It’s sea otter awareness week, which seems like a great time to reveal something heroic about this charismatic animal. A recent study from UC Santa Cruz concluded that sea otters are helping to save the ocean—with their appetites.

When you think of sea otters, you may think “cute and cuddly,” but these playful marine mammals are top predators, like great white sharks and tigers, and their hunt for food is helping to maintain ecosystem health along portions of California’s coastline.

The sea otter’s role in ecosystem management begins with one of its preferred foods: crabs. Sea otters eat crabs. Crabs in turn eat sea slugs and small crustaceans. The slugs and crustaceans eat algae off sea plants, keeping them green and healthy. It’s a relatively typical food web but now it’s clear: The healthier the crab-eating otter population is, the healthier the plants tend to be.

Sea plants, like eelgrass, along the west coast are important habitat for fish such as Pacific herring, halibut and salmon. They also protect shorelines from storms and waves, and they soak up carbon dioxide from seawater and the atmosphere.  Thus, a healthy coastal ecosystem has the right mix of otters eating crabs and invertebrates eating algae.

Unfortunately, seagrass meadows have been declining worldwide, partly due to excessive nutrients from agricultural and urban runoff entering coastal waters.  When sewage and agricultural waste like fertilizers spill into the sea, ecosystems suffer. Excessive nitrogen and phosphorus in the water spawns excessive algae growth, which can block sunlight and limit plant growth. Coastal areas that would otherwise be swaying in seagrass and kelp turn brown, murky, and barren of important marine species. But, not when sea otters are around.

Brent Hughes from the University of California, Santa Cruz and his colleagues studied 50 years’ worth of data, comparing areas with or without otters. The team discovered that otters trigger the above ecological chain reaction that protects seagrass meadows and can stave off algal blooms.

“The seagrass is really green and thriving where there are lots of sea otters, even compared to seagrass in more pristine systems without excess nutrients,” Hughes says.

Sea otters were hunted to near extinction in the 19^th and 20^th centuries. Populations on the California coast are slowly recovering now, and one of those places otters have called home since the 1980s is Elkhorn Slough, an estuary in Monterey Bay. Hughes and his colleagues determined that the re-colonization of that estuary by sea otters has been an important factor in the seagrass comeback.

In Tomales Bay, a nearby inlet with far lower levels of incoming nutrients, but no otters, the beds don’t look nearly as good. Hughes told Ed Yong of National Geographic:

The seagrass looks relatively unhealthy: it’s brown, covered in algae, and slumped over. The crabs are four times more abundant and 30 percent bigger than they are in Elkhorn Slough.

The findings in Elkhorn Slough suggest that expansion of the sea otter population in California and re-colonization of other estuaries will likely be good for seagrass habitat—and coastal ecosystems—throughout the state.

“This provides us with another example of how the strong interactions exerted by sea otters on their invertebrate prey can have cascading effects, leading to unexpected but profound changes at the base of the food web,” Hughes says. “It’s also a great reminder that the apex predators that have largely disappeared from so many ecosystems may play vitally important functions.”

The study was published last month in the Proceedings of the National Academy of Sciences.

(Sea otters also play a heroic role in the next Academy planetarium show! Currently in production and set for a fall 2014 opening date, the latest production from our visualization studio will highlight complex relationships in ecosystems—and how humans fit into the picture.)

Jami Smith is a science geek-wannabe and volunteers for Science Today.

Image: Robert Scoles/NOAA

The period on our planet between 540 and 520 million years ago when most modern animal groups appeared is also known as evolution’s Big Bang. Prior to the Cambrian explosion, life was much simpler on Earth—single-celled organisms dominated the landscape.

But how did so many different organisms develop in such a short period of time? “The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life,” says Michael Lee of the University of Adelaide. “Darwin himself famously considered that this was at odds with the normal evolutionary processes.”

Lee and his colleagues decided to look into “Darwin’s dilemma,” focusing on arthropods (insects, crustaceans, arachnids and their relatives), the most diverse animal group in both the Cambrian period and present day.

“It was during this Cambrian period that many of the most familiar traits associated with this group of animals evolved, like a hard exoskeleton, jointed legs, and compound (multi-faceted) eyes that are shared by all arthropods,” explains team member Greg Edgecombe of the Natural History Museum of London. “We even find the first appearance in the fossil record of the antenna that insects, millipedes and lobsters all have, and the earliest biting jaws.”

The team quantified the anatomical and genetic differences between living animals, and established a timeframe over which those differences accumulated with the help of the fossil record and intricate mathematical models.

“In this study we’ve estimated that rates of both morphological and genetic evolution during the Cambrian explosion were five times faster than today—quite rapid, but perfectly consistent with Darwin’s theory of evolution,” Lee says.

ScienceNOW offers the numbers:

The creatures’ genetic codes were changing by about .117% every million years—approximately 5.5 times faster than modern estimates.

Unusual, perhaps, but in line with natural selection, the team indicates. The study appears in the recent edition of Current Biology.

Perhaps Darwin can get some rest now.

Image: Michael Lee

What’s going on with the oceans and what can we do?

As carbon dioxide (CO₂) rises in our atmosphere, the oceans absorb roughly a quarter of the amount. This lowers the pH level in the seawater, making the oceans more acidic. How this affects life in and out of the sea is continually studied.

This week, ocean acidification is the topic of several scientific papers. We thought we’d highlight a few of them here.

Nature Climate Change has two papers—one about the affect of acidification on several different species, and the other on how ocean acidification causes even more global warming.

For the first paper, German researchers surveyed previous studies that dealt with the consequences of ocean acidification for marine species from five animal taxa: corals, crustaceans, mollusks, fish, and echinoderms. By the end, they had compiled a total of 167 studies with the data from more than 150 different species.

Their findings? Different species are affected in different ways and to different extents, but all species are negatively affected by ocean acidification. “Our study showed that all animal groups we considered are affected negatively by higher carbon dioxide concentrations. Corals, echinoderms, and mollusks above all react very sensitively to a decline in the pH value,” says lead author Astrid Wittmann, of the Alfred Wegener Institute.

The second study demonstrates that the negative effects of ocean acidification aren’t just limited to marine life. The authors discovered that rising ocean acidity has the potential to amplify climate warming in general, through the decreased production of a biogenic sulfur compound.

Phytoplankton in the ocean produce dimethyl sulfid (DMS). As DMS is released into the air, it creates atmospheric sulfur—which increases the reflectivity of the atmosphere to incoming radiation, reducing surface temperatures. Warming acidic oceans means the phytoplankton produce less DMS, causing an even warmer planet.

In addition to the Nature papers, Philosophical Transactions of the Royal Society B has an ocean acidification-themed issue this week, with nine papers studying its effects. The papers describe three distinct effects on marine life due to ocean acidification: species interactions, decreased ecosystem functions, and adaptations. Andrew Revkin has a great summary of them on his Dot Earth blog in the New York Times.

“It’s great that some of these papers are looking at entire ecosystems,” says Aaron Pope, the Academy’s sustainability manager who works tirelessly to communicate ocean acidification issues. “There’s been lots of research in the past on individual species impacts, but data on entire natural systems was missing. Now we can start to talk about what will really happen in marine ecosystems as ocean acidification gets worse.”

One paper of the group (from local researchers at San Francisco State University) looks at tiny coccolithophores. These single-celled algae are able to sequester oceanic carbon by incorporating it into their shells, providing ballast to speed the sinking of carbon to the deep sea. The little organisms are central to the global carbon cycle, a role that could be disrupted if rising levels of atmospheric carbon dioxide and warming temperatures interfere with their ability to grow their calcified shells.

This paper might provide a bit of hope among the rest: “At least in this experiment with one coccolithophore strain, when we combined higher levels of CO₂ with higher temperatures, they actually did better in terms of calcification,” says co-author Jonathon Stillman, of SF State.

Here’s to hoping that all of these papers findings will create more awareness of ocean acidification that will lead to more solutions.

Coccolithophore image: Alison R. Taylor/PLoS Biology