A couple of weeks ago, New York Times’s science writer Carl Zimmer wrote about a recent review by Hays et al (2018), which asked: how important are jellyfish in marine ecosystems?
The answer: important. In fact, probably more important than we have been giving them credit for.
Jellyfish (often referring to both cnidarians and ctenophores) have typically been considered trophic dead ends, meaning they are usually not eaten by anything and thus effectively act as a “dead end” to the movement of energy in a food web. All of the carbon from what jellies eat and use to build their tissues returned to the ecosystem only after jellyfish die and are biochemically recycled (i.e. decomposed) by microbes.
Only a few vertebrate animals were known to be jellyfish specialists, including the massive sunfish (Mola mola) and the endangered leatherback turtle (Dermochelys coriacea; Heaslip et al 2012). The massive size of sunfish and leatherback sea turtles suggested that jellyfish-specific consumers necessitated the need for a large “belly full of jelly”, meaning a jellyfish-specific predator must maintain a large body size (and therefore a large stomach) to both consume enough jellyfish to be nutritionally ‘worth it’ and also protect the consumer from its own prey. There is an assumption that not all jellyfish sting people, but in reality humans are too large for most jellyfish to sting us, so a 2,000 lb turtle or 5,000 lb fish should be well protected.
Recent work on deep-sea octopuses (Hoving and Haddock 2017), sea birds such as albatross (McInnes et al 2017), penguins (Theiebot et al 2017), European eel larva (Ayala et al 2018), and deep-sea food webs (Choy et al 2018) show a wider diversity of animals may act as opportunistic jellyfish consumers. Current tools to observe jellyfish consumption are relatively modern, including long-term video documentation from cameras attached to marine animals as well as isotope analysis and DNA barcoding to identify stomach contents. And even though all of these papers mention that jellyfish have been poorly studied as major contributors, all suggest that may not be the right perspective.
Video from Monterey Bay Aquarium Research Institute (MBARI): video editing: Susan von Thun, Kyra Schlining, script and narration: Anela Choy, music: Imua Garza (www.imuagarza.com), production support: Steve Haddock, Bruce Robison, Lonny Lundsten, Linda Kuhnz
Video from OluwaPlenty A.
One likely reason for jellyfish’s crucial role in pelagic foods webs (meaning in the water column) lies with their diversity and quantity. Within cnidarians, medusozoans (species that possess a medusa stage) are an incredibly diverse group found all across the globe. Many live as benthic (on the ocean surface), asexual polyps as well as pelagic medusae, comfortable living off large prey such as fish or smaller zooplankton either as a solitary animal or in a colony. Jellies can be found in every kind of marine environment across the globe, as well as some estuarine and freshwater environments, and thrive in a variety of temperatures, salinities, and depths. When they bloom, medusae persist in incredibly high numbers in relatively small spaces.
Jellies also make up a substantial proportion of biomass in the ocean. In a recent census of the world’s biomass and how it is distributed, cnidarians make up about 0.1 gigatons of carbon (Gt C). That doesn’t sound like a lot, but consider that humans only make up 0.06 Gt C! It’s no surprise that these animals have the potential to play a critical role in our oceans, but their gelatinous composition and boom-bust cycles have made them incredibly hard to study in both open waters and the guts of other animals.
In fact, jellies have been shown to be more easily digestible in comparison to other prey items such as shrimp (Arai et al 2003), which means jellyfish tissue would not be found in the gut contents of their predators unless they had recently been consumed them. The utility more modern techniques circumvent this issue by looking identification in even digested leftovers, certain chemical isotope traces and DNA. It is important to understand if jellies are even opportunistically consumed, because quick digestion may mean jellies are an easy way to get quick energy for little effort (providing you don’t get stung). Jellies do not provide many calories but perhaps offer some other nutritional advantage we do not know of, which would be important for understanding ecological relationships. There are also current concerns about the bioaccumulation of microplastic readily ingested by medusae and thus transferred upstream to their predators.
If jellies are so abundant and easy to digest, why are leatherback turtles and sunfish some of the only vertebrate jellyfish specialists? Aside from ‘hypodermic needle-like, venom-containing cells covering their bodies’, Hays et al (2018) suggests that low metabolism might be key. Because jellyfish come in highly concentrated blooms and then remain absent for long periods of time from the water column, jelly-exclusive predators must maintain a feast-and-fast strategy to survive, which means a lower metabolism. Feed intensely when there is plenty to go around and then wait for the next bloom, be that weeks or months later.
It has been suggested that high consumption predators could be used to estimate jellyfish populations. In an interview on Science Friday last year, Dr. David Grémillet talked about modeling jellyfish consumption by sunfish in Western Europe, which totaled up to 20,000 metric tons of jellies daily (Grémillet et al 2017). His argument: sunfish are a lot easier to spot than jellyfish. And he has a good point, there are still many issues with tracking jellyfish medusae. And what about the polyps? Even with well-studied species in relatively well-understood and well-mapped environments, the wonderfully titled “Where are the polyps?” (van Walraven et al 2016) says it all. We do not have a good idea where these animals are, how many there are, or good predications about when they might bloom.
While Hays et al (2018) rightly suggests more work needs to be done to establish the nutritional contributions of jellyfish from an organismal and ecological perspective, we still need a firmer understanding of jellyfish ecology overall. Jellyfish blooms are receiving more attention, especially now that several scientists have raised the alarm to the potential “jellyfish joyride” (Richardson et al 2009) and “rise of slime” (Grémillet et al 2017), but our understanding of jellyfish ecology over their entire life cycle is still poor. How can we determine the impact of disruptive jellyfish blooms if we can’t establish their importance to the local ecosystem as a food source? How can we make estimates of their population size if we cannot determine the location of polyps? How do we know what polyps are eating, or what is eating them.
While research continues to determine what is eating jellies, maybe give jellyfish a try. You know, see if this belly full of jelly thing seems like a good idea.
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