Effects of salinity on swimming performance, routine activity and resting metabolic rate on the euryhaline killifish, Aphanius dispar
Itai Plaut
Poster presented in the Fifth International Congress of Comparative Physiology and Biochemistry,
Calgary, Alberta, Canada,  Aougust 23-28, 1999

 
 

Introduction

Most fishes are capable of tolerating only a narrow range of salinities (Hanrey and Nordly, 1997). However, some euryhaline fishes live in water bodies which experience wide range of salinity fluctuations. The killifish, Aphanius dispar (Rüppell) (Cyprinodontidae) is found in a wide range of salinities, from freshwater to >500% seawater in springs around the Dead Sea and in salt ponds in Atlit, Israel (Lotan, 1969, 1971; Lotan and Skadhauge, 1972;  Skadhauge and Lotan, 1974). Lotan (1971) reported that A. dispar is capable of maintaining its body osmotic pressure and ionic concentration within a relatively narrow range against salinity changes from freshwater to 300% seawater in the external water.
Increase in external osmotic pressure may effect other physiological parameters such as osmoregulation costs, body permeability etc. Such a situation may effect the physiological capability, and reduce ecological fitness of the fish.
In the present study I exposed A. dispar (Fig. 1) to different levels of salinity, from freshwater to 500% seawater for four weeks and measured the effects on swimming performance, routine activity level and resting metabolic rate.

Fig. 1: Aphanius dispar female (above) and male (below)

Methods

Fish collection, holding and acclimation:
A. dispar specimens [standard length (SL), 31.9±0.5 mm, body mass (BM), 0.72±0.04 g]. Fish were collected in the salt ponds of Atlit, Israel. The salinity in the fish ponds at the time of collection was 57 ppt (about 160% seawater), but can fluctuate between 36 to 170 ppt (pers. observ.) . Fish were divided to five groups, each acclimated to different salinity. The aquaria initially contained water from the natural habitat. A week after collection, salinity acclimation began means of adding artificial sea salt or diluting with dechlorinated tap water (change of 25% seawater a day). After 14 days there were 5 salinities: freshwater (<1 ppt salt), 100% (35 ppt), 200% (70 ppt), 300% (105 ppt), 400% (140 ppt) and 500% (175 ppt) seawater. Four weeks later experiments began. Temperature in the aquaria was 25±1°C and fish were fed daily with Tetramin® flasks and live Daphnia magna.

Measurements of critical swimming speed (Ucrit):
Swimming performance was measured as critical swimming speed (Ucrit) in a water tunnel (Fig. 2). Fish were introduced into the water tunnel (containing the relevant acclimation salinity) 2 hours before the experiment began at water velocity of 4 cm sec-1 [about 1 SL s-1]. Water velocity was then increased in increments of 4 cm sec-1 at 5 min intervals, until the fish fatigued. Ucrit was calculated according to the equation (Brett, 1964):

Ucrit = Ui +  [Uii(Ti/Tii)]
where Ui is the highest velocity maintained for the whole 5 min (cm sec-1), Uii is the velocity increment (e.g. 4 cm s-1), Ti is the time elapsed at fatigue velocity (s) and Tii is the interval time (5 min).

Fig. 2: Water tunnel

Measurements of routine activity rate:
Routine activity of the grouped of A. dispar at different experimental salinities was measured in 6 identical aquaria, 28X14X18 cm (length, width and height respectively) at 25±1°C. Each aquarium contained different experimental salinity. A group of six fish (of a certain experimental salinity) were placed in each relvant aquarium 72 h before the measurement began. Each aquarium was equipped with three IR beam projectors with a photocell along the aquarium wall. On the opposite aquarium wall a reflector was placed. Every 15 min number crosses were recorded on a PC. Photoperiod was 12:12 L:D. measurement lasted 72 h.

Measurements of oxygen consumption:
Oxygen consumption rate was measured in a semi-closed respirometer (Plaut, 1999). Postabsorptive fish was placed in the cylindrical respirometer and left undisturbed for 3 h to recover from handling in the relevant salinity. Then, the respirometer was sealed without disturbing the fish and oxygen depletion rate was measured (YSI Model 5300 Biological Oxygen Monitor).
 
 

Results

Critical swimming speed:
No significant differences were found among fish acclimated to freshwater, 100% and 200% seawater (Fig 3, Table 1). In contrast, Ucrit of fish acclimated to 300% seawater was significantly lower than the values above, but significantly higher than Ucrit of fish acclimated to 400% seawater. Some of the fish acclimated to 400% seawater did not survive the test, hence, sample size in this category was reduced.
 

Table 1. Linear regression of Ucrit against standard length (±se) in Aphanius dispar acclimated to different salinities. Intercepts sharing same superscript letter are not significantly different (ANCOVA, p>0.05).


Fig. 3: Critical swimming speeds of Aphanius dispar acclimated to different salinities.


 

Routine activity rate:
A. dispar showed a daily circadian activity, being active during the light period and less active during dark period (Fig. 4). Routine activity rates showed the same trend as Ucrit. Fish acclimated to freshwater, 100% and 200% seawater showed similar rates of activity, both in light and dark periods. Fish acclimated to 300% and 400% were significantly less active, while those acclimated to 400% seawater was less active than those acclimated to 300% seawater.


Fig. 4: Routine activity rate of Aphanius dispar acclimated to different salinities.

Oxygen consumption:
Oxygen consumption of A. dispar was similarly high at freshwater, 100% and 200% seawater, but decreased significantly in 300% and 400% seawater (Fig. 5).



Fig. 5: Oxygen consumption of Aphanius dispar acclimated to different salinities.
 
 

Conclusions

  1. 1. Salinity above ~250% seawater seems to decrease the swimming     capabilities of A. dispar, and hence, its routine activity rate.
  2. 2. Decrease in resting metabolic rate at high salinities may indicate a stressful situation for the fish, as shown in other species (Haney and Nordlie, 1997; Plaut, 1999a,b).
  3. 3. It seems that, although A. dispar is commonly found in water of various salinities, up to 500% seawater, it is best adapted to salinities of 0-250% seawater. Above this concentration, the fish functional capabilities decrease.
  4. 4. The results are consistent with the hypothesis  that critical swimming speed (Ucrit) is a useful measurement for general physiological condition.


References


 
  1. Brett, J. R. (1964). The respiratory metabolism and swimming performance of young sockeye salmon. J. Fish. Res. Board. Can., 21:1183-1226.
  2. Haney, DC and FG Nordlie (1997). Influence of environmental salinity on routine metabolic rate and critical oxygen tension of Cyprinodon variegatus. Pysiol. Zool., 70:511-518.
  3.  Lotan, R (1969). Sodium, chloride and water balance in the euryhaline teleost Aphanius dispar (Ruppell) (Cyprinodontidae). Z. vergl. Physiologie. 65:455-462.
  4.  Lotan, R (1971). Osmotic adjustment in the euryhaline teleost Aphanius dispar (Cyprinodontidae). Z. vergl. Physiologie. 75:383-387.
  5.  Lotan, R and E Skadhauge (1972). Intestinal salt and water transport in a euryhaline teleost, Aphanius dispar (Cyprinodontidae). Comp. Biochem. Physiol. 42A,  303-310.
  6. Plaut, I. (1999a). Effects of salinity on survival, osmoregulation, and oxygen consumption in the intertidal blenny, Parablennius sanguinolentus. Copeia, 1999: 774-778.
  7. Plaut, I. (1999b). Effects of salinity acclimation on oxygen consumption in the freshwater blenny, Salaria fluviatilis, and the marine peacock blenny, S. pavo. Mar. Freshwat. Res. (in press).
  8.  Skadhauge, E and R Lotan (1974). Drinking rate and oxygen consumption in the euryhaline teleost Aphanius dispar in waters of high salinity. J.exp.Biol. 60:547-556.


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