Imagine mapping your neighborhood, only to return six months later and find entire populations have vanished without a trace. This isn't science fiction—it's exactly what's happening on the ocean floor in sub-Arctic waters, where researchers have documented one of the most dramatic seasonal disappearances in marine ecosystems.

The Mystery Beneath the Waves

In the cold waters of Newfoundland and Labrador, scientists embarked on an unusual year-long mission: to watch the seafloor change with the seasons. What they discovered challenges everything we thought we knew about mapping ocean habitats.

Using underwater cameras deployed throughout 2020 and 2021, researchers documented something extraordinary. During summer surveys, the seafloor teemed with fuzzy sea cucumbers—nearly three individuals packed into every square meter. These sausage-shaped creatures, about the size of a large cucumber, dominated the landscape alongside brittle stars, sea urchins, and sand dollars.

But when autumn arrived, the sea cucumbers performed a vanishing act that would make Houdini jealous.

Three Million Gone in Weeks

By fall, sea cucumber densities had plummeted by 99.9%. From peak summer populations of nearly three per square meter to less than one in every fifty square meters, millions of these creatures seemingly evaporated from the 23 square kilometer study area in Holyrood Bay.

The disappearance was so complete that entire biological communities reorganized around their absence. Where once "blue assemblages" dominated—ecosystems defined by dense sea cucumber populations—the seafloor transformed into completely different habitats dominated by brittle stars and other species.

"Without the incorporation of spatio-temporal dynamics into community studies, entire species can be missed or misrepresented," the research team noted in their findings.

Where Did They Go?

The million-dollar question remains: where do these sea cucumbers disappear to each fall?

Scientists suspect these mobile filter feeders burrow beneath the sediment to wait out the harsh winter months, similar to behavior observed in Antarctic sea cucumber species. Unlike fish that can swim away or crabs that can walk to new areas, sea cucumbers face limited options when conditions deteriorate.

The timing offers clues. In this region, phytoplankton blooms peak in April, showering the seafloor with organic matter that suspension feeders like sea cucumbers depend on for food. When this seasonal feast ends and water temperatures drop, the cucumbers likely retreat underground until spring returns.

Intriguingly, subsea observatory footage that captures five minutes of video every hour shows no evidence of sea cucumber movement across the seafloor surface, supporting the hypothesis that they burrow rather than migrate horizontally.

A Seafloor in Flux

The sea cucumber disappearance was just the most dramatic change observed. The entire ecosystem underwent seasonal transformations that rivaled changing landscapes on land.

Northern sea urchins shifted their depth distribution, moving from deeper waters in summer to shallower zones in fall and winter—possibly following food sources or seeking breeding grounds. Common sand dollars, those familiar flat creatures beachcombers love to find, dropped to near-zero densities in fall and winter from their summer abundance.

The research team identified eight distinct biological assemblages across the seasons, ranging from sea star and sand dollar communities in shallow waters to decapod-dominated deeper zones. Some assemblages appeared in certain seasons and vanished in others. Winter presented the starkest landscape, with just two main community types compared to five in spring and summer.

Total organism density dropped dramatically from over five animals per square meter in summer to less than one in winter—a ghost town compared to the bustling summer seafloor.

The Technology Behind the Discovery

The study employed sophisticated acoustic mapping combined with underwater video surveys across four seasons. Multibeam sonar created detailed maps of seafloor depth and hardness, while underwater cameras drifted above the bottom for two minutes at each of 78 carefully selected sites, repeated each season.

Scientists used machine learning algorithms to connect what they saw in the videos with environmental variables like temperature, salinity, depth, slope, and seafloor texture. These models achieved 75 to 94% accuracy in predicting which communities would occur where—but only when built separately for each season.

Remarkably, even though physical features like slope direction never changed, their importance in predicting biological communities varied wildly by season. This suggests that the same physical habitat can host radically different species depending on the time of year.

Why This Matters

The findings carry profound implications for ocean management and conservation.

Currently, most seafloor habitat maps are created from single survey snapshots, typically conducted during summer when weather permits easier data collection. These maps form the basis for marine protected area design, fisheries management, and environmental impact assessments.

But if millions of organisms appear and disappear seasonally, those single-time-point maps may be fundamentally misleading.

"Without accurate representation, entire species can be missed or misrepresented since the same physical habitat can host different species depending on the season," the researchers emphasized.

Consider the consequences: a summer survey might identify a location as critical sea cucumber habitat requiring protection, while a winter survey of the same spot would find virtually none. Which represents reality? The answer is both—and that's precisely the problem with temporal snapshots of dynamic ecosystems.

Beyond the Sub-Arctic

While this study focused on cold Canadian waters, seasonal changes affect virtually all marine environments globally. Mediterranean ecosystems, tropical reefs, and temperate coastlines all experience seasonal shifts in temperature, food availability, and species composition.

The researchers found biological assemblages similar to those documented in other Newfoundland studies, suggesting their findings may represent broader regional patterns. Communities dominated by brittle stars, sea urchins, and sand dollars appear across multiple bays in the area.

What remains unknown is whether similar seasonal disappearances occur elsewhere. The fuzzy sea cucumber phenomenon may be unique to sub-Arctic regions, or it could be happening worldwide without documentation because seasonal surveys are so rare.

The Climate Connection

Understanding natural seasonal fluctuations takes on added urgency as climate change accelerates.

Marine ecosystems worldwide are experiencing warming waters, shifting currents, and altered productivity patterns. Distinguishing between natural seasonal variation and climate-driven changes becomes impossible without baseline knowledge of normal seasonal cycles.

The researchers note that "capturing the natural fluctuations of sub-Arctic coastal ecosystems will be crucial in the early detection of perturbations caused by climate change."

If we don't know how communities naturally shift through the year, we can't recognize when those patterns break down—the early warning signs of ecosystem stress.

Rethinking Ocean Mapping

The study calls for fundamental changes in how we approach marine habitat mapping.

Rather than single surveys, scientists recommend seasonal ground-truthing to capture natural variability. When repeated surveys aren't feasible due to cost or harsh conditions, maps should clearly state they represent only the specific season surveyed—not year-round conditions.

One promising approach involves identifying representative proxy sites where intensive seasonal monitoring could inform understanding of broader regional patterns.

The researchers also suggest exploring other temporal scales beyond seasons. Do these patterns repeat consistently year after year, or do they vary? How do rare events like particularly warm or cold years affect community dynamics?

Looking Forward

The underwater cameras revealed glimpses of unexpected diversity beyond the main study species. Observations included snow crabs—an important commercial species—though at low densities, along with occasional Atlantic plaice aggregations and even anthropogenic trash scattered across the seafloor (an average of 14 items per season ranging from glass bottles to tires).

Over the course of the year, researchers documented 61 different morphological types of organisms, with species richness peaking in spring following the phytoplankton bloom.

The question now is whether similar seasonal dramas play out across other ocean habitats. Do coral reef fish communities reorganize seasonally? Do deep sea assemblages shift with changing currents? We simply don't know because we haven't looked.

This study opens a window onto the dynamic nature of seafloor ecosystems that challenges our static view of the ocean. The benthos—the communities living on and in the seafloor—may be far more dynamic than previously imagined.

As one season fades into the next, millions of creatures appear and disappear in patterns we're only beginning to understand. The ocean floor, it turns out, is less like a stable landscape and more like a stage where the cast changes with each act of the seasonal drama.

The fuzzy sea cucumbers will return when spring arrives and food rains down from the productive surface waters above. But where they spend their winter months—buried in sediment, waiting in some undiscovered refuge, or perhaps engaged in behaviors we haven't yet observed—remains one of the ocean's many mysteries.

Publication Details

Published: 2025
Journal: Marine Ecology Progress Series
Publisher: Inter-Research
DOI: https://doi.org/10.3354/meps14803

Credit and Disclaimer

This article is based on original research published in Marine Ecology Progress Series by researchers from the Fisheries and Marine Institute of Memorial University of Newfoundland and Labrador and Fisheries and Oceans Canada. The content has been adapted for a general audience while maintaining scientific accuracy. For complete technical details, comprehensive data, full methodology, and in depth analysis, readers are strongly encouraged to access the original peer reviewed research article through the DOI link provided above. The researchers have made their video transect data and associated metadata publicly available through the Federated Research Data Repository (https://doi.org/10.20383/103.0964). All factual information, data interpretations, and scientific conclusions presented here are derived from the original publication, and full credit goes to the research team and their institutions.