Razor edges

I stepped into the water at the lake’s edge, and only the thin plastic soles of my water shoes protected my feet from the razor edges of the mussels clustered on the rocky bottom of the lake – I could feel hundreds of shells crunching beneath my heels with each step. It was 1993, the year zebra mussels invaded Lake Champlain, Vt., and the striped shells of the mussels were everywhere, coating dock pilings, moorings, and rocks, sharp enough to leave nasty cuts on bare feet. Zebra mussels are small, less than two inches across, and native to the Black and Caspian Seas and the Sea of Azov in Eastern Europe. They’ve now spread throughout much of the Great Lakes and Mississippi River drainages, and where they’ve gone, they’ve taken over.

Zebra mussels were first detected in Oneida Lake, N.Y., about 200 miles southwest of Lake Champlain, in 1991. As reported recently in the journal PLoS ONE, a team of scientists from Cornell University and SUNY Buffalo State conducted a survey of the molluscs – mostly gastropods and bivalves – in Oneida Lake, which they compared to historical surveys conducted every few decades beginning in 1915.

The lake experienced more than one drastic change due to human activity during that time – in particular, the researchers note a period of high nutrient levels and low water clarity, the height of which was in 1967, and the zebra mussel invasion in 1991.

In one bay, the scientists found that by 2012, the number of bivalves – mussels and clams – was ten times as great as it had been in 1915-17. Not surprisingly, almost all of those bivalves were exotic, or non-native, and at least one family of native freshwater mussels appeared to be gone completely – no individuals from that family were found in the 2012 surveys. 

Bivalve density - the number of individuals found in one square meter - increased dramatically between 1915 and 2012; most of that increase was due to two exotic species, the zebra mussel, which invaded Lake Oneida in 1991, and the quagga mussel, which first appeared around 2005.

Most of the change in exotic gastropod density was driven by one species, Bithynia tentaculata, a small snail.

Source: Data from Karatayev et al., 2014.

(Figure by Emily Benson)

Zebra mussels are filter feeders: they eat algae from the lake water around them, and, in the process, they increase water clarity. The researchers suggest that this is the mechanism behind the recovery of underwater gastropods – snails and slugs – that they found; after the number of gastropods in Oneida Lake declined in the 1960s, when water clarity was at its worst, gastropod numbers and diversity rebounded to close to 1917 levels by 2012. Poor water quality during the 1960s, the authors argue, limited the growth of algae on the bottom of the lake, the main source of food for gastropods. When zebra mussels invaded, they ate the algae suspended in the lake, which allowed more sunlight to reach the lake bottom. The scientists suspect the increase in light led to an algae boom on the lake bed, and the subsequent revitalization of the gastropod population of Oneida Lake.

Zebra mussels don't just make life more difficult for bare-foot swimmers – they also clog water pipes, corrode underwater pilings, and choke boat engines. By altering the ecosystems they invade, they may also, at least in the case of Oneida Lake, contribute to the recovery of other organisms. 

Zebra mussels grow on any hard surface they can find, even other mussels.

(Image by U.S. Fish and Wildlife Service via Wikimedia Commons)

Salmon steaks

Picture a salmon steak, maybe marinated in olive oil and spices, grilled just long enough to crisp the edges, served with roasted asparagus and wild rice – that’s a dinner I would be happy to serve (and eat).

Salmon have long been a key source of protein for the people living in the Pacific Northwest and Alaska, in addition to being culturally and economically significant. Yearly runs of salmon returning from the ocean to reproduce in the freshwater rivers and streams where they were born represent a pulse of nutritiously rich food that supports river ecosystems as well as people – adult salmon die after they spawn, releasing nutrients incorporated from the ocean into the freshwater systems where the fish originated.

In rivers, the first recipients of this marine bounty are the macroinvertebrates that colonize and consume salmon after they’ve spawned (and which will eventually serve as food for the juvenile fish that hatch from the salmon eggs). As reported recently in the journal Aquatic Sciences, a team of scientists from the University of Notre Dame investigated how the precise location of salmon carcasses influences the macroinvertebrates that devour them.

The researchers, working in streams on Prince of Wales Island, Alaska, set out salmon steaks in four different areas of each stream – buried in the streambed, resting on top of it, lying out of the water on a gravel bar, and placed above the water among the trees next to it. They found that different types of insects were present in the four habitats – as they expected, terrestrial locations (the gravel bar and the riverbank) featured macroinvertebrates that specialize in consuming carrion, while underwater locations were dominated by generalist consumers, aquatic insects that were likely already living in those locations and opportunistically consumed the salmon steaks.

Despite high rates of decomposition, the scientists also found that, by the end of their experiment (two weeks in some locations, four weeks in others), the remaining salmon was still high in nutrients. They interpret this to mean “that salmon carcasses may provide a resource supporting a succession of consumers over an extended period of time.” In other words, the pulse of energy that a salmon run brings to a river may be less of a spike and more of a slow release, sustaining different types of organisms as it plays out.

Salmon are more than a source of fillets and steaks for human plates – they also nourish the river ecosystems where they’re born by returning there to spawn at the end of their lives. 

Salmon are born in freshwater streams, grow large in the ocean, then return to their natal streams to spawn and expire. 

(Image by Todd Gordon Brown via Wikimedia Commons)

Mysteries of mayfly molting

Scientific experiments are usually meticulously planned out – every detail accounted for in order to avoid the influence of uncontrolled factors that could skew results. Still, even with the best-laid plans, surprising outcomes sometimes occur.

“It was truly an exciting and unexpected find! The first time we observed a molt, we could not figure out what was happening or why the data output was looking so different,” Allison Camp, a third-year PhD student studying environmental toxicology and aquatic insects at North Carolina State University and the lead author of a recent paper published in Freshwater Science, said in an email.

Camp and her co-authors had intended to study the affects of increasing temperature on mayfly larvae oxygen consumption. Mayflies and other insects are a fundamental part of aquatic ecosystems – among other functions, they provide a major food source for fish and other predators.

When Camp and her co-authors began monitoring how much oxygen mayfly larvae consume, they noticed something odd. Most of the mayflies were taking in more oxygen as the temperature increased, but some were exhibiting a completely different pattern – a steep drop in oxygen consumption, followed by an abrupt peak. It was as if some of the larvae were holding their breath, then gasping to make up for lost time.

As Camp said of one of the mayflies that displayed the unusual breathing pattern, “it wasn’t until we opened our respirometry chambers at the end of the experiment that we realized there was a molted exoskeleton in the chamber.”

Molting is an intense process during which a larva sheds its entire exoskeleton, including the lining of its respiratory system. Insects don’t circulate oxygen throughout their bodies via blood vessels like humans do – instead, they have an extensive network of respiratory tubes, called tracheoles, which deliver oxygen directly to different tissues.

The shed exoskeleton of a mayfly larva. The branched, feathery structure at the base of the head is part of the lining of the respiratory system. 

(Image by Allison Camp)

As Camp and her colleagues found, molting larvae show a distinctive respiration pattern – for three to four hours before they shed their skin, their respiration rate increases, then it plummets as the molt begins, when their breathing is restricted for 45-60 minutes as they shed their outer skin (and along with it, the lining of their tracheoles). Fifteen to 30 minutes after the lowest level of oxygen consumption, the rate spikes to roughly twice that of larvae that aren’t molting, followed by a gradual reduction and then recovery, until the breathing rates of the molting and nonmolting larvae converge. Camp says this process can take several hours. 

As the researchers point out, molting is a risky process for mayflies – it’s an energetically costly endeavor, and it’s associated with an increased risk of dying. They write, “molting is highly disruptive to respiratory physiology and is a far more challenging process than we had previously imagined.”

Moving forward, Camp plans to continue studying molting and the affects of temperature on mayfly development. 

“We are looking at growth rates and growth efficiencies across different rearing temperatures right now, and molting is an important piece of that puzzle because it requires so much energy,” she said in an email. Perhaps another unexpected result will help Camp and her colleagues further unravel the mysteries of mayfly molting.

Cloeon dipterum, the mayfly Camp and her colleagues studied, is a member of the baetidae family, a group commonly known as the small minnow mayflies. 

(Image by Allison Camp)

Whale hormones

The largest mammal on Earth – the whale – has long captured the imaginations of humans. Indigenous North Americans hunted whales for thousands of years, using them for food and fuel, before commercial harvesting began in the early 1600s. The huge demand for products derived from bowhead whales (primarily lamp oil made from blubber, and buggy whips, umbrella ribs, and corset stays made from baleen, the long keratin plates that bowhead whales have instead of teeth) led to a drastic decline in their worldwide population, from 30,000-50,000 individuals before commercial whaling began to 3,000 in 1921, when large-scale hunting was banned.

Today, commercial whaling is still largely banned, but a small number of bowhead whales are taken in subsistence hunts each year.

Despite humanity’s long relationship with bowhead whales, there are some areas in which our basic knowledge of their life history is lacking, including reproduction. Scientists estimate that mature female bowhead whales go about three or four years between having calves. That estimate is largely based on three direct observations – in other words, an extremely small sample size.

Recent research conducted at the New England Aquarium in Boston suggests that a new technique might allow us to further investigate bowhead whale calving rates. A team of scientists analyzed discs of baleen, taken from 16 bowhead whales harvested during subsistence hunts in Alaska, for levels of progesterone, a hormone that can indicate pregnancy.

Because baleen grows longer over time (just as our fingernails do), the tissue closest to the gum line – the most recently grown tissue – reflects recent hormone levels, while tissue further out can serve as a record of the whale’s hormone levels in the past. The research team estimated that baleen from bowhead whales could record hormone levels from as long as 25 years ago, depending on the age and size of the animal.

Of the seven mature female bowhead whales the scientists studied, four were pregnant at the time of harvest and had elevated progesterone levels in their most recently grown baleen. All of the other whales – the non-pregnant females, immature females, and males – had lower levels of progesterone in the same area.

The researchers also found elevated progesterone levels in older sections of baleen from two of the mature females who weren’t pregnant when they were harvested, which suggests that they might have been pregnant in the past. (The researchers analyzed up to four samples from each whale, so they likely missed some baleen sections that might have shown evidence of additional past pregnancies.)

The method isn’t perfect – the scientists point out that baleen can grow at different rates in different individuals, and estimating calving intervals from baleen would require knowing the growth rate of the mother’s baleen. However, given the current scarcity of information on bowhead whale reproduction, analyzing baleen could be a fruitful area of research. It could also help scientists figure out if reproduction rates have changed since historical times, an important question with implications for conservation efforts. As the researchers note, “of particular interest is the availability of historical baleen samples in museum archives (samples collected during the era of commercial whaling) that could be used for comparisons with present-day population data.” 

Though still nowhere near as large as it once was, the bowhead whale population has rebounded since the commercial whaling moratorium in 1921; currently, there are 7,000-10,000 bowhead whales worldwide. Baleen harvest was one of the primary motivations behind the commercial exploitation of whales in the past – perhaps now we can use baleen to inform our conservation efforts, rather than in our umbrellas.

Most research labs don't have a tank large enough for a bowhead whale - adults typically weigh between 150,000 and 200,000 pounds and are 35 to 40 feet long - much less a research budget large enough to feed one, making it difficult to study bowhead whales in a controlled environment. 

(Image by NOAA Photo Library via Wikimedia Commons)