observed and the small size of eggs ready to be spawned indicates that the one year life cycle previously proposed for Illex illecebrosus is feasible.

R. D. Durward

Papers

Maintenance and maturation of squid (illex illecebrosus in a 15 meter circular pool