Metabolic Rates of Aquatic Arthropods from Different Habitats

Whereas innumerable adaptations of structure and habit are known, fitting specific animals to their natural environments, we have as yet very little knowledge of the physical and chemical causes or of the physiological effects of animal distribution.

In the case of aquatic animals, Krogh and Leitch (1919), and more recently Root (1931), have shown that the bloods of fishes which inhabit waters often deficient in oxygen contain haemoglobins with a greater oxygen affinity than those of fishes living in swift streams or in the open waters of the sea. In trout and mackerel the oxygen is readily given up from blood to tissues, permitting of an active life, but when oxygen is deficient in the water a sufficient quantity cannot be secured by the haemoglobin. Carp, on the other hand, lead a more sluggish life ; their haemoglobin parts reluctantly with its oxygen, but the blood can capture this gas from waters containing very small amounts in solution, and thus enable the fish to survive. The work of Gardner et al. (1914, 1922), indicates, in effect, that in aerated water trout consume more oxygen than carp. Redfield, Coolidge and Hurd (1926) have shown that there are arthropods comparable with Krogh and Leitch’s fishes. The haemocyanin in the blood of the hardy king crab has a high oxygen affinity compared with that of the active but delicate squid.

Temperature limits the distribution of innumerable poikilothermal animals for reasons which we have hardly commenced to understand as yet. One cause of this limitation may again be the nature of the respiratory pigments. The oxygen affinity of these substances varies with temperature. At a higher external temperature than that at which a given species occurs in nature, it may well be that its haemoglobin or chlorocruorin would take up an insufficient amount of oxygen to supply the tissues adequately, while at too low a temperature the oxygen would not be unloaded from blood to cells (Fox, 1932).

Of recent years one of the most fruitful lines of attack on this problem of the physiology of adaptation has concerned the invasion of fresh waters from the sea. Pantin (1931) has made a thorough study of the water and electrolyte exchanges between animal and environment in the planarian worm, Gunda ulrae, when it is subjected alternately to salt and fresh water, an event which occurs daily in its life. Schlieper (1930) and Beadle (1931) have shown that when euryhaline marine invertebrates, such as crabs, Nereis and Gunda, pass from sea water into brackish water their rate of oxygen intake increases. Similarly young eels (Anguilla anguilla) consume more oxygen in brackish water than in sea water, and still more in fresh water (Raffy and Fontaine, 1930).

Fresh water is a more difficult environment for an animal than sea water, for in the former medium a more considerable difference in composition must be maintained between body fluid and the surrounding water than in the latter. The increased oxygen consumption of euryhaline animals in dilute sea water suggested to Schlieper (1930) that perhaps fresh-water animals have necessarily a higher rate of metabolism than their marine relatives. The practically complete absence of data in this connection suggested a part of the investigation to be described below. We have compared the oxygen intake of the marine amphipod species Gammarus marinus and G. locusta with that of their fresh-water relative G. pulex, and of the marine isopod Idotea neglecta with the fresh-water Asellus aquaticus.

Data are equally scarce concerning the rate of metabolism of related species of animals living in well-oxygenated fresh waters, that is to say rapid streams, and in still waters, which at times are deficient in oxygen. Measurements of the rate of oxygen consumption have therefore been made with the following pairs of animals :

The survival times of the ephemerids and trichopterids in waters deficient in oxygen, and the rates of heart beat of the ephemerid and amphipod species were also studied.

In all these comparisons of oxygen consumption, heart rate, etc., animals of the same size and sex were used. In the case of Asellus aquaticus the same species from different fresh-water habitats could be compared. In the remaining instances the species selected were as nearly related as possible.

The investigation was done in Birmingham, marine animals having been sent from Plymouth. The “swift stream” from which Asellus aquaticus, Baetis rhodani and Hydropsyche sp. were obtained is at Blakedown in Worcestershire. The average velocity of the current in the middle of the stream is one metre per second. The “slow stream” from which A. aquaticus was taken is Spring Brook at Earlswood, Warwickshire. Chloeon dipterum and Molanna sp. came from a pond at Alvechurch. Gammarus pulex (kindly identified by Mrs E. W. Sexton) was derived from two sources, namely from the stream at Blakedown, and from a slow stream close to the University¹ Whereas, however, A.aquatiois, B. rhodani and Hydropsyche were found in the rapid water in the middle of the stream at Blakedown, Gammarus pulex occurred in sand near the banks, where the stream is slow.

The alkali reserves of these waters are as follows : Blakedown, 0·0026–0·0032 N ; Earlswood, 0·0022 N ; Alvechurch, 0·00265–0·0029 N ; University stream, 0·0010 N. Birmingham tap water, which was used in certain experiments, has a value of 0·0003 N-The waters had the following values of pH: Blakedown, 7·3–7·9 ; Earlswood, 6·9; Alvechurch, 6·8–7·6; University stream, 6·9–7·4.

Oxygen consumption was determined with a Barcroft manometer, the animals being in sea water or distilled water containing an anaesthetic. In all cases the animals recovered completely on being subsequently replaced in pure water. The Barcroft apparatus was slowly shaken in a water bath. After each experiment the dry weight of the animals was determined, and all oxygen consumptions are referred to dry weight.

The oxygen consumption of three species of Gammarus was studied, namely G. marinus and G. locusta, which are littoral marine species, and the fresh-water G. pulex taken from two different situations. All animals used were males, of approximately the same size, namely 13 mm. in length. Forty animals were used in each experiment. The anaesthetic employed was 0·75 per cent, urethane. The solution was adjusted to pH 8-0 with NaHCO₃. The temperature was io° C. In the cases of G. marinus and G. locusta, and of G. pulex from the University stream, the oxygen consumption of each set of animals was measured on two successive days. This was not possible with G. pulex from the Blakedown stream, for although on being placed in stream water after the experiment recovery from the anaesthetic was complete within 15 to 20 min., more than 50 per cent, of the animals failed to survive until the next day, even in aerated water.

The results are given in Table I. It is clear that the fresh-water species, G. pulex, has a higher rate of respiration than either of the marine species. The average oxygen intake of the former is $112$ times that of the latter. G. pulex from the University stream showed a greater oxygen consumption than its relatives from Blakedown. It may or may not be significant in this connection that the alkali reserve of the University stream is one-third of that of the Blakedown water.

As another indication of metabolic rate, the frequency of heart beat was studied. Females were used in this case. No significant difference was found in the rate of heart beat in the three species of Gammarus.

The oxygen consumption was measured of the littoral marine species Idotea neglecta, and of the fresh-water Asellus aquaticus (kindly identified by Dr J. Omer-Cooper) from two different habitats. Twenty males were used in each experiment. All animals were approximately 10 mm. in length. The anaesthetic was 2 per cent, urethane and the temperature 10° C. In most cases the oxygen intake of the same animals was measured on two successive days.

The results are shown in Table II, from which it is seen that the rate of metabolism of A. aquaticus from the swift stream is $112$ times that of the same species from the slow stream. This considerable difference between the same species of Asellus in two habitats suggests the existence of different races. It is hoped that breeding experiments now being undertaken will decide whether or not there is an inherited difference in metabolic rate. It is further apparent from the table that the average rate of metabolism of the fresh-water A. aquaticus is three times that of the marine Idotea neglecta.

Two species of ephemerid nymphs were studied, namely the stagnant water form Chloeon dipterum and the swift water form Baetis rhodani.

The gills of C. dipterum move incessantly. In water deficient in oxygen the rate of this movement is increased, while in water through which oxygen has been bubbled it is decreased. The gills of B. rhodani are usually motionless, and movement is not induced by lack of oxygen.

The oxygen consumption of B. rhodani was compared with that of C. dipterum. The nymphs of both species were of the same average size, the dry weight of 150 B. rhodani being 41· mg. and that of 150 C. dipterum 41·9 mg. In each experiment 150 animals were used. They were anaesthetised with 0-5 per cent, urethane.

The results are shown in Table III, from which it is seen that the oxygen consumption of the stream species at 10° is four times, and at 16° nearly three times, that of the pond species.

In view of this, it was to be anticipated that B. rhodani would succumb more rapidly in water deficient in oxygen. The results of typical experiments made to test this are shown in Table IV. For each experiment, boiled and aerated water was mixed, and the oxygen content of a sample identical with that in which the animals were placed was determined by the Winkler method. Five animals in 120 c.c. water in a stoppered bottle were used each time. The temperature was 16° C. It is seen that B. rhodani is considerably more sensitive to oxygen lack than C. dipterum.

During these experiments the pH of the water in the bottles fell owing to the accumulation of CO₂. The maximum drop, in the case of animals which survived over 30 hours, was from pH 7·9 to 6·7. The change in hydrogen-ion concentration, however, was not responsible for deaths, since further experiments showed that neither species was sensitive to changes in pH between 6 and 9.

The frequency of the heart beat was next measured in the two species. The animals were anaesthetised as before and the experimental temperature was 15·16° C. For each animal the time taken for 25 heart beats was measured at intervals during 1 hour. The averages of these measurements are given in Table V, which shows that the rate in B. rhodani is three times that in C. dipterum.

Hydropsyche sp. has a larva which constructs a silken snare on the undersurface of stones in running streams. The larva of Molanna sp. is a case-bearing form found in stagnant water. The oxygen consumption of these two animals was compared. All specimens used were of the same average size, namely 16 mm. in length. The larvae of Molanna were removed from their cases. In each experiment 40 animals were used, the anaesthetic being 0.25 per cent, chloretone and the temperature io°. The results are given in Table VI, which shows that the rate of metabolism of Hydropsyche is $112$ times that of Molanna.

Further experiments were made to test the endurance of oxygen lack by the two species. The experimental conditions were the same as described for the ephemend nymphs. The temperature was 15o C. Table VII shows that Molanna is considerably more resistant than Hydropsyche.

1. The oxygen consumption, resistance to oxygen lack and rate of heart beat of closely related arthropods, of like size but from different aquatic habitats, have been compared under standard conditions.

2. The oxygen consumption of the fresh-water amphipod Gammarus pulex is 11 times that of the marine G. marinus and G. locusta, and the oxygen consumption of the fresh-water isopod Asellus aquaticus is three times that of the marine Idotea neglecta. So far as this evidence goes, it supports the suggestion that fresh-water animals may necessarily have greater oxygen requirements than their relatives in the sea.

3. The oxygen consumption of Asellus aquaticus from a swift stream is $112$ times that of the same species from a slow stream.

4. The oxygen consumption of the nymph of the ephemerid Baetis rhodani, living in a swift stream, is 3-4 times that of Chloeon dipterum, from a pond. The former is more sensitive to oxygen lack than the latter.

5. The ratio of the rate of heart beat of B. rhodani to that of C. dipterum is 3 : 1.

The larva of the trichopterid Hydropsyche sp. from a swift stream has an oxygen consumption which is $112$ times that of Molanna sp. from a pond. The former is more sensitive to oxygen lack than the latter.

The difference in oxygen consumption found between marine and fresh-water amphipods and isopods might conceivably have been due to a greater susceptibility of the marine species to anaesthetics. In order to test this, additional experiments have since been made with unanaesthetised Gammarus. The oxygen consumption (c.mm./gr./hr.) of two lots of G. marinus was (i, ist day) 682, (1, 2nd day) 525, (i, 3rd day) 501, (2, ist day) 580, (2, 2nd day) 524. That of three lots of G. pulex was (1) 1242, (2) 993, (3, ist day) 1182, (3, 2nd day) 976. The average value for G. pulex is twice that for G. marinus.