Physiological tolerances across latitudes: thermal sensitivity of larval marine snails (Nucella spp.)
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- Zippay, M.L. & Hofmann, G.E. Mar Biol (2010) 157: 707. doi:10.1007/s00227-009-1354-3
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A critical step in understanding how temperature will affect biodiversity in coastal ecosystems is to gain insight into how the tolerances, and ultimately survival, of early life history stages will influence the distribution and abundance of adults. We assessed the thermal tolerance of encapsulated veliger-stage larvae of a common dogwhelk, Nucella ostrina, that occur in the rocky intertidal zone on the west coast of North America. Results showed that veligers collected from northern latitudes in Washington State were less tolerant of heat stress than those from central sites in California. For all sites, we found there to be a subtle difference between the temperatures at which veligers first began to die compared to when veligers reached 100% mortality. On a biogeographic scale, the LT50 temperatures, a measure of larval sensitivity, for N. ostrina veligers displayed a strong latitudinal trend. These findings provide a conservative measurement of the upper thermal limits of encapsulated veligers while illustrating how these early life history stages could be physiologically compromised under future climate warming scenarios.
Although larval forms have been traditionally recognized as a vulnerable stage in the life history of marine organisms (Wilson 1973; Jackson and Strathmann 1981, but see Hamdoun and Epel 2007), few studies have explored the tolerances of embryos or larvae in broad ecological terms (Fernández et al. 2007). Existing research has focused on thermal tolerance differences among developmental stages (e.g., Pedersen and Tande 1992; Sewell and Young 1999; Lu et al. 2004) and temperature effects on the pelagic larval duration (PLD) and their ecological consequences (O’Connor et al. 2007). The importance of understanding the vulnerabilities of larval life history stages in marine invertebrates lends recognition to the emerging field of ecological development (see Sultan 2007). In this study, we tested the thermal tolerance of veligers of the marine dogwhelk, Nucella ostrina, a common, broadly distributed inhabitant of the rocky intertidal of the Northeastern Pacific. Additionally, we tested whether there were any differences in physiological tolerance that could be linked to biogeographic distribution.
The dogwhelks of the Northeastern Pacific coast are an ideal system with which to address the role of physiological capacities, such as thermal tolerance, in setting species range boundaries in the rocky intertidal zone. These intertidal snails lack a planktonic larval stage and develop in egg capsules to juveniles that hatch out and ‘crawl away’ (Morris et al. 1980). Females lay yellow, stalk-like egg capsules that contain numerous developing embryos (Spight and Emlen 1976), fixed on benthic substrates (Thorson 1950). Thus, these egg capsules are routinely exposed to elevated air temperatures during emersion and cooler seawater temperatures during immersion, making them an ideal biological indicator for environmental thermal stress. Nucella ostrina are commonly found in the mid-high intertidal zone within and among mussel beds and barnacles. In coastal sites along the US Pacific Northwest, female N. ostrina can produce egg capsules year-round, although they are more abundant during the winter and spring when food supply is more favorable (Seavy 1977; Moran and Emlet 2001). During development to trochophore and later to veliger stages, embryos feed on nurse eggs (unfertilized eggs or non-developing embryos) within the egg capsule (LeBoeuf 1971). After 2.5–4 months of encapsulated development, individuals hatch as metamorphosed juvenile snails (Costello and Henley 1971; Morris et al. 1980). Unlike juvenile and adult stages of Nucella, the sessile egg capsules are not able to actively avoid abiotic stressors (e.g., desiccation, temperature) by moving to less stressful microhabitats. As a result, developing larvae may represent a significant stage of vulnerability for this marine gastropod. Thus, understanding the physiological responses underlying the development of encapsulated embryos is a key step in defining environmental parameters that can contribute to setting a species range boundaries.
Previous studies have shown that survivorship of early life history stages can be significantly impacted by environmental factors and ultimately influence adult populations (barnacles, Connell 1985; polychaetes, Qian and Chia 1994; oysters, Roegner and Mann 1995; snails, Gosselin and Qian 1997; Moran and Emlet 2001). Additionally, a few studies have looked at the effects of temperature on encapsulated development (Boon-Niermeijer and Van de Scheur 1984; Przeslawski 2005; Fernández et al. 2007); however, to our knowledge, none have addressed survivorship during encapsulated development in Nucella. For later stages, mortality data on N. lamellosa juveniles estimate that 90–99% die within the first year (Spight 1976) and only 1–2% of N. lapillus survive the first few months of life (Feare 1970). Other investigators have addressed potential biotic and abiotic sources of mortality (e.g., salinity— Pechenik 1983; desiccation—Gosselin and Chia 1995a; Rawlings 1999; sun exposure—Moran and Emlet 2001; predation—Spight 1976; Gosselin and Rehak 2007; wave exposure—Etter 1996; Gosselin and Rehak 2007; ultraviolet radiation—Rawlings 1996) in juvenile and encapsulated individuals. However, temperature, in particular, has not been well studied with respect to earlier stages of development. In regard to temperature tolerances of N. ostrina, very little is known about how thermal stress affects embryonic survivorship. Gosselin and Chia (1995b) found that N. ostrina (formerly N. emarginata, Marko 1998) hatchlings around Barkley Sound (Vancouver Island, Canada) were tolerant of temperatures as high as 26°C; all hatchlings survived 8 h of exposure at 22 and 26°C. Mortality among hatchlings did occur at 30°C, whereas adults survived all temperature treatments. This study exemplifies the dichotomy that can exist between the impacts of temperature on juvenile and adult marine invertebrates.
To further explore the relationship of thermal tolerance and early life history stages, we characterized the temperature tolerance of encapsulated veligers of N. ostrina. This is the first study to measure the temperature tolerance of encapsulated Nucella veligers. In addition, given that N. ostrina has an expansive range distribution, from Yakutat, Alaska to Pt. Conception, CA (Vermeij et al. 1990; Collins et al. 1996), we tested the thermal tolerance of veligers collected from multiple sites within the species’ biogeographic range in order to assess the relationship between latitudinal distribution and thermal tolerance. By identifying the upper thermal limits for these embryos, we hoped to contribute to our understanding of how the physiological tolerances of larval stages maps onto the distribution of the adult population.
Materials and methods
Collecting sites for Nucella ostrina
Site of collection
Cattle Point, WA
Boiler Bay, OR
Strawberry Hill, OR
Coos Bay, OR
Bodega Marine Reserve
Bodega Cove, CA (BML)
Rancho Marino Reserve
Cambria, CA (RM)
Thermal tolerance trials
Temperature incubation regimes for Nucella ostrina egg capsules for a particular site
1-h incubation temperatures (°C)
13, 15, 17.5, 19.5, 21.5, 23.5, 25.5, 27.5, 29.5, 31.5, 33.5
13, 19.5, 25.5, 29.5, 31.5, 34.5
13, 19.5, 21.5, 25.5, 29.5, 31.5, 34.5
13, 17.5, 19.5, 21.5, 25.5, 29.5, 31.5, 33.5, 34.5
13, 19.5, 21.5, 23.5, 25.5, 29.5, 31.5, 33.5
13, 19.5, 21.5, 25.5, 29.5, 31.5, 34.5
13, 16.5, 19.5, 21.5, 25.5, 29.5, 31, 34
Methodologically, temperature exposures of 1–2 h in duration are commonly used to analyze the physiological performance of larvae (Boon-Niermeijer and Van de Scheur 1984; Sewell and Young 1999; Brown et al. 2004). Although it is likely that Nucella egg capsules might experience more prolonged exposures to thermal stress in the field, we took a conservative approach in this study and chose a 1 h period for the assessment of survivorship. It is important to note that only advanced veliger-stage larvae were used for this experiment (as described by Gallardo 1979). Namely, if the larvae did not have distinct velar lobes, appearance of shell, eyespots, formation of foot, and operculum, then the whole egg capsule was eliminated from the experiment. These thermal tolerance trials were designed to measure veliger mortality across different temperatures, and these data were used to calculate the LT50 for that particular site. LT50 refers to the temperature at which 50% of the total experimental population was killed by the heat treatment (Stillman and Somero 2000; Hamdoun et al. 2003).
To assess if the observed LT50 was a true indication of larval mortality as opposed to a temporary state of torpor (i.e., a temporary state of decreased physiological activity) induced by the acute heat stress, a second experiment was conducted on N. ostrina larvae from Strawberry Hill, OR. Thirty-five egg capsules were collected from the field and subjected to a lethal (34.5°C) and an acute (31.7°C) heat shock for 1 h and allowed to recover for up to 1 h at the ambient seawater temperature for this population (11°C). N. ostrina veligers were exposed to 31.7 and 34.5°C because these temperatures represent the LT50 and lethal temperature, respectively, for the Strawberry Hill, OR site.
Using the same thermal tolerance protocol as above, egg capsules (n = 35) were exposed to a 1 h heat shock, in triplicate, at the two temperatures (LT50 = 31.7°C and lethal = 34.5°C). Immediately following the exposure, seven egg capsules were dissected and larvae survival rates were determined. The remaining 28 egg capsules for each temperature were placed in the seawater table to recover. Every 15 min up to 1 h, seven egg capsules were removed from the recovery seawater table and survival rates were determined. To obtain a baseline measurement of larval condition in the egg capsules with no heat stress (“control”), survival rates of larvae from 21 N. ostrina egg capsules were quantified prior to any high temperature exposures.
For the thermal tolerance assays, analyses were performed using JMP 7.0 statistical software (SAS Institute Inc.). The LT50 of Nucella for each site was calculated using a 95% confidence interval where upper and lower limits of the 50% mortality were given. Analyses for the recovery experiment were performed using SigmaStat 3.0 statistical software. The average mortality was calculated for each recovery time point for veligers that were heat shocked at the LT50 temperature of 31.7°C and those that were heat shocked at 34.5°C. All values were Box-Cox transformed to meet the assumptions of normality. The mean mortality values for each heat shock temperature were compared using one-way analyses of variance.
Survivorship of Nucella ostrina veligers
Relationship of thermal tolerance to site of collection
Discussion and conclusions
In this study, we assessed the thermal tolerance of veliger larvae of Nucella ostrina, an intertidal dogwhelk, across its biogeographic distribution and measured survivorship in laboratory trials across a range of ecologically relevant temperatures. There were two major findings in this study: (1) N. ostrina veligers displayed a sharp change in survivorship within just a few degrees at all sites, and (2) we found a significant relationship between veliger thermal tolerance and latitude.
Larval thermal tolerance
In the thermal trials, there was a rapid, linear increase in mortality occurring over a few degrees of temperature. Specifically, there was only ~3–5°C difference between the temperature at which we first began to see mortality and the temperature where nearly 100% mortality was achieved across all the sites. Given the large-scale distribution of Nucellaostrina, a broader range of LT50 temperatures might be predicted. However, our data revealed a narrow window of thermal tolerance. For example, we observed a very small gap between survival and mortality, where the LT50 temperature for N. ostrina veligers collected from Strawberry Hill was 31.7°C when compared to the recorded lethal temperature of 34.5°C. These slight differences in thermal tolerance indicate that there is an abrupt threshold for mortality that could have profound affects on survival where “ecological surprises” of sudden increases in temperature could precipitate mass mortality events (Harley et al. 2006).
While our study is the first to assess thermal tolerance, our observation of the narrow window between tolerance and mortality is consistent with other abiotic stressor studies of Nucella that also found broad range of tolerance followed by a sudden decline in survivorship of encapsulated veligers. Although the nature of the egg capsule can mitigate some of the abiotic stresses inherent to life in the intertidal (desiccation stress, osmotic shock, and UV radiation) (Pechenik 1982, 1983; Hawkins and Hutchinson 1988; Rawlings 1990; Russell and Phillips 2009a, b), there are thresholds where the protective capacity of the capsule fails. For example, in a study on desiccation stress, late stage veligers (in the formally N. emarginata, now referred to as N. ostrina, Marko 1998) could withstand up to 80% water loss from the capsule chamber before suffering substantial mortality (Rawlings 1995). In other studies, it has been shown that capsule wall thickness provides some protection against UV radiation. In a comparative study, the thicker-walled capsules of N. emarginata (now referred to as N. ostrina) were more resistant to UV stress and protected from UV-A radiation when compared to the thinner-walled lower intertidal species, N. lamellosa (Rawlings 1996, Pechenik 1999).
Our finding of an acute increase in N. ostrina mortality at elevated temperatures is not isolated within larval biology; similar observations of sudden shifts in survivorship in response to thermal stress have been made in early life history stages of other marine invertebrates. Boon-Niermeijer and Van De Scheur (1984) found that a 1°C increase (39.5–40.5°C) in heat exposure for 3-day-old Lymnaea stagnalis, a gastropod mollusk, shifted mortality from 40 to 100%. Similarly, in a non-marine invertebrate, larvae of two species of Drosophila, D. melanogaster and D. simulans, had percent survivorship that shifted from 100% to near zero when exposure temperatures were increased from 37 to 39.5°C (Krebs 1999). Responses of developing embryos to temperature, and the particular temperatures encountered during the normal breeding season, have been most extensively investigated in echinoderms (Andronikov 1975; Strathmann 1987; Fujisawa and Shigei 1990). Purple sea urchin (Strongylocentrotus purpuratus) embryos displayed abrupt changes in survival over just a 2°C span for both gastrulae and 4-arm larvae (L. M. Hammond, unpublished results). In an early study on five tropical species of sea urchins, Rupp (1973) found that the upper thermal limits of cleavage were near 34°C, whereas egg fertilization was more thermally resistant and not affected until 36°C, suggesting that larval development would cease if larvae were trapped in flat reef pools or lagoons where temperatures exceed 36°C. Similarly, Sewell and Young (1999) reported that the ability for blastulae and later staged tropical sea urchin larvae (Echinometra lucunter) to survive short periods (2 h) of elevated temperatures (4–5°C higher than the 34°C upper limit for normal development) might be important if the larvae are carried to shallow waters of the Caribbean reef flats where seawater temperatures can reach 40°C. In another experiment, significant increases in mortality were observed in the larvae of a brooding tropical coral, Porites astreoides, when exposed to slight increases in temperature from 28 to 33°C (Edmunds et al. 2001). Finally, in a polar species, successful larval development was achieved in a 3°C window, from +6 to +9°C, for the sub-Antarctic crab, Paralomis granulosa (Anger et al. 2003).
While all the above examples help to explain the importance of understanding how temperature limitations influence early developmental stages of larvae, they also highlight the scarcity of data on the temperature impacts on non-planktonic larvae. From an ecological perspective, this study contributes to our understanding of how conditions in the intertidal might influence larval survival. In a more global context, these data also suggest that increased air and seawater temperatures due to global climate change may also increase mortality during extreme heat events (Hoegh-Guldberg 1999; Pandolfi et al. 2003; Denny et al. 2009).
Relationship of thermal tolerance and latitude
Our data suggest a strong latitudinal relationship between calculated LT50 temperature and site. Larvae from more southerly distributed populations were more thermally tolerant than northern populations. For N. ostrina veligers collected from Cattle Point, the LT50 was 30.1°C when compared to an LT50 of 33.9°C of veligers collected at the more southerly location of Rancho Marino Reserve (Table 2). Few other studies have addressed the relationship of biogeography and the physiological tolerances of larval forms. One such study found a modest correlation between thermal tolerance and biogeography for echinopluteus larvae of Strongylocentrotus purpuratus. Larvae spawned from Oregon adults (LT50 = 29.7°C) were more thermally sensitive than those larvae collected from Baja California, Mexico urchins (LT50 = 31°C) (L. M. Hammond, unpublished results).
In contrast to the limited data on larval forms, the correlation of thermal tolerance with latitude and environment has been documented in adult forms of many marine invertebrates (see review Vernberg 1962; Stillman and Somero 2000; Somero 2005; Sorte and Hofmann 2005; Jansen et al. 2007) and in other ectotherms (e.g., Drosophila melanogaster—Hoffmann et al. 2002, mosquitoes—Zani et al. 2005, Fundulus heteroclitus—Fangue et al. 2006). Similar to our results, Urban (1994) found a correlation between LT50 temperature and geographical distribution for adults of 10 bivalve species from Peru and Chile over 22° of latitude.
In summary, the current study showed that Nucellaostrina veligers displayed different thermal tolerances as a function of the latitude at which they were collected. The data presented here are a conservative measurement of the upper limits of physiological thresholds of encapsulated larvae. Overall, these data suggest that larval stages of this intertidal invertebrate may not be a hyper-vulnerable life stage (see also Hamdoun and Epel 2007). This observation is further supported by other studies, for example, on an intertidal bivalve which found that early life stages and adults had similar thermal tolerances. The oyster, Ostreola conchaphila, brood their young and adults and veliger larvae survive the same lethal temperatures of 38.5–39°C (Brown et al. 2004). However, changing thermal patterns due to climate change may significantly increase mortality during larval development and result in population fragmentation based on the latitudinal timing of low tides and tendency for local adaptation (Sanford et al. 2006).
We would like to thank our field assistants (Tim Crombie, Jessica Dutton, Dr. Sarah Henkel, Elizabeth Hoaglund, and Dr. Chris Osovitz) for their help with intertidal collections. We are grateful to Drs. Mary Sewell and Kathy Foltz for helpful comments during the preparation of this manuscript. This research was conducted at the University of California Natural Reserve System Bodega Marine Laboratory/Reserve, Coal Oil Point Natural Reserve, Kenneth S. Norris Rancho Marino Reserve, and Scripps Reserve and supported by a Mildred E. Mathias Graduate Student Research Grant from the University of California Natural Reserve System (to MLZ). Partial support also came from an Alan J. Kohn Graduate Student Fellowship from Friday Harbor Laboratories at the University of Washington (to MLZ). Preparation of this manuscript was partially supported by the University of California Marine Council, Coastal Environmental Quality Initiative Graduate Student Fellowship to MLZ and by the US National Science Foundation grant OCE-0425107 to GEH. This is contribution number 346 from PISCO, the Partnership for Interdisciplinary Studies of Coastal Oceans funded primarily by the Gordon and Betty Moore Foundation and David and Lucile Packard Foundation. The described experiments and specimens collections comply with the current laws of the United States of America and the regulations of the California Department of Fish and Game.
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