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

Several recent studies (O’Dor, 1982; Freadman, Hernandez & Scharold, 1984; Webber & O’Dor, 1985) have now shown that the energetics of squid can be studied in swim-tunnel respirometers like those used for fish. Such dynamic swimming invites an examination of the relationship between the intra-mantle pressures and thrust during squid swimming suggested by Johnson, Soden & Trueman (1972) based on Trueman & Packard’s (1968) studies of tethered cephalopods. The present experiments examine the key role of pressure generation in the mantle during dynamic swimming using telemetered pressures from both squid in respirometers and freely swimming squid. The mechanics of the mantle musculature producing this pressure is now reasonably clear (Gosline, Steeves, Harman & DeMont, 1983; Gosline & Shadwick, 1983; Bone, Pulsford & Chubb, 1981), and evidence will be presented that nearly every aspect of squid locomotion, in contrast to that of fish, is subject to direct analysis. Although this paper deals primarily with the empirical observations, the telemetering differential pressure transducer that is described here should allow detailed analysis of the bioenergetics of this nektonic predator from the level of muscle mechanics to natural behaviour in the sea. Squid and fish have many functional similarities (Packard, 1972), but recent studies have shown that they have dramatically different energy strategies (O’Dor & Webber, 1986). Detailed studies of squid locomotion should not only provide insights into squid life history strategies, but also, by comparison, into those of fish, which use a propulsion system much less easily studied in either the laboratory (Blake, 1983) or nature (Rogers, Church, Weatherly & Pincock, 1984; Priede, 1983; Bainbridge, 1958).

Squid, Illex illecebrosus, LeSueur 1821, were obtained from mackerel box traps between August & November, from 1982 to 1985, from Mill Cove, St Margaret’s Bay, Nova Scotia. They were gently removed from the pursed trap by dipnet and placed either in portable aerated 2501 polyethylene tanks (20 per tank) or in 701 buckets surrounded with ice (five per bucket) containing sea water at 1–3 °C previously saturated with oxygen for the 1-h trip to the Aquatron Laboratory. The icing technique ‘calmed’ the squid, was more convenient, and gave at least as good a survival rate (70–100 %) as the ambient temperature method. At the laboratory, the animals were maintained in a pool tank of 700 m³ volume and 15 m diameter under conditions previously described (O’Dor, Durward & Balch, 1977).

Mass and length of the 37 animals used ranged from 0·204–0·672 kg and 0·34–0·52 m. Females constituted 75% of the animals used and ranged in mass from 0·244 to 0·672kg. Males ranged from 0·204–0·496kg. Maximum mantle cavity volume was estimated by filling the mantle cavity of anaesthetized squid with water. A 1·5 % solution of ethanol in sea water was used to anaesthetize the animals for this and for cannula and transmitter insertion.

The respirometer was’ designed by the Department of Engineering, Guelph University as a recirculating water tunnel constructed of acrylic plastic (19·2 cm i.d.). The original design (see Farmer & Beamish, 1969) was extensively modified for squid swimming and respiration measurements. Total volume was reduced from 166 to 921 and the swimming chamber extended to 85 cm in length to accommodate large animals better. Since the maximum cross-sectional area of the largest squid measured (551g) did not exceed 11 % of the cross-sectional area of the swimming chamber and most animals did not exceed 8% of the cross-sectional area, an adjustment for swimming speed was not required. Details of respirometer operation and oxygen consumption measurements are given elsewhere (Webber, 1985; Webber & O’Dor, 1985).

Continuous monitoring of intra-mantle pressures of animals swimming in the respirometer was done with cannulae (1·1 mm i.d. polyethylene tubing) 50–70 cm long inserted through the mantle, positioned lateroventrally, 3–4cm posterior to the edge of the mantle and inhalant aperture (Fig. 1). Two tightly fitting rubber washers (0-8 cm diameter) on either side of the mantle prevented the cannula from slipping. The cannula was attached to a Statham Model P23AC pressure transducer which was connected to an integrating analogue-to-digital (A/D) converter (ADALAB from Interactive Microware Inc., State College, Pennsylvania) with 12bit accuracy (–2048 to +2048), sampling at a rate of 22 analogue measurements per second. A microcomputer (Apple 11 + ) continuously recorded pressure data. The frequency response of the transducer and cannula ranged from 30 to 25 Hz for a cannula of 50–70 cm, respectively.

A telemetry system was developed to monitor continuously intra-mantle pressure of free-swimming animals. Pressure was measured differentially using a semiconductor strain gauge bridge mounted in a stainless-steel housing filled with light mineral oil. The diaphragm faced into the mantle and external pressure caused by depth was compensated for by pressure transferred through an oil-filled 22 gauge needle, as shown in Fig. 1. Output from the transducer was integrated electronically and telemetered ultrasonically at 69 kHz by a miniature transmitter (Voegeli & Pincock, 1981) designed with Vemco Ltd, Shad Bay, Nova Scotia. The transducer (5·3× l·4× 1·4cm) was positioned inside the mantle cavity ventral to the organs, 4–6 cm posterior to the edge of the mantle with the needle and a threaded rod protruding through the mantle as anchors (Fig. 1). A signal at 69 kHz was transmitted to a CR-40 hydrophone-receiver (Communication Associates, Inc., Huntington Station, NY) which sent a digital signal to the microcomputer. The interval between successive signals was proportional to pressure. A 1000 Hz digital timer in the microcomputer was used to measure the interval between digital signals. The transducer was made neutrally buoyant by gluing a strip of rigid plastic foam along each side. This increased the width to 2·0 cm and gave a total volume of 17·5 ml. The transmitter displaced 12·7–6·1 % of the maximum mantle volume for animals ranging in weight from 0·348 to 0·672 kg.

Both methods (cannula and ultrasonic) were calibrated in kiloPascals (lkPa = 10³Nm⁻² = 10·2cmH2O) using a water column of known height. The cannula system was calibrated four or five times for each animal. The calibration was linear and did not change during this time. For normal and burst swimming the gain on the amplifier was set at 2·4× 10⁻³ V and 9·8× 10⁻⁴V for a pressure change of 0·1 kPa. This resulted in a change of 10 and 4 decimal units, respectively, after conversion by the A/D converter. For the ultrasonic transmitter, a pressure change from 0 to 20 kPa resulted in a frequency change from 4·2 to 16·7 Hz, respectively. For both methods, jet pressure was integrated and an average value for pressure was computed for each swimming speed by dividing the sum of pressures by the total number of measurements. The average pressure value accounts for both jet pressure and frequency.

All experimental animals were fasted for 24 h before being transferred in water from the pool tank to the respirometer. The digestion time for I. illecebrosus was 16 h for average meals (Wallace, O’Dor & Amaratunga, 1981). The 24-h interval should be sufficient to ensure that oxygen consumption was no longer elevated because of digestion. An effort was made to minimize excitement by reducing movements in the room and reducing light intensity. To allow comparisons of the energetics and swimming performance of squid and fish, the routine used by O’Dor (1982) for Loligo opalescens, which was a shorter version of that developed by Brett (1964) for fish, was adopted. After a brief exposure to the full range of speeds (0 to 0 ·88 m s⁻¹ ), the speed was decreased to 0·07ms⁻¹, and the animal either settled to the bottom in the ‘resting posture’ (Bradbury & Aldrich, 1969) or swam spontaneously for 2–3 h. After this rest period the squid were forced to swim for 60-min periods at fixed speeds. Commencing at 0·28ms⁻¹ the speed was increased in increments of 0·10ms⁻¹ until the animal collapsed against the downstream grid. Whenever possible, oxygen consumptions and pressures were recorded for each animal in resting posture before and after recovery from exercise. In several cases, video recordings of swimming animals were made using a Sony SLO-323 recorder and an RCATC2011/N low-light camera. Continuous recordings of mantle cavity pressure were synchronized with filmed swimming movements by voice audio and by audio output from the ultrasonic receiver. This method enabled frame-by-frame (30 Hz) video analyses of individual jet cycles and fin activity to be made. Trials in the respirometer were all at 15°C except for squid with transmitters. To allow direct comparison between respirometer and tank, the ambient temperature in the pool was used in these cases.

Squid with transmitters were released in the pool tank for recovery and observation before swim-tunnel trials. A thin, 1·0 cm² aluminized reflector was attached with cyano-acrylate glue to the dorsal surface of the head to aid in recognition when the squid rejoined the school, which generally required only a few minutes. At intervals during this period the animal and the school were restricted to one half of the pool covered by a ceiling-mounted, low-light video camera with a wide-angle lens. During these periods mantle cavity pressure was continuously recorded. To determine swimming speed, a 2·0 m grid was marked on the pool bottom. As well, a 2·0 m stick was positioned at various locations, within the viewing field of the camera, 2 m from the bottom and on the surface. This calibration was necessary to account for the parallax effect of the camera lens. Data were first collected for 1–3 h while the animals were cruising at speeds between 0·10 and 0 ·20 m s⁻¹. Then the animals were disturbed by waving a dipnet in the water, and data were collected at swimming speeds greater than 0·20ms⁻¹.

A 0·412-kg animal was used to compare the cannula and the transmitter methods. The animal was anaesthetized and both a cannula and transmitter were inserted. The signal from the Statham transducer (cannula) was split into both the A/D converter and a Gould thermal recorder (Model TA 600) with a frequency response of 1 ·2 kHz. This permitted a direct comparison of jet cycles measured by the Gould thermal recorder and the A/D converter (Fig. 2A). Despite the lower frequency response of the A/D converter (22 Hz), the peak pressure and shape of the pressure curve were similar for both methods for jets of various sizes. It was also possible to compare jets measured by the Gould recorder and the transmitter (Fig. 2B). During this test the frequency response of the transmitter was only 2 Hz at 0 kPa. The peak pressures and shapes of individual jet cycles were different for the two methods, but the integrated pressure values were similar. The ultrasonic transmitter integrates pressure before it is transmitted, and, since average pressure is derived from the integration of a jet cycle, it is the same for a cycle measured by the Gould recorder or the transmitter.

Regression analyses were performed using Minitab (Ryan, Joiner & Ryan, 1976). Programs to measure analogue pressure and ultrasonic pressure data and to integrate raw pressure data were developed by DMW and may be obtained by writing to the authors.

Illex illecebrosus is negatively buoyant and apparently uses its small fins primarily for control. During normal cruising it maintains its body at an angle such that the thrust from its jet is balanced between pushing the body fins-forward through the water and countering gravity. In the tube, at speeds below 0·28 ms⁻¹, the animal would either drop to the bottom in resting posture or swim gently with the body angled upwards, the arms touching the bottom of the tube and the posterior tip of the mantle striking the upstream grid. Thus, no accurate data could be obtained for the metabolic cost of low-speed swimming using the respirometer. At higher speeds, swimming was oscillatory and varied considerably among animals, but most swam relatively uniformly after the pre-trial conditioning period, seldom bumping against either grid. Most animals avoided contact with the tube, remaining in the area of microturbulent flow, and almost all animals swam horizontally. Even the few animals whose fins angled upwards seldom rubbed against the tube. At speeds from 0·28 to 0·38ms⁻¹, most animals used the fins at or near the jet frequency. As swimming speed increased, the strength and frequency of mantle contractions increased and the use of the fins usually decreased. At the higher speeds (0·48–0·88ms⁻¹), animals frequently tucked their fins under and against the mantle, probably to minimize drag. This behaviour was also observed during high-speed swimming in the pool. Under these conditions, the jet alone controls both the angle of attack and the forward momentum.

For smooth swimmers, jet frequency usually increased most dramatically at speeds from 0·28 to 0·48ms⁻¹. At higher speeds, frequency usually either increased less or reached an asymptotic value. Frequencies ranged from 0·60 to l·17jets s⁻¹ for animals from 0·220 to 0·491 kg (Table 1). Generally, smaller animals exhibited higher frequencies than larger animals, for a particular speed. Analysis using the ultrasonic transmitter permitted determination of jet frequency for a 0·491-kg animal freely swimming in the respirometer and in the pool, for a wide range of speeds from 0·12 to 1·22ms⁻¹ (Fig. 3). For this animal, jet frequency increased as speed increased for low speeds but reached an asymptote at speeds above approximately 0·70ms⁻¹.

A detailed analysis of swimming performance and aerobic metabolism at various swimming speeds from these experiments has been given elsewhere (Webber & O’Dor, 1985), but the results are summarized in Table 2 for later use in the comparison of oxygen consumption and pressure data as indices of metabolic rate. The table includes data on a well-studied fish for reference; the comparison of a 0·4-kg squid to a 0·5-kg salmon was chosen because the average mass of water carried in the squid’s mantle is about 0·1 kg. Fish can swim nearly twice as fast using less than half the energy required by squid, as reflected in the cost of transport figures. Maximum oxygen consumption in this squid is higher than any marine poikilotherm of this size range (M, kg) and temperature reported in the literature. As has been reported in most empirical studies on fish (Jones & Randall, 1978), oxygen consumption increased exponentially with swimming speed (U, ms⁻¹). Since the exponential relationship resulted in a higher correlation coefficient (r) than the linear or power relationships for individuals, this form has been adopted for squid (r always >+0·92 and >+0·98 in a majority of cases). The values given in Table 2 are calculated from a multiple regression for 97 determinations of oxygen consumption at various speeds in 24 squid. This yielded the relationship:

The standard rate of metabolism (extrapolated oxygen consumption at 0ms⁻¹) and the active metabolism (extrapolated value at critical speed), as derived by Brett (1964), can be calculated from equation 1. The difference between these is the metabolic scope for locomotion, an indication of the capacity of the animal to do work. Oxygen consumption for an animal resting on the bottom of the tube was always lower than the standard rate of metabolism, presumably reflecting the energetic cost of ‘hovering’ for the negatively buoyant squid. The regression lines for 0·2- and 0·5-kg squid are plotted in Fig. 5B to allow estimations of swimming speeds from pressure.

The speed of the A/D converter (22 Hz) allowed characterization of the pressure waveform developed in the mantle cavity for jet cycles at various swimming speeds. Fig. 4 illustrates the pressure of a smooth swimmer. Animals that had difficulty maintaining position gave more variable patterns, but Illex illecebrosus can clearly generate graded pressure jets. Pressure reaches a maximum in 0·09–0·13 s for all jets and then quickly falls to zero. At this point in the cycle, the mantle cavity is at minimum volume (Gosline & Shadwick, 1983). Both the rise and fall of pressure occur during the contraction phase and the period of this phase appears to decrease slightly as swimming speed increases. The recoil of compressed ‘collagen springs’ (Gosline et al. 1983) then causes pressure to become negative for a brief period (0·06–0·09 s). The magnitude of the negative pressure increases with the pressure of the preceding jet. The negative pressure presumably occurs before the inhalant apertures fully open to allow water into the mantle cavity. The mantle cavity then refills while the pressure is zero before the start of the next jet. A squid increases both jet frequency (Fig. 3) and jet pressure (Fig. 4) with increasing swimming speed. The increase in frequency is primarily the result of a shorter refilling phase (Table 3). Thrust can be increased by increasing the volume of water in the jet, but, since there is little pressure differential to drive refilling, the maximum effective frequency (Table 1) will be limited by a balance between the amount of water taken in per cycle and the number of cycles. Thrust can also be increased by the higher jet velocities produced at higher pressures, and this seems to be the most important factor at higher speeds. It remains unclear how such graded pressures are produced by the ‘all-or-none (non-facilitating, non-summating) response’ (Packard & Trueman, 1974; Wilson, 1960) normally associated with the squid giant fibre system, but control of funnel diameter may contribute.

The transducer-transmitter has been used in 10 squid, and caused little apparent trauma. Two animals died under the anaesthetic, but seven of the eight remaining were exercised in both the respirometer and the pool tank. In the respirometer, speeds up to 0·78 ms⁻¹ were attained. Swimming behaviour in the tube was the same as that for non-cannulated and cannulated animals except that squid had difficulty maintaining higher speeds (0·58 and 0·78ms⁻¹) for the full hour, probably because of the reduced mantle cavity volume. When returned to the pool after respirometer trials, the animals swam with the school, were active and fed for up to three days until experiments were terminated due to battery failure.

Complete analysis of these results is in progress, including video analysis of changes in mantle volume and the interaction of fins and jet in a variety of natural behaviour patterns, but only data from a representative animal are presented here to show correspondence between respirometer results and those in free-swimming squid. A single 0·491-kg animal swam in the respirometer at speeds from 0·28 to 0·68ms⁻¹. In the pool, it swam from 0·12ms⁻¹ to l-4ms⁻¹ using consecutive escape jets. Although others performed similarly, this animal gave the most complete range of results in both the respirometer and the pool and it has been used in the analysis that follows. Fig. 6 shows pressure records for this animal swimming in the pool at 10°C. The shape of the pressure curve is the same as that produced by the cannulation method. The negative pressure is also apparent in the pool. The number of data points describing the pressure curve is less for the transmitter than for the cannula method; however, since the digital circuitry for both methods integrates pressure over time the value for average pressure is comparable for the two methods. As observed with the cannula method, both jet pressure and frequency increased as swimming speed increased.

The average pressures for swimming in the respirometer and pool are plotted against swimming speed in Fig. 7. The swimming speeds in the pool were calculated as the change in horizontal distance per unit time, and, since the video analysis did not detect vertical displacement, they may be slightly underestimated. The solid line in Fig. 7 is the relationship between pressure and speed derived from equations 1 and 3 for a squid of 0·5 kg by eliminating oxygen consumption as a variable:

Given these correlations, it should be possible to estimate energy consumption from pressure data collected for this animal swimming freely in the pool. In the pool the school cruises almost without interruption producing average pressures of 0·38 kPa. The cruising speed calculated from sequences of video images ranged from 0·12 to 0·19ms⁻¹. The predicted oxygen consumption for this pressure at 15°C is 160mlO₂h⁻¹. The appropriate correction to 10°C is still problematical, but, using an energy value for oxygen of 19Jml⁻¹ (Brett & Groves, 1980) and for typical crustacean prey one of 3800 J g⁻¹ (O’Dor, 1982), this equates to a daily food intake of 3·8 % body mass. At 15°C squid in this size range may consume as much as 6·7 % body mass per day and have a gross conversion efficiency of 35 % (O’Dor & Wells, 1986). This leaves a small proportion of food intake to support higher level activities such as escape and attack jetting, feeding and interactions within the school which video records indicate may occupy 5–10 % of a squid’s time.

Their high metabolic rates, voracious feeding, rapid growth and semelparous reproductive pattern all indicate that squid ‘live fast and die young’. This life history pattern may be a consequence of their inherently inefficient jet locomotion, which requires power outputs nearly quadruple those of fish to achieve similar speeds. It has been suggested that the resting rate of a metabolism adapted to produce such high output may also be proportionately higher. The resultant ‘hyperactivity’ may suit squid to life histories that take advantage of short-term variability in the ecosystem while their competitors, fish, live more economically, have adapted to longer lives and use more stable resources (O’Dor & Webber, 1986). The initial stimulus for quantitative studies on squid locomotion was a need for a better understanding of the ‘rules’ for the competition between squid and fish (O’Dor, 1982). The three studies to date (O’Dor, 1982; Freadman et al. 1984; Webber & O’Dor, 1985) all make it clear that squid have a severe handicap in their economy of locomotion. The present study was undertaken to develop a means of determining how and to what extent the natural behaviour patterns of squid compensate for this handicap.

As an example of the need for economy, there is clear evidence that Illex illecebrosus and the very similar Todarodes pacificus, which occur off Japan, make spawning migrations exceeding 2000km at average speeds of as high as 0·37ms⁻¹ (Dawe, Beck, Drew & Winters, 1981; Hurley & Dawe, 1980; Shevtsov, 1973). If locomotion takes as much energy as respirometer studies suggest, such travel would require that an Illex consume 6·5% of its body mass in prey per day. This is its ad libitum feeding rate in captivity (Hirtle, DeMont & O’Dor, 1981), and field data suggest that, even on the feeding grounds, such high rates rarely occur (O’Dor, Durward, Vessey & Amaratunga, 1980). If the squid resort to cannibalism, which is common among squid, and fuel their migration by eating others in the school, only 1 % of the squid could survive for 2000 km (O’Dor & Wells, 1986).

Many mechanisms of energy conservation have been suggested for fish, but few could be tested directly because fish do work against a large volume of water outside their bodies. The difference in the locomotor systems means that some mechanisms proposed for fish, such as the potential advantage produced by interacting vortices in properly spaced schools suggested by Weihs (1974), will not apply to squid. However, other mechanisms, such as retaining laminar flow or climb-and-glide swimming (Weihs, 1973), can be directly tested for squid with the system described here. Such tests will not only indicate how squid behave, but should also improve our understanding of the basic energy requirements of nektonic organisms. The potential for biomechanical and hydrodynamic modelling of squid is also high, but a complete analysis is beyond the scope of this paper.

The net cost of transport should reflect the efficiency of the locomotor process. From Table 2, the cost of transport for squid is 3·2 times that of salmon. If efficiencies are in the same ratio, the fish has an efficiency of about 15 %. Although it has not been possible to measure fish efficiency directly because of the diffuse power output, this is the generally accepted value based on a variety of indirect estimates (Blake, 1983; Alexander, 1977; Webb, 1975).

The pressures required for squid to swim at low speeds are somewhat higher than those predicted by the drag equation. This is probably due to the additional power required for the ‘acceleration reaction’ (Daniel, 1984). The oscillatory character of squid locomotion means that they are constantly accelerating and decelerating which increases power requirements. This effect is complex because the amount of boundary layer water that must be accelerated also changes with velocity. Daniel (1984) suggested that the overall effect may be smaller in fast-moving squid than in other slow animals using oscillatory modes of locomotion, but these data indicate that it may be significant for squid at cruising speeds. It is also interesting to note that these low speeds are the ones where the fins are used most. More evidence is certainly required, but, since pressures are relatively higher at low speeds, it could be argued that the jet, not the fins, is still the major contributor to locomotion, even at low speed.

Alexander (1977) has argued that the microturbulent flow in swim-tunnels should have little effect on the boundary layer turbulence, that conventional assumptions about laminar and turbulent flow at a particular Reynolds numbers apply, and that the power required to swim in static water should be no lower than that in a swimtunnel. Earlier workers suggested that swimming costs in nature under laminar conditions might be lower than those in the tunnel (Brett, 1963; Webb, 1975). Fig. 7 provides the first direct evidence on this controversy. The analysis above for squid assumes a turbulent boundary layer, even though the Reynolds number of the slowest squid studied was only 150000, below the transition speed for even a bluff body. Deciding which drag equation is appropriate would require a careful analysis of all the other factors outlined above and considerably more data; however, the pressure required to swim in the tunnel actually appears to be less than that required in the pool. This is not consistent with either assumption about drag. One explanation may be that the microturbulence in the tunnel thins the boundary layer and reduces the volume of water involved in the acceleration reaction. If this is true, the result could not be applied to fish which maintain a steady speed. This difference could also relate to the vertical component of swimming in the pool which was not accounted for but could be in future studies using this technique.

Although these experiments have, perhaps, produced more questions than answers so far, it seems clear that direct measurement of pressure-flow power output in freely swimming squid can provide key information for an analysis of the energetics of squid locomotion under both laboratory and field conditions. The relationship between power consumption (oxygen consumption) and power output (pressure) can be characterized for a variety of conditions, the alternation of powered vertical swimming with passive gliding, for example. Even without a complete analysis, the approach presented in Fig. 5 allows one to estimate the speed and oxygen consumption of squid based only on the average pressure data relayed by the transducer-transmitter. A reasonable approximation of average energy expenditure by squid in nature could be developed by simply releasing several squid carrying transducers in a small bay and recording pressure data over several days. For more detailed analysis, a system of sonobuoys, radio-linked to a shore-based computer, is under development which will also continuously triangulate the position of the squid. This system will allow individual squid to be ‘calibrated’ and compared. Pressure-oxygen relationships for an individual squid with a transducer can be measured in the field using a small portable respirometer of the sort described by DeMont & O’Dor (1984) before it is released. The pressure-speed relationship for this squid can then be established based on returned position and pressure data. Once these relationships for an individual are established, calibration curves like those in Fig. 5 can be drawn for subsequent analysis. A complete picture of energy consumption also requires information on vertical movement and ambient temperature; these data might also be collected by simultaneously releasing other squid, carrying depth, pressure and temperature transducers, as part of the same school.

This work was supported by grants from the Canadian Natural Sciences and Engineering Research Council and Fisheries and Oceans, Canada. We thank the Aquatron Laboratory staff for their continual support, R. E. Saunder for providing the respirometer and F. A. Voegeli of Vemco, Ltd for building the transducer we needed and teaching us how to use it.