Hitchhikers make life difficult for Antarctic sea spiders

Algae, barnacles, and more can grow on sea spiders in Antarctica. How cumbersome can these clingy companions get?

By Megan Chen
Published 24 Nov 2018, 08:33 GMT
An Antarctic sea spider ('Ammothea glacialis') about 15 centimetres (6 inches) long struggles under the weight ...
An Antarctic sea spider ('Ammothea glacialis') about 15 centimetres (6 inches) long struggles under the weight of numerous stalked barnacles growing on top of it. These pesky protrusions are not only heavy, but they also increase the chance of the sea spider getting swept off its feet by strong ocean currents.
Photograph by Steven J. Lane

In the ocean, a wide variety of hitchhikers hang on to other animals. Barnacles are among the most prominent, but there are actually hundreds of creatures that pursue this life strategy. Generally, these freeloaders—which can be seen covering everything from whales to sea turtles to horseshoe crabs—are seen as harmless and even beneficial in some cases.

But what if they become a real drag?

Sea spiders, an enigmatic group of creatures found all over the world, are not immune to such hitchhikers. New research shows that organisms encrusted on or protruding from their surface can affect their movement and interfere with their breathing—since they lack lungs and gills and absorb oxygen through their exoskeleton.

The paper, published in the journal Marine Biology, found that larger hitchhikers such as stalked barnacles did in fact increase the drag experienced by Antarctic sea spiders by two to three times, which increased the energy necessary to walk. The increased surface area also created a Mary Poppins umbrella effect where the sea spiders were more prone to being swept off their feet by ocean currents.

Encrusting organisms such as algae and bryozoans, also known as moss animals, had the potential to greatly reduce breathing locally by up to 50 percent. However, total coverage on most sea spiders was not enough to substantially affect breathing overall.

The study, by a team of scientists led by Steven Lane, a lecturer at Loyola University Maryland, took a look at how three types of sea spiders in Antarctica fare with their cohabitants.

One of the largest species of Antarctic sea spiders ('Colossendeis australis') has encrusting patches of white bryozoans, also known as moss animals, growing on its legs and mouthparts. This one is larger than a dinner plate with a leg span of 30 centimetres (12 inches). Also in view are two special appendages, called ovigers, with hooks at the end that males use to carry eggs and both sexes use to groom their legs.
Photograph by Timothy R. Dwyer PolarTREC 2016, Courtesy of ARCUS

Two of the species studied (Ammothea glacialis and Colossendeis megalonyx) range from 15 centimetres (7 inches) to over 30 centimetres (one foot) in length and resemble daddy long legs with long, thin legs and tiny bodies. The other species, Nymphon australe, are around 4 centimetres (two inches) long and have a bulkier build.

Over two summers at McMurdo Station, a U.S. research centre which Lane likens to a mining and college town combined, scientists spent days diving into the freezing waters of McMurdo Sound, a biologically rich region. During dive days, Lane and his team dove for 30 to 40 minutes at a time. “The limiting factor was how cold we got,” Lane says. The scientists ultimately collected around 200 sea spiders from the shallows to depths beyond 100 feet.

They also recorded video footage of how often sea spiders were found with organisms growing on them and how fast sea spiders with and without organism growth walked away from bright lights, a behaviour that may minimise their exposure to predators.

Back in the lab, scientists first measured how much oxygen could make it past encrusting organisms to the exoskeleton surface using an oxygen sensor on live sea spiders. Then, they placed an oxygen sensor within sea spider leg segments to see how much more oxygen diffused into their bodies.

To quantify the drag that protruding hitchhikers contributed, live sea spiders were dropped in a small tank in front of GoPro cameras and their falling speed recorded. The same sea spiders were dropped again after the protrusions were removed. The study suggests that barnacled sea spiders would be limited to areas with low flow to forage and mate, but it’s still unclear what impact this would have on Antarctic ecology as a whole.

“I’m sure [the sea spiders] are doing something important, but we just don’t know enough about them to say,” Lane says.

There have been rare cases of fish and crabs eating sea spiders and, in the field, some fish were seen grabbing a sea spider, but the would-be predators relented and spat them out.

But male sea spiders, which surprisingly carry the eggs, using specialised appendages called ovigers, are more vulnerable. Some types of shrimp will grab the males, flip them over and eat the eggs, before letting the adults go, Lane says.

Needless to say, there is much more to discover about sea spiders themselves, especially in a place as remote as Antarctica. Including how humans might be affecting them.

As the poles warm and acidify faster than the rest of the world, sea spiders and other creatures could be drastically affected. For example, the growth rate of algae and bryozoans often increase with temperature, potentially leading to a higher density of unwanted hitchhikers.

“Sea spiders have been around for a very, very long time,” Lane says, with the earliest fossils dating back at least 400 million years. “We think these Antarctic species are going to be the first affected by climate change because they’ve been living in such cold environments for so long, that as the ocean warms up, there’s no place for them to go,” Lane says.

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