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Observations on the responses of some southern california tidepool fishes to nocturnal hypoxic stress
OBSERVATIONS ON THE RESPONSES OF SOME SOUTHERN CALIFORNIA TIDEPOOL FISHES TO NOCTURNAL HYPOXIC STRESS JAMESL. CONGLETON* Department
of Marine
Biology.
Scripps
Institution
(Receiwd
of Oceanography,
I October
La Jolla, California,
U.S.A.
1979)
--I. Oxygen tension and pH declined rapidly in pools exposed by nocturnal low tides. 2. Minimum oxygen tensions recorded in tidepools at night (14-l 7 Torr) were lower than critical oxygen tensions (cP,,) established by respirometry for three common intertidal fish species. 3. Individual CIinocottus analis and Paraclir~us integripirwis ventilated with water from the surface film when tidepool PO, dropped below the critical level. 4. In comparison with C. analis and P. intrgr@irmis, Gibhonsia ekgans had a higher cPo, and a shorter survival time under anoxic conditions. G. elegans and other species intolerant of hypoxic conditions may avoid tidepools where low oxygen tensions occur during the season of nocturnal low tides.
Abstract
INTRODUCTION
Tidepool fishes were observed during five nocturnal low tides in the late summer of 1967 and 1968. Observations could be made by moonlight on some occasions; on moonless nights a gasoline lantern placed on the ground 5-6 m from the pools provided adequate light. Water samples for oxygen analysis were taken from Pool A at 30-90 min intervals during the early morning hours of August 7 and August 8, 1968. Pools B and C were also sampled several times so that oxygen tensions could be compared with those in Pool A. Sampling began soon after the pools were exposed by the receding tide, and ended when the pools were submerged by the incoming tide. Five milliliter glass syringes with attached polyvinyl chloride surgical tubing were used to withdraw samples from depths of 35cm (bottom of pool) and 1 or 3cm. A three-way syringe stopcock allowed flushing of the dead space. The sample syringes were chilled and transported to the laboratory, where oxygen tensions were measured with a polarographic oxygen electrode (Beckman Mode1 777), fitted with a 0.2 ml glass sample chamber. The oxygen electrode was calibrated with nitrogen gas and air at 100% humidity, cross-referenced against the Winkler titration. The electrode was maintained at 20 + 0.2”C in a water bath, and measured oxygen tensions were corrected to ambient tidepool temperature using the temperaturesolubility tables of Carpenter (1966). The pH of tidepool water was measured in situ at a depth of 6cm with a portable pH meter (Instrumentation Laboratories) calibrated against a high and a low buffer. Fishes used for respirometry were held for 1-3 weeks at 15 + 0.5”C and were not fed for 24 hr before transferral to the chamber. Once in the darkened chamber, the fishes were allowed I2 hr to recover from the stress of handling and acclimate to the chamber. A flow of aerated seawater was maintained throughout this period. The anodized aluminium respirometer had a volume of 83 ml; the design and procedures have been described in detail elsewhere (Congleton, 1974). Respirometric measurements were initiated by stopping the flow of aerated seawater through the chamber. Declining oxygen tension was monitored by an oxygen electrode (Beckman Model 777) and gentle stirring was provided by a magnetic stirring bar. A water bath regulated the temperature in the chamber at 15.0 + 0. I ‘C. Microbial respiration was suppressed by prefiltering seawater through a 0.4 micron sintered glass filter and treating with 100 mg of streptomycin sulfate per liter.
Oxygen concentrations in exposed intertidal pools containing algae or marine grasses typically increase during the day, due to the predominance of photosynthetic oxygen production over respiratory oxygen uptake (Stephenson, Zoond & Eyre, 1934; Pyefinch, 1943). At the same time, carbon dioxide uptake by photosynthesizing plants causes pH values to rise (Gall, 1919; Emery, 1946). During the nighttime hours photosynthesis ceases, and exchange of respiratory gases by tidepool plants and animals causes oxygen and pH values to decline (Stephenson rt al., 1934; Bovbjerb & Glynn. 1960; Emery, 1946). Llttle is known concerning the behavioral and physiological responses of tidepool animals to hypoxic stress. This paper reports field observations on the behavior of tidepool fishes exposed to low nocturnal oxygen and pH levels. Experiments were also carried out in the laboratory with several species of tidepool fishes to establish their respiratory response to progressive hypoxia and to determine tolerance for anoxic conditions.
MATERIALS
AND
METHODS
The study area was a wide, gently sloping shelf of calcareous sandstone near La Jolla, California (South Casa Beach area). Tidepools on this shelf varied in elevation from 18 to 28 cm above Mean Lower Low Water (MLLW), and contained dense growths of algae And surf grass. Three adjacent pools (elevation 23 cm MLLW) were selected for measurements of oxygen tension, pH and temperature during nocturnal tidal exposure. The largest pool (A) was rectangular with dimensions of 2.0 x 4.5 m and an average depth of 35 cm. Pool B was elongate, 0.8 x 3.5 m, with an average depth of 30cm. Pool C was circular, with a diameter of 0.6 m and a depth of 30 cm.
* Present address: Washington Cooperative Research Unit, College of Fisheries, University ington, Seattle. Washington 98195, U.S.A.
Fishery of Wash-
719
JAMES L. CONGLETON
720
To test the importance of access to the water’s surface, tidepool fishes were collected during nocturnal low tides and divided between two wire mesh cages (25cm each side). One cage was positioned with the upper side above the surface, while the second cage was positioned with the upper side 3-S cm below the surface. Tolerance for anoxic conditions was examined by sealing test animals into NO-ml Erlenmeyer Basks filled with deoxygenated water. Deoxygenation was accomplished by flushing with nitrogen gas; residual oxygen tension following this procedure was below the sensitivity limit of the oxygen electrode (approx 2Torr). Fishes were considered dead when coordinated respiratory movements had ceased.
RESULTS
Nocturnal oxygen and pH values Oxygen tension (P,J in Pool A began to decline near the surface (1 and 3 cm depths) as well as near the bottom (35 cm depth) immediately after the pool was isolated by the outgoing tide (Fig. I). Vertical mixing prevented formation of a pronounced PO2
gradient: 3-4 hr after initial exposure, PO1 at surface and bottom differed by only 3-4 Torr (mm Hg). Tidepool temperatures of 18.5-2O.O”C were I-2°C warmer than air temperatures on both August 7 and 8, so the pools did not stratify thermally. Temperatures at bottom and surface never differed by more than O.I”C, and declined slowly throughout the exposure period. of Pool A On both dates, P,2 near the bottom declined to approximately 15 Torr after 3 hr and remained at that level for the duration of the exposure period. Near-bottom PO, in Pools B and C corresponded closely to PO, in Pool A (Fig. I). Nearsurface P,_ varied from 17 to 20 Torr during the last few hours bf exposure. The decline in pH also stabilized after 3-4 hr, reaching minimum values of 7.35-7.45. Oxygen tensions near the bottom declined more rapidly on August 7 than on August 8, because diffusion of atmospheric oxygen into the pool was enhanced by a slight breeze (estimated at 3-5 km hr-‘) during the first few hours of sampling on August 8.
I50 f
1 80
I
+ 7.8
PH
i 76
4 .
PO2 at Icm
v
PO2 at 3cm,
, pool
n
Po,ot 35cm,pool
A A
A PO2 at 30cm,
pool B
+ PO2 at 30cm,
pool C
a
pH, pool
-8 2
pool A
A
- 7.8 PH -76
0100
0200
0300 Time
0400 (PST)
0500
0600
Fig. 1. Oxygen tension and pH changes in tidepools during nocturnal tidal exposure on August 7 (upper) and August 8 (lower). 1968. The vertical arrows near the abscissa indicate the beginning and end of the period of tidal exposure.
Nocturnal hypoxic stress in fish Oxygen tension changes similar to those shown in Fig. I were also recorded in Pools A, B, and C and other nearby pools during nocturnal iow tides in August and September of 1967. Although the technique used to measure oxygen con~ntrations in 1967 (Scholander Microgas AnaIyzer ; Scholander et ui., 1955) is not believed to be as accurate as the oxygen &e&-ode method used in 1368, the 1967 data indicated that tidepool oxygen tensions invariably declined to below 20Torr during nocturnal exposure. An exception occurred on one occasion when a strong wind, estimated at 40-50 km hr- ‘, arose suddenly after several hours of sampling. Samples taken a short time Iater indicated a marked increase in oxygen tension at depths of I and 15 cm, while oxygen tension continued to decline at a depth of 4Ocm. Observations offish behavior Few fishes could be seen in newly exposed tidepools. After an hour of exposure, individuals of the species CIinocottus anatis, the woolly sculpin, could be seen rising to the surface for a few seconds and then returning to deeper water. After 2-3 hr of exposure many specimens of this species began to move into shallow water around the margin of the pool. These fishes rested on the bottom and ventilated rapidly with mouths at the water’s surface. Often a small dimple in the surface film marked the ventilatory intake of water. Individual Pnraclirzus intlrgripimis, tidepool kelpfish, behaved similarly by flexing their bodies laterally to bring the mouth to the surface. Specimens of P. inlegripinnis were usually seen resting on their sides on floating masses of surf grass, rather than around the margins of the pool. Initially, fishes that had moved into shallow water would flee back to deeper water if disturbed by the too-near approach of a human observer. Later in the period of exposure, fishes did not respond even if touched or prodded. Gibbonsia &guns, one of the most common fishes of the southern California intertidal, was never seen in tidepoob at night during more than 30 hr of observation. Juveniles of Girella nigricans, also common in the intertidal during daytime iow tides, were seen on anly a few Qccasions. lrnportattce
qf surface access for survival
Fishes confined in the subsurface cage began to swim continuously upward against the upper side of the cage during the third hour of tidal exposure, when subsurface P,, had declined to 2O-25Torr. Fishes confined in the control cage swam with snuuts contacting the water’s surface. Ten specimens of C. analis
721
that were denied surface access died after 8O-140min (% = 107 min), while seven controls survived the entire exposure.
Specimens of C. an&s, P. infeqripp&is, and G. eleguns regulated oxygen consumption (I&,) over a wide range of oxygen tensions. The animals were not active while in the respirometer chamber, and there was little variation or change in PO, with falling ambient PO, until a critical oxygen tension level was reached, below which voio,declined sharply. The critical oxygen tension (cPoL) was arbitrarily defined as the PO, interval preceding (higher than) the PO, interval in-which I&, dechned to 85% or less of the lowest I&, recorded on the oxygen uptake “plateau.” Median ‘PO, values were 20 Torr for C. anuUs. 23 Torr for P. irzteyripitlnis, and 28 Torr for G. rlequrrs (Table 1).
Anoxic survival times (x i_ SE) for C. an&is, P. j~~~~~~~~i~~~~j‘~, and G. dqpns were, respectively: 44 rfi: 4 min (n = 8); 9.5 _t 22 min (rr = 5); and 14 ) 4 min (t? = 4). These fishes were similar in size to those used for respirontetry (Table I). Mean survival times determined for c‘. ar~ulis and P. in~qripinnis did not differ signi~ca~t~~. but the mean survival time found for G, elqans was significantly less than that of the other two species (P < 0.01: Mann-Whitney U Test), DISCUSSION
Oxygen tensions measured in exposed tidepools at night were below critical oxygen tension (cPo,) levels established by respirometry for the common tidepool fishes C. wufis, P. inregrrpirztlis, and G. rlegans. Further, field cPo, levels would have been somewhat higher than estimated, since respirometric measurements were made at a lower temperature (15°C) than temperatures prevailing nighttime tide-pool (18,5-2O’C), and li,* and cPo2 are known to increase with increasing temperature (e.g. Beamish, 1964). When ambient oxygen tensions fall below the cPo,, compensatory physiological mechanisms can no longer maintain a rate of oxygen uptake adequate to satisfy the metabolic demand. The consequence is increasingiy severe hypoxic stress (Hughes, 1973). C. analis and P. inteyripirzrzis mitigated lrypoxic stress by moving into shallow water and ventilating their gills with surface water. The significamz of this behavior was demonstrated by the death within 1-2.5 hr af specimens of C rrrralk confined in wire
Table I, Critical oxygen tensions icP,J and resting oxygen consumption rates (I&,,)for three species of intertidal fishes at 15’C Species
N
B_odyweight (gl x (range)
$0, (pl 02 g-1 F (range)
!,-I)
CPOf mrr> median (range) 1__-
Clinacottus analis
4
1.32 ~1.20-1.70)
83 (53-103)
20 (16-24)
Paraclinus integsippinnis
6
1.08 (0.52-2.51)
84 (62-119)
23 (20-26)
Gibbonsia ele~tans
4
2.78 (1.28-2.77)
102 (86-112)
28 (X-30)
712
JAMES L. CohCLbroN
mesh cages several centimeters beneath the water’s surface. Oxygen exchange between air and water occurs primarily by molecular diffusion through a thin, coherent surface film less than 100pm thick (Kanwisher. 1964). The oxygen tension gradient between atmosphere and tidepool water is largely limited to this thin film, since oxygen diffusing through the film is rapidly mixed into the more turbulent underlying water. To obtain relief from stressful oxygen tension levels. tidepool fish must draw ventilatory water from quite near the surface. The hypoxic tolerance of P. irlteyripirlnis and G. &~LIIIS was not tested in the field, but laboratory tests indicated that P. irlteyripirlrzis survived anoxia for a slightly. but not significantly, longer time than C. au&s. G. rlryu~ survived anoxia for a significantly shorter time than either C. cudis or P. ir7tegripirlnis. In addition, the cPo2 established for G. rleguns was higher than the c,Po2 values established for either of the other two species. Hence, G. elegur7.s appears to be less adapted for tolerance of nocturnal hypoxia than either C. ur7uli.s or f intrgripimix Specimens of G. &yar7,s were never seen in the study area at night during approximately 30 hr of observation in August and September of 1967 and 1968. but were abundant when tidepool collections were made during daytime low tides several months later. The apparent absence of G. &guns during the late summer may have been due to avoidance by this species of tidepools where hypxic conditions occurred. Nocturnal low tides falling below the MLLW datum occur from March to September on the southern California coast. Future studies may demonstrate that G. &+JII.S and other littoral fish and invertebrate species show seasonal changes in depth distribution
or habitat preference in response nocturnal hypoxic stress. Aclirlowled~erner7ts-I S. Smith
for critical
to the occurrence
thank Dr J. B. Graham reading
of
and Dr L.
of the manuscript.
REFEREhCES B~AMISH
F. W. H. (1964) Respiration ol’ fishes with special emphasis on standard oxygen consumption. III. Influence of oxygen. Cult. J. Zoof. 42. 35.5 366. B~VBJFKB R. V. bt GLYIZ~. P. W. (1960) A class exercise on a marine microcosm. Er~olog~ 41, 229- 232. CAKPI:NT~K J. H. (1966) New measurements ol’ouygen solubility in pure and natural waters. Lirnnol. Ocru,~. II. 264 277. CONC;LETOI J. L. (1974) The respiratory response to asphyxia of Typlrloyohius ~tr/i/i,rtlif,l.~i.s (Teleostei: Gobiidac) and some related gobies. Niol. Bull. 146, 186~205. EMERY K. 0. (1946) Marine solution basms. J. Grol. 54, 209 228. GAIL F. W. (1919) Hydrogen ion concentration and other factors affecting the distribution of fucus. Pugct Sound Marine Station Publ.. Vol. II. 287-301. HU<;HES G. M. (1973) Respiratory responses to hypoxia in tish. Am. Zool. 13, 475-489. KAP;WISHFR J. (1964) On the exchange of gases between the atmosphere and the sea. Dt~p SU Rc.\. IO, 195~207. PVkFlhCH
K. A. (1943) The intertidal ecology of Bardscy Island, North Wales, with special rererence to the recolonization of rock surfaces and the rockpool cnkironment. J. Af7ia7. Ed. 12, 82 108. SCHOLANDER P. F., VAN DAM L.. CLAFF C. L. & KANWISHFK J. W. (19.55) MIcrogasometric determination of dissolved oxygen and nitrogen. Biof. Bull. 109, 3X334. STEPHEMON T. A.. Zooho A. & EYRI: J. E. (1934) The liberation and utilization ot” oxygen by the population of rock-pools. J. e~p. Bid. I I, I6.! 172.
of Marine
Biology.
Scripps
Institution
(Receiwd
of Oceanography,
I October
La Jolla, California,
U.S.A.
1979)
--I. Oxygen tension and pH declined rapidly in pools exposed by nocturnal low tides. 2. Minimum oxygen tensions recorded in tidepools at night (14-l 7 Torr) were lower than critical oxygen tensions (cP,,) established by respirometry for three common intertidal fish species. 3. Individual CIinocottus analis and Paraclir~us integripirwis ventilated with water from the surface film when tidepool PO, dropped below the critical level. 4. In comparison with C. analis and P. intrgr@irmis, Gibhonsia ekgans had a higher cPo, and a shorter survival time under anoxic conditions. G. elegans and other species intolerant of hypoxic conditions may avoid tidepools where low oxygen tensions occur during the season of nocturnal low tides.
Abstract
INTRODUCTION
Tidepool fishes were observed during five nocturnal low tides in the late summer of 1967 and 1968. Observations could be made by moonlight on some occasions; on moonless nights a gasoline lantern placed on the ground 5-6 m from the pools provided adequate light. Water samples for oxygen analysis were taken from Pool A at 30-90 min intervals during the early morning hours of August 7 and August 8, 1968. Pools B and C were also sampled several times so that oxygen tensions could be compared with those in Pool A. Sampling began soon after the pools were exposed by the receding tide, and ended when the pools were submerged by the incoming tide. Five milliliter glass syringes with attached polyvinyl chloride surgical tubing were used to withdraw samples from depths of 35cm (bottom of pool) and 1 or 3cm. A three-way syringe stopcock allowed flushing of the dead space. The sample syringes were chilled and transported to the laboratory, where oxygen tensions were measured with a polarographic oxygen electrode (Beckman Mode1 777), fitted with a 0.2 ml glass sample chamber. The oxygen electrode was calibrated with nitrogen gas and air at 100% humidity, cross-referenced against the Winkler titration. The electrode was maintained at 20 + 0.2”C in a water bath, and measured oxygen tensions were corrected to ambient tidepool temperature using the temperaturesolubility tables of Carpenter (1966). The pH of tidepool water was measured in situ at a depth of 6cm with a portable pH meter (Instrumentation Laboratories) calibrated against a high and a low buffer. Fishes used for respirometry were held for 1-3 weeks at 15 + 0.5”C and were not fed for 24 hr before transferral to the chamber. Once in the darkened chamber, the fishes were allowed I2 hr to recover from the stress of handling and acclimate to the chamber. A flow of aerated seawater was maintained throughout this period. The anodized aluminium respirometer had a volume of 83 ml; the design and procedures have been described in detail elsewhere (Congleton, 1974). Respirometric measurements were initiated by stopping the flow of aerated seawater through the chamber. Declining oxygen tension was monitored by an oxygen electrode (Beckman Model 777) and gentle stirring was provided by a magnetic stirring bar. A water bath regulated the temperature in the chamber at 15.0 + 0. I ‘C. Microbial respiration was suppressed by prefiltering seawater through a 0.4 micron sintered glass filter and treating with 100 mg of streptomycin sulfate per liter.
Oxygen concentrations in exposed intertidal pools containing algae or marine grasses typically increase during the day, due to the predominance of photosynthetic oxygen production over respiratory oxygen uptake (Stephenson, Zoond & Eyre, 1934; Pyefinch, 1943). At the same time, carbon dioxide uptake by photosynthesizing plants causes pH values to rise (Gall, 1919; Emery, 1946). During the nighttime hours photosynthesis ceases, and exchange of respiratory gases by tidepool plants and animals causes oxygen and pH values to decline (Stephenson rt al., 1934; Bovbjerb & Glynn. 1960; Emery, 1946). Llttle is known concerning the behavioral and physiological responses of tidepool animals to hypoxic stress. This paper reports field observations on the behavior of tidepool fishes exposed to low nocturnal oxygen and pH levels. Experiments were also carried out in the laboratory with several species of tidepool fishes to establish their respiratory response to progressive hypoxia and to determine tolerance for anoxic conditions.
MATERIALS
AND
METHODS
The study area was a wide, gently sloping shelf of calcareous sandstone near La Jolla, California (South Casa Beach area). Tidepools on this shelf varied in elevation from 18 to 28 cm above Mean Lower Low Water (MLLW), and contained dense growths of algae And surf grass. Three adjacent pools (elevation 23 cm MLLW) were selected for measurements of oxygen tension, pH and temperature during nocturnal tidal exposure. The largest pool (A) was rectangular with dimensions of 2.0 x 4.5 m and an average depth of 35 cm. Pool B was elongate, 0.8 x 3.5 m, with an average depth of 30cm. Pool C was circular, with a diameter of 0.6 m and a depth of 30 cm.
* Present address: Washington Cooperative Research Unit, College of Fisheries, University ington, Seattle. Washington 98195, U.S.A.
Fishery of Wash-
719
JAMES L. CONGLETON
720
To test the importance of access to the water’s surface, tidepool fishes were collected during nocturnal low tides and divided between two wire mesh cages (25cm each side). One cage was positioned with the upper side above the surface, while the second cage was positioned with the upper side 3-S cm below the surface. Tolerance for anoxic conditions was examined by sealing test animals into NO-ml Erlenmeyer Basks filled with deoxygenated water. Deoxygenation was accomplished by flushing with nitrogen gas; residual oxygen tension following this procedure was below the sensitivity limit of the oxygen electrode (approx 2Torr). Fishes were considered dead when coordinated respiratory movements had ceased.
RESULTS
Nocturnal oxygen and pH values Oxygen tension (P,J in Pool A began to decline near the surface (1 and 3 cm depths) as well as near the bottom (35 cm depth) immediately after the pool was isolated by the outgoing tide (Fig. I). Vertical mixing prevented formation of a pronounced PO2
gradient: 3-4 hr after initial exposure, PO1 at surface and bottom differed by only 3-4 Torr (mm Hg). Tidepool temperatures of 18.5-2O.O”C were I-2°C warmer than air temperatures on both August 7 and 8, so the pools did not stratify thermally. Temperatures at bottom and surface never differed by more than O.I”C, and declined slowly throughout the exposure period. of Pool A On both dates, P,2 near the bottom declined to approximately 15 Torr after 3 hr and remained at that level for the duration of the exposure period. Near-bottom PO, in Pools B and C corresponded closely to PO, in Pool A (Fig. I). Nearsurface P,_ varied from 17 to 20 Torr during the last few hours bf exposure. The decline in pH also stabilized after 3-4 hr, reaching minimum values of 7.35-7.45. Oxygen tensions near the bottom declined more rapidly on August 7 than on August 8, because diffusion of atmospheric oxygen into the pool was enhanced by a slight breeze (estimated at 3-5 km hr-‘) during the first few hours of sampling on August 8.
I50 f
1 80
I
+ 7.8
PH
i 76
4 .
PO2 at Icm
v
PO2 at 3cm,
, pool
n
Po,ot 35cm,pool
A A
A PO2 at 30cm,
pool B
+ PO2 at 30cm,
pool C
a
pH, pool
-8 2
pool A
A
- 7.8 PH -76
0100
0200
0300 Time
0400 (PST)
0500
0600
Fig. 1. Oxygen tension and pH changes in tidepools during nocturnal tidal exposure on August 7 (upper) and August 8 (lower). 1968. The vertical arrows near the abscissa indicate the beginning and end of the period of tidal exposure.
Nocturnal hypoxic stress in fish Oxygen tension changes similar to those shown in Fig. I were also recorded in Pools A, B, and C and other nearby pools during nocturnal iow tides in August and September of 1967. Although the technique used to measure oxygen con~ntrations in 1967 (Scholander Microgas AnaIyzer ; Scholander et ui., 1955) is not believed to be as accurate as the oxygen &e&-ode method used in 1368, the 1967 data indicated that tidepool oxygen tensions invariably declined to below 20Torr during nocturnal exposure. An exception occurred on one occasion when a strong wind, estimated at 40-50 km hr- ‘, arose suddenly after several hours of sampling. Samples taken a short time Iater indicated a marked increase in oxygen tension at depths of I and 15 cm, while oxygen tension continued to decline at a depth of 4Ocm. Observations offish behavior Few fishes could be seen in newly exposed tidepools. After an hour of exposure, individuals of the species CIinocottus anatis, the woolly sculpin, could be seen rising to the surface for a few seconds and then returning to deeper water. After 2-3 hr of exposure many specimens of this species began to move into shallow water around the margin of the pool. These fishes rested on the bottom and ventilated rapidly with mouths at the water’s surface. Often a small dimple in the surface film marked the ventilatory intake of water. Individual Pnraclirzus intlrgripimis, tidepool kelpfish, behaved similarly by flexing their bodies laterally to bring the mouth to the surface. Specimens of P. inlegripinnis were usually seen resting on their sides on floating masses of surf grass, rather than around the margins of the pool. Initially, fishes that had moved into shallow water would flee back to deeper water if disturbed by the too-near approach of a human observer. Later in the period of exposure, fishes did not respond even if touched or prodded. Gibbonsia &guns, one of the most common fishes of the southern California intertidal, was never seen in tidepoob at night during more than 30 hr of observation. Juveniles of Girella nigricans, also common in the intertidal during daytime iow tides, were seen on anly a few Qccasions. lrnportattce
qf surface access for survival
Fishes confined in the subsurface cage began to swim continuously upward against the upper side of the cage during the third hour of tidal exposure, when subsurface P,, had declined to 2O-25Torr. Fishes confined in the control cage swam with snuuts contacting the water’s surface. Ten specimens of C. analis
721
that were denied surface access died after 8O-140min (% = 107 min), while seven controls survived the entire exposure.
Specimens of C. an&s, P. infeqripp&is, and G. eleguns regulated oxygen consumption (I&,) over a wide range of oxygen tensions. The animals were not active while in the respirometer chamber, and there was little variation or change in PO, with falling ambient PO, until a critical oxygen tension level was reached, below which voio,declined sharply. The critical oxygen tension (cPoL) was arbitrarily defined as the PO, interval preceding (higher than) the PO, interval in-which I&, dechned to 85% or less of the lowest I&, recorded on the oxygen uptake “plateau.” Median ‘PO, values were 20 Torr for C. anuUs. 23 Torr for P. irzteyripitlnis, and 28 Torr for G. rlequrrs (Table 1).
Anoxic survival times (x i_ SE) for C. an&is, P. j~~~~~~~~i~~~~j‘~, and G. dqpns were, respectively: 44 rfi: 4 min (n = 8); 9.5 _t 22 min (rr = 5); and 14 ) 4 min (t? = 4). These fishes were similar in size to those used for respirontetry (Table I). Mean survival times determined for c‘. ar~ulis and P. in~qripinnis did not differ signi~ca~t~~. but the mean survival time found for G, elqans was significantly less than that of the other two species (P < 0.01: Mann-Whitney U Test), DISCUSSION
Oxygen tensions measured in exposed tidepools at night were below critical oxygen tension (cPo,) levels established by respirometry for the common tidepool fishes C. wufis, P. inregrrpirztlis, and G. rlegans. Further, field cPo, levels would have been somewhat higher than estimated, since respirometric measurements were made at a lower temperature (15°C) than temperatures prevailing nighttime tide-pool (18,5-2O’C), and li,* and cPo2 are known to increase with increasing temperature (e.g. Beamish, 1964). When ambient oxygen tensions fall below the cPo,, compensatory physiological mechanisms can no longer maintain a rate of oxygen uptake adequate to satisfy the metabolic demand. The consequence is increasingiy severe hypoxic stress (Hughes, 1973). C. analis and P. inteyripirzrzis mitigated lrypoxic stress by moving into shallow water and ventilating their gills with surface water. The significamz of this behavior was demonstrated by the death within 1-2.5 hr af specimens of C rrrralk confined in wire
Table I, Critical oxygen tensions icP,J and resting oxygen consumption rates (I&,,)for three species of intertidal fishes at 15’C Species
N
B_odyweight (gl x (range)
$0, (pl 02 g-1 F (range)
!,-I)
CPOf mrr> median (range) 1__-
Clinacottus analis
4
1.32 ~1.20-1.70)
83 (53-103)
20 (16-24)
Paraclinus integsippinnis
6
1.08 (0.52-2.51)
84 (62-119)
23 (20-26)
Gibbonsia ele~tans
4
2.78 (1.28-2.77)
102 (86-112)
28 (X-30)
712
JAMES L. CohCLbroN
mesh cages several centimeters beneath the water’s surface. Oxygen exchange between air and water occurs primarily by molecular diffusion through a thin, coherent surface film less than 100pm thick (Kanwisher. 1964). The oxygen tension gradient between atmosphere and tidepool water is largely limited to this thin film, since oxygen diffusing through the film is rapidly mixed into the more turbulent underlying water. To obtain relief from stressful oxygen tension levels. tidepool fish must draw ventilatory water from quite near the surface. The hypoxic tolerance of P. irlteyripirlnis and G. &~LIIIS was not tested in the field, but laboratory tests indicated that P. irlteyripirlrzis survived anoxia for a slightly. but not significantly, longer time than C. au&s. G. rlryu~ survived anoxia for a significantly shorter time than either C. cudis or P. ir7tegripirlnis. In addition, the cPo2 established for G. rleguns was higher than the c,Po2 values established for either of the other two species. Hence, G. elegur7.s appears to be less adapted for tolerance of nocturnal hypoxia than either C. ur7uli.s or f intrgripimix Specimens of G. &yar7,s were never seen in the study area at night during approximately 30 hr of observation in August and September of 1967 and 1968. but were abundant when tidepool collections were made during daytime low tides several months later. The apparent absence of G. &guns during the late summer may have been due to avoidance by this species of tidepools where hypxic conditions occurred. Nocturnal low tides falling below the MLLW datum occur from March to September on the southern California coast. Future studies may demonstrate that G. &+JII.S and other littoral fish and invertebrate species show seasonal changes in depth distribution
or habitat preference in response nocturnal hypoxic stress. Aclirlowled~erner7ts-I S. Smith
for critical
to the occurrence
thank Dr J. B. Graham reading
of
and Dr L.
of the manuscript.
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