Oecologia (2005) 142: 511–520 DOI 10.1007/s00442-004-1755-4
E C O PH Y SI OL O G Y
Jose M. Pulgar Æ Francisco Bozinovic F. Patricio Ojeda
Local distribution and thermal ecology of two intertidal fishes
Received: 26 July 2003 / Accepted: 7 October 2004 / Published online: 24 December 2004 Springer-Verlag 2004
Abstract Geographic variability in the physiological attributes of widely distributed species can be a result of phenotypic plasticity or can reflect evolutionary responses to a particular habitat. In the field, we assessed thermal variability in low and high intertidal pools and the distribution of resident fish species Scartichthys viridis and transitory Girella laevifrons along this vertical intertidal gradient at three localities along the Chilean coast: Antofagasta (the northernmost and warmest habitat), Carrizal Bajo (central coast) and Las Cruces (the southernmost and coldest habitat). In the laboratory, we evaluated the thermal sensitivity of fish captured from each locality. The response to temperature was estimated as the frequency of opercular movements and as thermal selectivity in a gradient; the former being a indirect indicator of energy costs in a particular environment and the latter revealing differential occupation of habitat. Seawater temperature in intertidal pools was greatest at Antofagasta, and within each site was greatest in high intertidal pools. The two intertidal fish species showed opposite patterns of local distribution, with S. viridis primarily inhabiting the lower sectors of the intertidal zone, and G. laevifrons occupying the higher sectors of the intertidal zone. This pattern was consistent for all three localities. Locality was found to be a very important factor determining the frequency of opercular movement and thermal selectivity of both S. viridis and G. laevifrons. Our results suggest that S. viridis and G. laevifrons respond according to: (1) the thermal history of the habitat from which they came, J. M. Pulgar (&) Æ F. Bozinovic Æ F. P. Ojeda Center for Advanced Studies in Ecology and Biodiversity and Departamento de Ecologı´ a, Pontificia Universidad Cato´lica de Chile, Santiago, 6513677, Chile E-mail:
[email protected] Tel.: +56-2-6862797 Fax: +56-2-6862621 Present address: J. M. Pulgar Departamento de Ciencias de la Salud, Universidad Andre´s Bello, Santiago, Chile
and (2) the immediate physical conditions of their habitat. These results suggest local adaptation rather than plasticity in thermoregulatory and energetic mechanisms. Keywords Physiological diversity Æ Intertidal fish Æ Thermal sensitivity Æ Rock pool
Introduction Physiological constraints are important determinants of the distributional limits of species and populations (Bozinovic and Rosenmann 1989; Bozinovic et al. 1995; Gaston and Spicer 1998; Chown and Gaston 2000); however, processes associated with environmental tolerance explaining, at the local scale, differential habitat use or, at the geographical scale, species distribution patterns remain poorly understood. Indeed, latitude, altitude and depth all constitute gradients, which can generate physiological diversity in the responses of individuals and populations (Spicer and Gaston 1999). Among populations, variation in physiological traits can be environmentally induced, through non-genetic, reversible mechanisms of phenotypic flexibility or acclimatization (Piersma and Drent 2003), or can reflect genetic adaptation to local conditions, in which case differences among organisms from different habitats cannot be removed by common garden experiments (Stearns 1982; Spicer and Gaston 1999). Among vertebrate ectotherms, variation in physical factors, particularly temperature, has been documented as one of the most important determinants of distribution (Norris 1963; Spicer and Gaston 1999), affecting all biological processes and life history traits, such as body size (Tracy and Christian 1986; Cossins and Bowler 1987; Weise 1991; Lenski and Bennett 1993; Parsons 1993; Stephen and Porter 1993; Pulgar et al. 1999; Angilletta et al. 2002). Among marine animals with high dispersal capacity, latitudinal genetic differentiation of
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populations has occurred as an adaptation to local conditions, in spite of the absence of obvious barriers to gene flow (Lonsdale and Levinton 1985). Latitudinal physical variation may generate selective pressures that result in thermal tolerance, depending on the geographic origin of the populations (Spicer and Gaston 1999; Morritt and Ingo´lfsson 2000). Intertidal rock pools are characterized by their discrete nature in time and space, as well as by their daily and seasonal variability in temperature, oxygen concentration and salinity (Newell 1970; Stephenson and Stephenson 1972; Truchot and Duhanel-Jouve 1980; Metaxas and Scheibling 1993). These variations, particularly in temperature, depend on the position of the rock pool along the intertidal vertical gradient, which determines the length of time that a rock pool remains isolated from the subtidal system (Gibson 1972, 1982; Horn and Gibson 1988; Horn et al. 1999). Intertidal fish assemblages are composed of resident and transitory species (Horn et al. 1999). Gibson (1982) and Horn and Gibson (1988) predicted that intertidal fish zonation patterns should be similar to those observed among sessile individuals, being principally associated with physical conditions (Zander et al. 1999). In this sense, Thompson and Lehner (1976) determined that resident fish species usually occupy sectors of the intertidal zone closest to the subtidal zone. These fishes should occupy the lowest sectors of the intertidal zone, and, thus, the habitats that are the least variable in terms of physical conditions. Whereas transient fish usually occupy sectors most distal from the subtidal zone, which are the most variable in terms of physical conditions In spite of the fact that intertidal fish present an excellent study model for research on thermal physiology, most work on fish has centred on trophic ecology, the identification of guilds, differential habitat utilization and assemblage persistence and stability (Gosline 1966; Thompson and Lehner 1976; Yoshiyama 1980, 1981; Gibson 1982; Grossman 1982; Bennett et al. 1983; Bennett and Griffitths 1984, Collette 1986; Grossman 1986; Ralston and Horn 1986; Bennett 1987; Varas and Ojeda 1990; Prochazca and Griffitths 1987; Mun˜oz and Ojeda 1997, 1998; Aldana et al. 2002). Information on the thermal sensitivity of intertidal fish is scarce. Graham (1970) predicted that a fish’s geographic origin would determine its thermal tolerance and distribution along intertidal vertical distribution gradients. Davis (1977) indicated that in the fishes of the genus Gibbonsia, those with the greatest capacity for adaptation to high temperatures had the widest geographic distributions. Bridges (1993), Metaxas and Scheibling (1993) and Davis (2000) established that seasonal variation in the abundance of intertidal fish was mainly associated with seawater temperature and specific microhabitats. Furthermore, Pulgar et al. (1999) demonstrated that starvation did not modify the thermal selectivity of Girella laevifrons, and Herna´ndez et al. (2002) reported that small-sized individuals of Graus nigra had a greater tolerance to thermal stress than larger sized individuals,
a response that may explain their differential use of the intertidal vertical gradient. Thus, the distribution and abundance of intertidal fish appear to be strongly influenced by physical habitat conditions, especially temperature of the tidal pool (Davis 1977; Zander et al. 1999). Scartichthys viridis (Blenniidae) and G. laevifrons (Kyphosidae) (Nelson 1994), are the most abundant fish species inhabiting the rocky intertidal zone along the Chilean coast (Stepien 1990; Varas and Ojeda 1990). S. viridis is a permanent resident of the intertidal zone, with the exception of its planktonic larval stage (Varas and Ojeda 1990). G. laevifrons is a transitory species since it inhabits the intertidal zone only during the juvenile stage, later migrating to the subtidal zone (Varas and Ojeda 1990; Mun˜oz and Ojeda 1997). Given the wide geographic distribution of both fish species, extending from 18 to 40S along the Chilean coast (Ojeda et al. 2000), we predicted that populations inhabiting separate geographic localities would be subjected to strong variation in tidal pool temperature, and this might have generated more thermally sensitive populations at higher latitudes than at lower latitudes, and in those environments where the thermal extremes are more marked (low latitudes) fishes inhabiting low intertidal areas (i.e. thermally more stable) would not occur in the high intertidal zone (i.e. thermally more variable). We also predict that in the southern localities both fish groups could occupy the entire intertidal zone. Therefore, the objectives of this work were: (1) to record thermal variations in tidal pools of the low and high intertidal zones at three localities along the Chilean coast (Antofagasta and Carrizal Bajo in northern Chile, and Las Cruces in central Chile, each separated by ca. 5 latitude), (2) to characterize the intertidal vertical distribution of S. viridis and G. laevifrons at each locality, and (3) to evaluate the thermal sensitivity of S. viridis and G. laevifrons under laboratory conditions. Thermal sensitivity was estimated using two measurements: (1) the frequency of opercular movements as a function of increasing temperature, and (2) thermal selectivity within a thermal gradient. The frequency of opercular movements has been directly related to energy expenditure (see Roger and Weatherley 1993; Van Rooij and Videler 1996; Granterner and Taborsky 1998), while thermal selectivity reveals differential occupation of the habitat (Pulgar et al. 1999; Herna´ndez et al. 2002).
Materials and methods Thermal variability of the intertidal system To characterize thermal heterogeneity along the vertical intertidal gradient we placed a thermal logger (TIDBIT; Unisource) in one low and one high intertidal pool at each of the study locations: Antofagasta (2324¢S, 7035¢W), Carrizal Bajo (2804¢S, 7035¢W), and Las Cruces (3326¢S, 7141¢W). Seawater temperature in the
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pools was recorded at half-hour intervals between June 2001 (austral winter) and January 2002 (austral summer). The selected low intertidal rock pools were located no more than 1–2 m above the low tide mark. Thus, the seawater of these pools was renewed during every tidal cycle. The high intertidal pools were located at least 50 m above the breaking waves during low tide and, thus, the seawater in these pools was not renewed during most tidal cycles. At each locality, we registered maximum, minimum and average daily water temperatures in both low and high intertidal pools. At each geographic locality, specimens were captured from tidepools between June 2001 and January 2002, from high and low intertidal rock pools at three localities along the Chilean coast: Antofagasta (2320¢S, 7038¢W), Carrizal Bajo (2804¢S, 7108¢W) and Las Cruces (3326¢S, 7141¢W). Specimens were transported to the University Marine Season at Las Cruces (Coastal Marine Research Station) where they were maintained under laboratory conditions, with constantly circulated filtered sea water, constant aeration and daily feeding ad libitum. Frequency of opercular movements in resident S. viridis and transitory G. laevifrons In the laboratory, we evaluated the frequency of opercular movement in response to increasing seawater temperature (from 13 to 30C at a rate of 0.1C/min over a 2-h period) for fish collected from the three different localities. In these tests we used six similarly sized fish from each locality [i.e. total length (Lt)=5 cm for G. laevifrons and Lt=7 cm for S. viridis]. Fish were placed one per trial, in a 12·18·9-cm aquarium, which was submerged in a larger 22.5-l (30·30·25 cm) tank. The aquarium contained filtered seawater and was equipped with constant aeration and illumination. This design allowed frontal filming during 2-h periods, using a Sony Handycam 8 XR video camera. All experimental assays began at 1200 hours and were preceded by a 1-h acclimation period in the boxes, at constant water temperature (13C). Subsequently, we observed the recordings and counted the number of opercular movements per individual per 10 min during the 2-h period. Controls were conducted at constant seawater temperature (13C) using three similarly sized individuals per locality for each species, due to the low number of individuals that finally survived. Thermal acclimation Fish from each locality were separated into two groups. One group was maintained for 1 month at a constant temperature of 14±1C, while the other group was kept at 25±1C. These acclimation temperatures are the thermal extremes in natural conditions. Both groups were kept under identical laboratory conditions with constant aeration and replacement of seawater and unlimited access to food. Ten individuals of each species
from each geographical locality were subjected to one of the two thermal treatments. Five individuals were used for each control during the thermal selectivity assays. Since S. viridis is a bottom-dwelling species, its thermal sensitivity was evaluated in an horizontal gradient, while G. laevifrons being a water-column species, was evaluated in a vertical thermal gradient. S. viridis All of the S. viridis specimens used in the thermal selectivity experiments were similarly sized (6–7 cm). The thermal selectivity of S. viridis was evaluated in a 80·15·15-cm aquarium which was filled with seawater to a depth of 2.5 cm. This depth does not allow vertical thermal stratification within the aquarium, and also permits free movement of the fish. The aquarium was split into eight equally sized 10-cm compartments, and both ends were placed on two 15·15·15-cm aquaria filled with seawater. A thermoregulated bath calibrated to 5C was installed at one end of the tanks, while a heater set to 40C was placed in the other extreme of the tank. After a period of 45 min, a thermal gradient was established, ranging from 10.1±0.25C to 28.6±0.49C along the 80·15·15-cm aquarium. Each assay on an individual fish specimen lasted 1 h and was conducted using the following protocol. First, each fish was released in the mid-section of the aquarium; after 30 min (i.e. sufficient time for the fish to explore the gradient), we registered the position of the fish within the thermal gradient. Preliminary assays indicated that fish explored the entire experimental system within 20 min (Pulgar et al. 1999). Additional observations of position were made at 5-min intervals during the following 30 min, thus resulting in a total of seven observations per fish. An identical protocol was followed with the control fish, which were subjected to a situation with a constant temperature of 14C. Prior to the commencement of thermal selectivity assays we continuously measured temperature in the gradient using a digital thermocouple confirming the stability of the gradient over this time period. G. laevifrons For this species temperature selectivity experiments were performed in an aquarium (30·30·60 cm) with a vertical thermal gradient. The aquarium was split into six 10-cm cells, separated by perforated acrylic sheets, which permitted fish movement. The bottom of the aquarium was immersed in a water bath at 10C. A heater was placed on the top of the aquarium to maintain the water temperature at 31C. The temperature of each cell was monitored for 2 days by means of a digital thermocouple to record the stability of the temperature gradient. The average temperature registered at each cell in the thermal gradient was (±1SD): bottom cell 1=10.3C
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(0.3), cell 2=12.9C (1.3), cell 3=16.4C (1.76), cell 4=18.6C (2.56), cell 5=25.3C (1.11), upper cell 6=30.9C (1.22). Prior to the commencement of thermal selectivity assays we continuously measured temperature in the gradient using a digital thermocouple confirming the stability of the gradient over this time period. In each temperature assay one fish was introduced at the top of the thermal gradient. After 30 min, we recorded the total time spent by the fish in each cell of the thermal gradient for a total of 20 min. The same protocol was applied to the control fish, but the system was maintained at a constant temperature of 14C.
Statistical analyses A significance level of P £ 0.05 was considered throughout the study. Differences in the frequency of occurrence of S. viridis and G. laevifrons were compared between high and low tidepools and between localities using a v2-test applied to a 3·2 contingency table (Zar 1996). At each locality, we evaluated the relationship between temperature and opercular frequency for each experimental fish specimen exposed to temperature increments and each control fish using linear regression. The slopes obtained from these regressions were then compared between localities and treatments using a twoway ANOVA (Zar 1996). In order to correct the confounding effect between time and the temperature of each experimental group (control and experimental), the opercular frequency of control fishes was subtracted from the opercular frequency of experimental fishes. Later, the slopes of the relationship between temperature and opercular frequency was evaluated between localities using a one-way ANOVA. In order to analyse the amount of time spent by G. laevifrons specimens in each of the cells of the thermal gradient, we summed the time spent in the two cells with the highest temperature (i.e. cells 5 and 6) separately from the time spent in the two cells with intermediate temperature (3 and 4), and the two cells with the lowest temperature (1 and 2) (Table 1). To evaluate thermal selectivity of fish from each locality and acclimation treatment group, we calculated a standardized temperature (ST) as follows:
Table 1 Results of the two-way ANOVA evaluating opercular frequencya in Girella laevifrons. Factors are: locality (three levels) and treatment (two levels) Effect
df effect
MS
F
P
Locality Treatment Locality·treatment Error
1 2 1 21
403.811 2.37 32.44 2.31
174.93 1.02 14.05
0.0001 0.37 0.00012
a Opercular frequency of G. laevifrons depends on interaction between locality and treatment (i.e. control fishes versus fishes exposed to different temperatures)
locality (three levels, Antofagasta, Carrizal Bajo and Las Cruces) and acclimation treatment (two levels, 14 and 25C). To evaluate thermal selectivity between experimental fish (i.e. those exposed to the thermal gradient) and control fish (i.e. those maintained at constant temperature), we utilized the Shannon-Weaver diversity index (H) (Ricklefs 1990), calculated as a measure of fish time distribution: H ¼ Rpi loge pi where pi is the proportion of time spent in each sector of the aquarium. Statistical differences in H values were analysed using a one-way ANOVA, considering the experimental condition as a fixed factor with two levels (treatments and controls). A similar experimental protocol and statistical analyses were followed for S. viridis. In this case, however, we registered the number of times that each fish was observed in each of the four sectors of the aquarium.
Results Intertidal thermal sensitivity
The mean maximal daily temperature recorded in the low and high intertidal pools at each locality was higher in summer than in winter (Fig. 1a, b). Maximal temperatures were significantly higher in Antofagasta than in Carrizal Bajo and Las Cruces (Fig. 1a, b). On the other hand, the minimum daily temperature diminishes from summer to winter in Las Cruces and Carrizal Bajo (Fig. 1c, d). The average daily temperature was higher in summer than in winter in both intertidal bands (Fig. 1e, f). ððT C1time1ÞþðT C2time2ÞþðT C3time3ÞÞ ST¼ Totaltime
where, TC 1=the highest temperature of the experimental gradient; time 1=duration the fish spent at the highest temperature; TC 2=average temperature of the experimental gradient; time 2=duration the fish spent at the average temperature, TC 3=coldest temperature of the experimental gradient; time 3= duration the fish spent at the coldest temperature; Total time=time of each assay (1,200 s). ST values were compared using a two-way ANOVA considering the factors geographical
Vertical distribution of S. viridis and G. laevifrons along the intertidal gradient at different localities The distributions of G. laevifrons and S. viridis in the rocky intertidal zone proved to be strikingly different. G. laevifrons was significantly more abundant in high intertidal pools at all geographical localities (v2-test, v2(0.05,2)=25.69; P
