What is a clinging jellyfish?

The clinging jellyfish (Gonionemus vertens) is a small, nearly transparent jellyfish about the size of a dime, with more than 60 tentacles. It carries a neurotoxin, and its sting can be more severe than a common sea-nettle sting. It lives in shallow back bays and estuaries, clinging to eelgrass and submerged vegetation, not in the open surf.

Where in New Jersey are clinging jellyfish found?

Monitoring has recorded them most often in back-bay waters such as the Metedeconk River, the Shrewsbury and Navesink Rivers, the Cape May and Wildwood back bays, and parts of Barnegat Bay. They are a late-spring and early-summer animal in shallow, vegetated water up to about four feet deep.

Does this monitoring data tell me a beach is safe?

No. These are point-in-time sample observations, not a live hazard map, and many samples were logged without a count. The absence of a record does not mean clinging jellyfish are absent. Use the data as background and stay cautious around eelgrass in shallow back-bay water in early summer.

Clinging Jellyfish in Shrewsbury and Navesink Rivers

Recurring detectionsMonmouth County

Monitoring summary, current as of the 2026 season. This is a record of past samples, not a live conditions report.

Clinging jellyfish keep turning up in the Shrewsbury and Navesink Rivers. Of the samples here where a count was written down, 20 held the species, and the largest came to 100.

By the numbers

samples with clinging jellyfish

sampled, none found

111

sampled, no count recorded

150

total samples (2016–2024)

Largest single count: 100 · most recent detection: 2023

The counted samples split into 20 with the species and 19 without; 111 others were logged without a count and are set aside.

Year by year

'16

'17

'18

'19

'21

'23

'24

jellyfish counted sampled, none found no count recordedSampling effort varies year to year, so bar height reflects how much crews sampled, not risk.

The recorded detections have thinned out in the more recent samples. Sampling effort varies year to year, though, so this is a note about the record, not a verified decline.

When to be careful

Clinging jellyfish are largely a late-spring and early-summer animal here, showing up as back-bay water warms. The biggest single count in the Shrewsbury and Navesink Rivers, 100, was recorded on June 8, 2017, which lands squarely in that window.

Because clinging jellyfish show up here repeatedly, it is sensible to be careful in the shallows and around eelgrass during late spring and early summer. The species sticks to back-bay vegetation, not the ocean surf.

Where these samples were taken

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Scroll to load jellyfish monitoring map

What the clinging jellyfish is, and what to do if you are stung

The animal

About the size of a dime — roughly 1 inch across — and nearly transparent, so it is easy to miss.
Has more than 60 tentacles and carries a neurotoxin; stings can be painful and, for some people, cause symptoms beyond the skin.
Lives in shallow estuaries and back bays, typically in water up to about 4 feet deep, where it clings to eelgrass and submerged vegetation. It is not an open-surf animal.
This is not the common sea nettle; it is much smaller and its sting can hit harder.
In New Jersey it is most often encountered in late spring and early summer, when back-bay water warms toward the low 80s°F.

If you are stung

If you are stung, get out of the water and seek medical care. Reactions to a clinging jellyfish sting can be more severe than a common sea-nettle sting.

For treatment guidance call the NJ Poison Control Center at 1-800-222-1222, or seek emergency care for severe symptoms.

NJ Poison Control: 1-800-222-1222

Towns nearby

Communities on or near the Shrewsbury and Navesink Rivers include Sea Bright, Monmouth Beach, Rumson, and Oceanport, and each one has a full guide on the site.

How to read this page

A note on how to read this page. Each figure comes from individual sample observations collected by Montclair State University and the NJ Department of Environmental Protection between 2016 and 2024. A count reflects what a crew recorded at one spot on one day; many samples were logged without a count, and those are left out of the tallies rather than treated as zero. Areas are grouped from point samples, so boundaries are approximate. Most importantly, the absence of a record is not proof the species is absent. Data current as of the 2026 season.

Source: NJDEP Bureau of GIS / Montclair State University — Clinging Jellyfish Monitoring. NJDEP data layer.

Frequently asked

Have clinging jellyfish been found in the Shrewsbury and Navesink Rivers?: Yes. Clinging jellyfish have been recorded in the Shrewsbury and Navesink Rivers — 20 samples since 2016, most recently in 2023.
Where in the water are clinging jellyfish a concern?: They live in shallow estuaries and back bays, typically in water up to about four feet deep, where they cling to eelgrass and submerged vegetation. They are not found out in the ocean surf.

Other monitored waters

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