By Dr. Becker
Larvaceans are a type of zooplankton that resemble tadpoles and offer incredible filtering benefits to the ocean waters in which they live. Giant larvaceans, which are sometimes referred to as “sea tadpoles,” live in the upper 400 meters of the water column, building balloon-like “houses” daily. The houses are made of a gelatinous or mucous-like casing and are quite large, reaching diameters of more than 1 meter.
Giant larvaceans are filter-feeders and they use their houses to help them eat. In so doing, researchers revealed the extraordinary service they’re providing to the marine environment. To understand how the process works, it’s first important to know how filter feeders eat. These aquatic animals feed by filtering tiny organisms or particles of organic matter from the water and include animals such as clams, sponges and baleen whales, along with flamingos and many fish.
Each animal has a specialized filtering structure that allows it to feed this way, and giant larvaceans are no exception. Their “filter” is their house, and they use their tales to move seawater through them — that is, until it gets clogged. At that point, they vacate to build a new filtering house, leaving the old one to sink to the ocean floor.
Giant Larvaceans Are the Fastest Known Zooplankton Filter Feeders
A recent study published in Science Advances used new technology to reveal the role of giant larvaceans in oceanic carbon cycling, or the way carbon from the atmosphere is transported from surface waters to the deep. The researchers wrote.1
“The biological pump drives the transport of carbon through the ocean’s depths, and the rates at which carbon is removed and sequestered are often dependent on the grazing abilities of surface and midwater organisms. Some of the most effective and abundant midwater grazers are filter-feeding invertebrates.
Although the impact of smaller, near-surface filter feeders is generally known, efforts to quantify the impact of deeper filter feeders, such as giant larvaceans, have been unsuccessful.”
That is, until now. The researchers developed an instrument called DeepPIV, which was deployed from a remotely operated vehicle in order to measure the filtration rates of the fragile giant larvaceans living near California’s Monterey Bay. “The rates measured for giant larvaceans exceed those of any other zooplankton filter feeder,” according to the results, with researchers noting that the animals can filter all of the water in the bay from 300 to 1,000 feet deep in as little as 13 days. According to The New York Times:2
“In doing so, the creatures help transfer carbon that has been removed from the atmosphere by photosynthesizing organisms to the deep sea, where it can be buried and stored long term. And given their abundance in other parts of the world, these organisms likely play a crucial role in the global carbon cycle.”
Giant Larvaceans May Also Transfer Microplastics to the Seafloor
It’s not only carbon that cycles through the environment courtesy of giant larvaceans and other filter feeders — pollutants may also get cycled through. Plastic pollution is, sadly, now ubiquitous in the world’s oceans. Not only are large items, like plastic bottles and bags, problematic but so too are tiny bits of plastic debris from broken-down plastics or microbeads.
Microplastic particles, defined as those 10 to 600 micrometers (μm) in diameter, are ingested by filter feeders like giant larvaceans, according to another study published in Science Advances.3
The tiny particles were also detected in the animals’ fecal pellets and readily affix to their houses, which then sink to the seafloor beneath. So while plastic pollution is often regarded as a surface problem, it’s becoming increasingly apparent that it’s a deep-sea problem as well, with unknown consequences. While smaller larvacean species may discard and construct new “houses” on an hourly basis, giant larvaceans may do so daily.
This represents a significant accumulation of nutrient- and possibly microplastic-rich material being rapidly deposited on the bottom of the ocean. Writing in Science Advances, researchers explained why this is concerning, including for creatures that prey on larvaceans and their discarded houses:
“Microplastics arguably exert the largest ecosystem-scale pollution due to their small sizes, which allow for ingestion by pelagic organisms ranging from zooplankton, micronekton, and even large predatory fishes. However, the biological impacts on organismal feeding, growth, reproduction, and mortality are poorly known, in part because there is a lack of in situ data on the ingestion of microplastics and subsequent digestive processing.
Nevertheless, there is evidence that the chemical burdens associated with microplastics present ecotoxic hazards to marine animals … Larvaceans have been shown to be the primary prey for many planktonic carnivores and larval fish and could thus serve as an important trophic node for the transfer of microplastics through marine food webs.
In addition, the discarded houses of larvaceans are widely consumed by a diversity of mesopelagic and bathypelagic animals, as well as benthic organisms on the seafloor. The same is likely to be true for larvacean fecal pellets … A number of other highly abundant pelagic filter feeders and suspension feeders … are potential consumers of microplastics and could also transfer plastics through food webs.”
Ultimately, it’s clear that there’s much more to learn about the many creatures living in the ocean and their symbiotic relationship with the environment in which they live. Even the tiniest creatures play an important role, one that’s infinitely larger than their small size would reveal.