This Sightless Invertebrate Is Probably Doing More To Fight Climate Change Than You

Giant larvaceans such as this have no true spine, no eyes, and only a sort of brain analog, but they're playing a big part in fighting climate change. 2017 MBARI

If you think of politicians who won't do anything about climate change as brainless invertebrates incapable of vision, you may be overly generous. Giant larvaceans (Bathochordaeans), a genuine invertebrate genus related to sea squirts, have been found to play an important role in removing carbon dioxide from the upper oceans, helping to keep the planet from overheating.

Some of the carbon dioxide released into the atmosphere gets absorbed by the upper layers of the oceans, causing ocean acidification, and preventing further absorption. Fortunately, some falls into the depths, where it is far less harmful, but this happens much slower than we would like. Moreover, there are big gaps in our understanding of the processes.

A team at the Monterey Bay Aquarium Research Institute have revealed Bathochordaeans are major contributors, filtering organic material at a rate higher than any other zooplankton filter feeder.

Much of the carbon dioxide absorbed by the oceans becomes incorporated into phytoplankton, forming the basis of the marine food chain. However, since these species float, this does nothing on its own to reduce carbon levels in the upper ocean in the long term. When eaten by animals, however, things can change. Extensive studies have investigated the rate at which surface dwellers consume phytoplankton, and how much of their excretions or corpses escapes to the depths. However, we know much less about the animals that live slightly further down.

Using the Deep particle image velocimetry (DeepPIV) instrument, Dr Kakani Katija and colleagues studied deeper water plankton feeders. In Science Advances, they report that Bathochordaeans are collecting huge amounts of carbon in the upper 400 meters (1,500 feet) of the ocean.

The authors describe larvaceans as “tadpole-shaped... their morphology consists of a trunk (or head) and tail. Rhythmic motion of the tail is used to pump water-borne food particles through a complex mucus filtering structure that is attached to the trunk. This mucus structure is called a 'house' because the animal lives inside it.” While most larvaceans are 2-8 millimeters long (0.08-0.3 inches), Bathochordaeans are more than 10 times that size, and their houses can grow to more than a meter (3 feet), filtering up to 76 liters (20 gallons) an hour.

When the feeding structure gets clogged, the larvacean will abandon it and build a new one. The smaller houses tend to degrade in shallow water, but those built by Bathochordaeus sink to the ocean floor, taking carbon with them. The authors conclude this can account for as much as a third of the carbon removed from surface waters to the depths. The Bathochordaeus population of Monterey Bay might take as little as 13 days to filter and dispatch all the particles in the Bay between 100 and 300 meters (330-100 feet) down.

Monterey Bay Aquarium Research Institute's DeepPIV illuminates bubbles with a laser beam to study their movements.  Paul McGill © 2015 MBARI

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