An estuary stingray (Hemitrygon fluviorum) shifting sand as it sifts through it for morsels of food.

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The tide is low in Wallis Lake, and the water pulls back just enough to expose the flurry of activity beneath. Small crabs scuttle sideways as snails leave thin trails in the sediment. Nearby, a stingray lifts a flap of sand and disappears again; if you didn’t know any better, you might assume it is hunting something obvious. One of those crabs that hid away, perhaps, or a buried shellfish. Something you can see and name. But what if most of what fuels this ray cannot be seen at all?

Estuaries are often described as nurseries of the sea, but they are also some of the most heavily altered ecosystems on the planet thanks to coastal development, pollution and aquaculture reshaping them in ways that are not always immediately visible. When habitats shift so do, inevitably, food webs. And when food webs change, animals can suddenly find themselves at risk. You can see this play out at Wallis Lake, which flows into the sea between the towns of Forster and Tuncurry in New South Wales, Australia. Wallis Lake estuary supports extensive seagrass meadows and is the northern most stronghold of the seagrass species (Posidonia australis) commonly referred to as strapweed, in New South Wales. And it’s here that researchers focused on two species, the estuary stingray (Hemitrygon fluviorum) and the common stingaree (Trygonoptera testacea). Both are benthic feeders (meaning they forage along or within the sediment) and are considered ecosystem engineers, constantly disturbing the seafloor as they hunt, which in turn reshapes habitats for other organisms. Yet despite their ecological importance, a basic question remained unclear. What are they actually eating, and what ultimately supports their nutrition?

To answer that, the researchers turned to stable isotope analysis, a method that traces the chemical signatures of food through an animal’s body. Instead of watching what a ray eats in one exact moment, this approach reveals what it has been assimilating over time. And the results were anything but expected. While one might assume they would eat something like oysters, which are abundant in many estuaries, they actually contributed very little to the diets of either species. For the estuary stingray, oysters made up only about 5 percent of its assimilated diet while for the common stingaree, it was around 8 percent. Not insignificant numbers, but they are far from dominant. Instead, the estuary stingray relied more heavily on benthic fish and crustaceans. The common stingaree showed a strong connection to small gastropods, particularly Nassarius snails (better known as Zombie snails). Makes sense when you consider how these animals forage, sifting through sediment and targeting buried prey.

But the most surprising find was that both species derived nearly half of their nutrition from particulate organic matter! This is the fine, often invisible mix of decomposing material, microorganisms and detritus that drifts through the water or settles into sediments. Not something you would easily point to as “food,” but it forms the backbone of many aquatic food webs. Particulate organic matter is shaped by everything happening in an estuary. Changes in vegetation, runoff from land, nutrient inputs from human activity and even aquaculture operations can all influence its composition. If these inputs shift, the base of the food web shifts with them. So what does it mean for an animal to depend so heavily on something so diffuse and easily altered? For the rays, that could have cascading effects. Even if their immediate prey (like crustaceans or snails) stay, their nutritional quality could change if the underlying energy source is altered. We may be underestimating how sensitive these animals are to environmental change by focusing only on what they eat, rather than on the energy pathways that sustain their prey. Looking at diet in terms of visible prey items offers only part of the picture since those prey species are themselves supported by underlying sources.

The study also looked at how these rays fit into the broader food web by comparing them to a common estuarine fish, the yellowfin bream (Acanthopagrus australis). Using isotopic niche modeling, the researchers found a substantial overlap, with more than 70 percent of the estuary stingray’s niche overlapping with that of the bream. However, the reverse was not as strong showcasing that the bream occupied a broader niche overall (i.e., more flexibility in what it can eat and where it can find food). This means that the estuary stingray appears to rely on a more limited range of resources. If the environment changes, a species with a broad niche may adapt… but a species with a narrow niche may struggle. The estuary stingray, according to this study, falls into the latter category. That does not mean it is doomed, but it does suggest a potential vulnerability that has not been fully appreciated.

It is easy to track visible impacts like habitat loss or species abundance. It’s a little bit harder to see shifts in nutrient pathways or the composition of particulate organic matter. But these invisible changes can ripple upward, affecting animals like stingrays in ways that can be significant over time. The authors note that future work should include methods like stomach content analysis and assessments of prey availability to get a better idea of what we are looking at.

In the end, yes, the animals we see are important for an ecosystem, but we cannot forget the microscopic and detrital world that supports everything beneath the surface. This study forces us to ask one very hard question: If the foundation of that system shifts, would we even notice in time to respond? With how conservation is currently set up, I don’t think so.