Tag Archives: Snails

Carbon dioxide’s complex personality

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Carbon dioxide (CO2) deservedly gets a lot of bad press because it is responsible for much of the global warming Earth is currently experiencing.  Less publicized, but perhaps equally important, CO2 is acidifying oceans, thereby threatening the continued existence of some critical biomes such as coral reefs and kelp forests (acid interferes with the ability of many marine organisms to build their shells).  But carbon dioxide also has a kinder, gentler side, as it is an essential resource for plants, and in some cases higher CO2 levels can increase a plant’s ability to carry on photosynthesis.  Sean Connell and his colleagues explored this complex personality by studying a marine ecosystem that experiences naturally varying levels of CO2. High CO2 levels and acidity exist near CO2-emitting vents at the study site – a volcanic island (Te Puia o Whakaari) off the coast of New Zealand.

White_Island_James Shook [CC BY 2.5 (https-::creativecommons.org:licenses:by:2.5)], from Wikimedia Commons

The volcanic Te Puia o Whakaari off the coast of New Zealand’s north island. Credit: James Shook [CC BY 2.5 (https-//creativecommons.org/licenses/by/2.5)], from Wikimedia Commons.

The major players in this ecosystem are the kelp, Ecklonia radiata, several species of turf-forming algae, and two grazers, the snail, Eatoniella mortoni, and the urchin, Evechinus chloroticus.  The typical vegetation in the region is a mosaic of kelp forest, some scattered small patches of algal turf, and sea urchin barrens – hard rock without significant vegetation, a result of overgrazing by sea urchins.  In contrast, extensive algal mats carpeted the rocks near these vents, and the researchers hypothesized that high CO2 levels caused this shift in dominant vegetation.

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Sean Connell collects data in a habitat dominated by algal turf (and numerous fish). Credit: anonymous backpacker.

Connell and his colleagues chose two vents and two nearby control sites at a depth of 6-8 meters. The CO2 levels and acidification near the vents were approximately equal to the amount projected for the end of the 21stcentury, but there were no differences between vents and controls in temperature, salinity or nutrient concentrations. The researchers estimated photosynthetic rates for kelp and turf algae by measuring the rate of oxygen production. They also estimated snail consumption rates by caging them for 3 days and measuring how much algal turf they removed.  They used an analogous approach to measure sea urchin consumption rates.

Conditions at vents had a major impact on both producers and consumers.  Kelp production decreased slightly, while turf production increased sharply at vents (Figures A and B below).  Urchin density declined (almost to nonexistence) while gastropod density increased markedly at vents (Figures C and D).  Lastly, consumption rates (on a per individual basis) by urchins plummeted, while consumption rates by snails increased sharply at vents (Figures E and F).

ConnellFig3

Comparison of production and consumption at control sites vs. carbon dioxide emitting vents.

These patterns converted the normal mosaic of kelp forest, small algal turf patches and urchin barren into turf-dominated habitats.  Algal turf increased in size and frequency near the vents, while kelp forest shrank into near oblivion.

ConnellFig2

Frequency of patches of turf (light gray bars), urchin barren (medium gray) and kelp (black) in relation to patch size (diameter in meters) at control sites (top graph) and sites near vents (bottom graph).

These results can be pictured visually by the graph below.  Under conditions of present-day pH and CO2 levels, gross algal production is relatively low and urchin consumption is relatively high, which results in negligible net algal turf production (net production = gross production – urchin and gastropod consumption).  High CO2 levels sharply increase gross algal turf production while dramatically decreasing consumption by urchins.  Even though gastropod consumption increases slightly at vents, the overall effect on vents is a dramatic increase of net algal turf production. Consequently, the ecosystem experiences regime shift from kelp to algal turf domination.

ConnellFig1

Summary of effects of CO2 release by vents (bottom) vs Controls (top). Net algal production (red circle) = Gross algal production – urchin and gastropod consumption.  Net algal production in dark green zone is predicted to be turf-dominated (as is found near vents), light green is a mosaic, while white zone represents urchin barrens (low production and high consumption). Error bars are 1 standard error. 

Under current conditions, kelp is the dominant producer over turf algae in the near offshore ecosystem. High consumption by urchins keep the turf algae in check.  But near CO2 emitting vents, high levels of carbon dioxide have a dual effect on this ecosystem, disproportionately increasing turf algae production rate and decreasing urchin abundance and consumption rate.  This allows the competitively subordinate turf algae to replace the competitively dominant kelp, resulting in a dramatically changed ecosystem.  This occurs in the absence of an increase in ocean temperature.  Given that ocean temperature will increase sharply by 2100 (along with CO2 levels), many species interactions are expected to change in the next century, and ecosystem structure and functioning will be very different from what we observe today.

note: the paper that describes this research is from the journal Ecology. The reference is Connell, S. D., Doubleday, Z. A., Foster, N. R., Hamlyn, S. B., Harley, C. D., Helmuth, B. , Kelaher, B. P., Nagelkerken, I. , Rodgers, K. L., Sarà, G. and Russell, B. D. (2018), The duality of ocean acidification as a resource and a stressor. Ecology, 99: 1005-1010. doi:10.1002/ecy.2209 Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Seaweed defense – location, location, location.

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If you’re ever feeling sorry for yourself, you should know that things could have been much worse; you could have been the brown seaweed, Silvetia compressa. So many problems!  Ocean waves come crashing over you, threatening to pull you off your life-sustaining substrate.  Ocean tides recede, exposing you to harsh sun and dangerously dry conditions. Perhaps worst of all, the fearsome predator Tegula funebralis eats away at your body, and you are powerless to defend yourself from its savage ravages.

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Tegula snails chomp away on Silvetia seaweed in northern California. Credit: Emily Jones.

As it turns out, Silvetia is not so powerless after all.  After being partially grazed by Tegula, the seaweed can induce defenses that reduce its palatability.  From prior work, Emily Jones noticed that seaweed from northern California shorelines was much more sensitive to grazing than was seaweed from southern California shorelines.  It took fewer grazing snails to elicit palatability reduction in northern Silvetia than it did in southern Silvetia. She decided to focus her PhD work with Jeremy Long on documenting these geographic differences, and figuring out why they exist.

MoatCreek_surveys

Emily Matthews (near) and Grace Ha (far) survey snails and seaweed in a northern California site. Credit: Emily Jones.

Environmental conditions vary along the California coast.  Northern seaweed populations experience cooler temperatures (air ~5-20 °C; water ~10-12 °C) and more nutrients (nitrate levels up to 40 umol/L) than do southern populations (air 5-37 °C; water ~14-20 °C; nitrate levels < 2 umol/L). In addition, Jones and Long surveyed Tegula abundance at three northern California and three southern California sites, counting every snail in 20 quadrats placed in the low, mid and high intertidal zone at each of the six sites (360 0.25 X 0.25m quadrats in total) .  They discovered that seaweed was much more likely to encounter Tegula along northern coastlines.

JonesFig1

Percent of plots with Tegula snails in northern sites (Stornetta, Moat Creek and Sea Ranch – blue bars) and southern sites (Coast, Calumet and Cabrillo – orange bars). High, Mid and Low refer to location within the intertidal zone (high is closest to shore and regularly exposed at low tide).

Given these differences in snail abundance, we can now understand why Silvetia is more sensitive in its northern range to Tegula grazing.  But how strong are these differences in sensitivity? Jones and Long developed a simple paired-choice feeding preference assay to test for differences in palatability. At each location (north and south), the researchers gave test snails a choice between feeding on seaweed that had been previously grazed by either 1, 4, 7, 10 or 13 Tegula snails, or to feed on seaweed with no grazing history.  The test snails grazed for five days, and the researchers measured the amount of seaweed consumed for each group. They discovered that even a little bit of previous grazing (the 1-snail treatment) made northern test snails prefer non-grazed northern Silvetia, while only high levels of previous grazing (the 10 and 13-snail treatments) had similar effects on southern snails tested on southern Silvetia.

JonesFig2

Amount of previously-grazed and non-grazed Silvetia eaten by Tegula in paired choice tests. (Top) Northern Selvetia, (Bottom) Southern Silvetia. Error bars are 1SE. * indicates significant differences in consumption rate.

These findings raised the question of whether the cooler and more nutrient-rich environmental conditions at the northern site were somehow causing this difference in consumption of previously-grazed seaweed.  The researchers designed a series of common garden experiments at the Bodega Marine Laboratory, in which seaweed from both locations were tested in the same environment.  Silvetia was exposed to grazing by two snails, or by no snails for 14 days. When test snails were given the choice of non-grazed or previously-grazed northern Silvetia, they much preferred eating non-grazed Silvetia. In contrast, they showed no preference when given a similar choice between non-grazed or previously-grazed southern Silvetia. This indicates that seaweed from the north are responding more to grazing by reducing palatability than are seaweed from the southern locations.

JonesFig5

Amount of previously-grazed and non-grazed northern and southern Silvetia eaten by Tegula in paired choice tests.

In theory, there could be a tradeoff between induced defenses, such as reduction in palatability in response to grazing, and constitutive defenses, which an organism expresses all of the time.  Examples of constitutive defenses are thorns or spines in plants, and cryptic coloration or body shape in many insects.  Jones and Long found no evidence for such a tradeoff; in contrast southern Silvetia actually had lower levels of constitutive defenses, as both northern and southern Tegula strongly preferred eating southern Silvetia in paired choice tests.

JonesFig6

Amount of northern and southern Silvetia eaten by northern and southern Tegula in paired choice tests.

These geographic differences in seaweed sensitivity to grazing are probably due to long-term differences in environmental history.  Southern Silvetia seaweeds live in stressful conditions (high temperatures and low nutrients), and the physiological cost of mounting an induced defense against low and moderate levels of grazing may be too high to be worthwhile. We also don’t know what the overall grazing rates are in the north versus the south, and importantly, how variable the grazing rates are in each location.  Highly variable grazing rates would select for a strong set of induced responses, which could be turned on and off as needed, allowing seaweed, or any plant, to defend itself against new or more hungry herbivores moving into their environment.

note: the paper that describes this research is from the journal Ecology. The reference is Jones, Emily and Long, Jeremy D. 2018. Geographic variation in the sensitivity of an herbivore-induced seaweed defense. Ecology. doi: 10.1002/ecy.2407. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Snails grow large to fight fear

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In a recent post (Jan 12), I discussed research showing that song sparrow parents reduce provisioning to their offspring when threatened by predators, ultimately reducing offspring survival rates.  But in a turnabout that highlights the natural world’s dazzling diversity, a recent study by Sarah Donelan and Geoffrey Trussell revealed a very different impact of fear on the development of snail offspring. Donelan had worked as Trussell’s laboratory technician for two years and became fascinated by the egg capsules laid by the carnivorous snail Nucella lapillus, an ecologically important species in rocky intertidal communities. Earlier work had shown that predator-induced fear reduced snail feeding and growth rates, so Donelan decided that for her PhD work she would see how predator-induced fear influenced offspring development.

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Adult Nucella alongside ca. 100 egg capsules. Credit: Sarah Donelan.

The researchers recognized that the fear environment experienced by parents before or during reproduction, and by the embryos during early development, could influence growth and development of those embryos. At their research site along the Massachusetts, USA coast, the predatory green crab, Carcinus maenas, can be a source of fear for these adult and embryonic snails. Donelan and Trussell exposed snails to fear by housing separately one male and one female snail in adjacent protected perforated containers (with six blue mussels in each container to feed them) that were set within a large plastic bucket. This bucket also had a somewhat larger perforated container (the risk chamber) containing the dreaded green crab (and two snails to feed it). The control risk chamber had two snails, but no crab.

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Experimental setup with buckets containing egg capsules in perforated cages experiencing different exposure to fear. Credit: Sarah Donelan.

In late spring of 2015 and 2016, field-collected female and male snails were matched to create a total of 80 parental pairs. Donelan and Trussell set up experiments to explore the effects of parental experience with predation risk, embryonic experience with predation risk, and duration of embryonic experience.

Parent snails were exposed to a risk chamber (with a crab in the experimental group, and without a crab in the control group) for three days, and then placed together for four days (without risk) to mate. If an egg capsule was laid, the researchers removed it, and immediately exposed it to an experimental or control risk chamber for a week. Embryonic risk duration was further manipulated by continuing to expose half of the egg capsules to risk for a total of six weeks. The table below summarizes the treatments received by parents and offspring.

DonelanTableGood

DonelinFig1

Mean (+ standard error) shell length (top graph) and tissue mass (bottom graph) of snail embryos exposed to predation risk. Parents were either exposed (solid circles) or not exposed (open circles) to risk before mating.

 

When parents were not exposed to risk, but their offspring were exposed, these offspring had shorter shells and reduced tissue mass compared to all other groups. When both parents and offspring were exposed to risk, offspring shell length increased by 8% and offspring mass increased by a whopping 40% over risk-exposed offspring whose parents were not exposed to risk (left data points in figures a and b). If embryos were not exposed to risk, parental exposure had no significant impact on embryonic development (right data points on figures a and b). Embryonic risk duration had no impact on development.

 

In addition, risk-exposed offspring of risk-exposed parents emerged from their egg capsules an average of 4.1 days sooner than other offspring.

Donelanfig4

Mean (+standard error) number of days until emergence of snail offspring that experienced the presence or absence of predation risk during early development.  Their parents were exposed to risk (solid circle) or no risk (open circle) before mating.

What could be causing these differences in size and rate of development? Donelan and Trussell hypothesized that embryonic snails could grow larger and more quickly if they were somehow able to reduce their metabolic rate. With a reduction in metabolic rate, more energy could be diverted to growth and development, resulting in larger and faster-growing snails. The researchers used an oxygen meter to measure oxygen consumption rates of individual egg capsules (from the eight different treatments in the first experiment) six weeks after deposition, about a week before embryos would begin to emerge. They exposed some of the capsules to predation risk during the experiment (current risk graph below), and left other capsules unexposed. When tested under risky conditions, capsules from parents who were exposed to risk, and that experienced risk as embryos during early development, had 56% lower metabolic rates than the other three groups (left graph), and similarly low metabolic rates as capsules tested without risk (right graph).

Donelanfig2

Mean (+ standard error) respiration rate of egg capsules that were (left graph) or were not (right graph) exposed to current predation risk.  During early development, the embryos in these capsules experienced risk or no risk, and were produced by parents exposed to risk (solid circles) or no risk (open circles) before mating.

Overall, parental experience with predation risk enhances offspring growth and development in the presence of risk. If the parents lack this exposure, risk-exposed offspring suffer the costs associated with small size and slower development. Currently Donelan and Trussell are trying to figure out what these costs are. Smaller snails have less energy reserves, may feed on a less diverse group of prey, and are less likely to remain in safer habitats than are larger juveniles. But we still don’t know whether these effects on early stages of life have lasting impacts as a snail gets older and larger. More generally, we don’t know whether there are similar types of interactions between parental and embryonic experiences of other stressors, most notably environmental stresses that are already being imposed by climate change.

note: the paper that describes this research is from the journal Ecology. The reference is Donelan, S. C. and Trussell, G. C. (2018), Synergistic effects of parental and embryonic exposure to predation risk on prey offspring size at emergence. Ecology, 99: 68–78. doi:10.1002/ecy.2067. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.