Jellyfish

Overview

Jellyfish are a natural part of marine ecosystems, but in recent decades, scientists have observed a global increase in large-scale jellyfish blooms. These blooms are often linked to human-induced pressures such as overfishing, rising sea temperatures, and habitat degradation. In some cases, ecosystems can reach a tipping point where they shift from being dominated by fish to being dominated by jellyfish, making recovery extremely difficult.

The increase in jellyfish populations is closely connected to the decline of fish species. Many fish, such as sardines and anchovies, feed on jellyfish or compete with them for food. When these fish populations are reduced, particularly due to overfishing, jellyfish face fewer natural controls and can rapidly expand. At the same time, jellyfish prey on fish eggs and larvae, further preventing fish populations from recovering.

Destructive fishing practices, including bottom trawling, can also contribute to jellyfish blooms by damaging seabed habitats and removing key species from the ecosystem. In addition, areas of low oxygen create conditions in which jellyfish can thrive while other marine life struggles to survive.

As jellyfish populations increase, they can disrupt fisheries, alter food webs, and reduce overall biodiversity. Addressing the root causes of jellyfish blooms, particularly overfishing, habitat destruction, and climate change, is essential to restoring balance in marine ecosystems and protecting the health of our seas.

Partners

Archipelagos is a partner of the Mediterranean Science Commission (CIESM) JELLYWATCH program, which gathers baseline data on the frequency and extent of jellyfish outbreaks across the Mediterranean Sea for the first time.

The threat of jellyfish blooms

Oceans around the world are being exposed to devastating destruction due to anthropogenic threats. Ecosystems commonly struggle to find stability, faced with overfishing, increasing sea temperatures and more. As key species struggle to cope with these factors, biodiversity finds itself susceptible to more primitive species groups such as jellyfish.

 

There are many trophic levels by which, when threatened, provide space for jellyfish blooms to infiltrate. Jellyfish thrive when oceans experience the following:

  • A collapse of small fish that live closer to the sea surface, such as sardines or anchovies. These are species that feed on jellyfish and when their populations decrease, predation pressures on jellyfish plummet. Impacts are later amplified as jellyfish numbers begin to increase, remaining individuals of fish populations are often out competed for resources of zooplankton.
  • Overfishing. As fish stocks are depleted due to human action, jellyfish populations increase. This initial action perpetuates the permanent removal of commercial fish from our oceans. Jellyfish prey upon the eggs and larvae of fish species, making it impossible for populations to not fall inferior to the already increasing number of jellyfish. 
  • Destructive fishing practices. Trawling and other methods of fishing that damage the seabed and all of its inhabitants, create optimal conditions for jellyfish. By removing all predators while simultaneously leaving behind a rocky damaged habitat, jellyfish are able to outcompete all remaining life.

Species

Common name

Mauve Stinger

Scientific name

Pelagia nocitiluca

CLASS

Scyphozoa

Family

Pelagiidae

Life span

2-6 months

SIZE

6-10cm diameter, 10m tentacle length

Population trend

Mauve Stinger populations have been observed to be increasing in the Mediterranean. Since 1990, persistent blooms have occured in the western Mediterranean quasi-annually.

Habitat

Pelagic zone as well as benthic adn temperate coastal habitats.

Diet

Zooplankton, small fish, crustaceans, other jellyfish and fish larvae.

Status (IUCN)

Least Concern (LC)

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Common name

Moon Jelly

Scientific name

Aurelia aurita

CLASS

Scyphozoa

Family

Ulmaridae

Life span

8-12 months

SIZE

40-50cm diameter; 10-12.5 length

Population trend

Moon Jelly populations are observed showing an increasing frequency of blooms.

Habitat

Inshore, existing in coastal waters, estuaries and bays.

Diet

Plankton, small fish and larvae.

Status (IUCN)

Deficient Data (DD)

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Common name

Portuguese Man O’ War

Scientific name

Physalia physlis

CLASS

Hydrozoa

Family

Physaliidae

Life span

~1 year

SIZE

2m; 50m tenticle length

Population trend

Habitat

Ocean surface in tropical and sub-tropical areas.

Diet

Young fish and small adult fish, as well as less frequently, cetaceans.

Status (IUCN)

Least Concern (LC)

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Common name

Flower Hat Jelly

Scientific name

Olindias formosa

CLASS

Hydrozoa

Family

Olindiidae

Life span

4-6 months

SIZE

12-15cm diameter

Population trend

Habitat

Semi-benthic coastal waters, living around 33-35m below surface waters.

Diet

Zooplankton and diatoms.

Status (IUCN)

Least Concern (LC)

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Common name

Mediterranean Jelly

Scientific name

Cotylorhiza tuberculata

CLASS

Scyphozoa

Family

Cepheidae

Life span

6-12 months

SIZE

40cm diameter; 5-10cm arm length

Population trend

Mediterranean Jelly populations remain steady; however, are potentially threatened by increasing acidification and sea temperatures.

Habitat

Coastal waters between depths of 0-7m.

Diet

Plankton

Status (IUCN)

Deficient Data (DD)

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Common name

Compass Jellyfish

Scientific name

Chrysaora hysoscella

CLASS

Scyphozoa

Family

Pelagiidae

Life span

>1 year

SIZE

30m diameter

Population trend

Habitat

Coastal waters between depths of 0-30m.

Diet

Small fish, crustaceans and other jellyfish.

Status (IUCN)

Deficient Data (DD)

All Media © Archipelagos Institute of Marine Conservation