Gambierdiscus australes Confirmed on Spain’s Mediterranean Mainland Coast

A toxin-producing tropical microalga has been verified for the first time on Spain’s Mediterranean mainland coastline.

Its presence broadens the known distribution of a group associated with seafood poisoning and changes the areas that authorities need to monitor.

Discovery of Gambierdiscus australes

Routine seawater samples taken off Dénia and Jávea, on Spain’s east coast, revealed the tropical species in waters where it had not previously been reported.

After analysing the samples, César Bordehore of the University of Alicante identified cells consistent with Gambierdiscus australes, a tropical microalga capable of producing fish-poisoning toxins. The finding establishes the genus on the western Mediterranean’s continental coast.

Stored samples from the same section of coast contained no evidence of the organism a little more than a decade earlier, suggesting it has only recently expanded into these waters.

This development introduces a toxin-producing organism that was previously absent into an active fishing area, prompting closer examination of its behaviour in this new environment.

How toxins pass through fish

The illness is known medically as ciguatera, which results from eating fish containing particular hazardous marine toxins.

When grazing animals feed on algae growing on rocks and seaweed, toxins accumulate progressively as they move into larger fish.

In humans, poisoning may cause vomiting, diarrhoea, numbness and confusion over hot and cold sensations. Cooking does not destroy these compounds.

Because the toxins have neither a detectable taste nor smell, prevention relies on monitoring potentially risky fish before they enter shops.

What the sampling revealed

Two surveys, conducted in March and September 2023, covered 12 locations between approximately 250 metres and 1 kilometre from shore.

At every site, technicians mixed surface water with water collected at a depth of 5 metres before counting cells in the laboratory.

The microalga appeared in 75% of spring samples and in 100% of September samples, at concentrations of 20 to 140 cells per litre.

The higher summer figures suggested that warmer conditions encouraged cell multiplication, although overall concentrations remained low.

Warmer waters support expansion

Summer temperatures across the western Mediterranean now reach levels once typical of seas farther south.

Warmer conditions accelerate cell growth and may extend the period during which Gambierdiscus australes can persist on coastal rocks and plants.

A long-term indicator records warming in Mediterranean waters over recent decades, while temperatures in the cooler months were around 15 to 16°C.

Water temperatures of nearly 27°C in late summer may make survival along the coast easier, increasing the length of shoreline that requires monitoring.

A species living on the seabed

Unlike algae responsible for surface blooms, Gambierdiscus australes is benthic, meaning it lives on the seabed or attached to plants.

Rather than floating through open waters, its cells adhere to seagrass blades and rocks, where grazing animals feed.

Powerful waves and currents may dislodge cells, which means standard water samples can still detect them even when no visible bloom is present.

This unobtrusive way of living complicates early detection and makes sampling around reefs, harbours and seagrass particularly valuable.

Maintaining robust seafood checks

Seafood safety depends on consistent procedures, and the discovery led local authorities to reassess their monitoring arrangements.

If laboratories identify dangerous toxin levels, inspectors can withhold catches, while prompt notifications to authorities help direct the response.

“Our job is to inform the relevant authorities, as we have already done, and take the necessary measures at all times to maintain food safety,” said Bordehore.

He said that current screening programmes prevent fish with elevated toxin levels from reaching the market, meaning seafood presently on sale remains safe for consumers.

Previous Mediterranean indications

Before the Alicante finding, scientists had already recorded the same algal group in other parts of the Mediterranean, commonly near protected islands and coves.

Further from the coast, it can be carried by currents or transported on ship hulls before becoming established in new coastal habitats.

Researchers at Spain’s Institute of Agrifood Research and Technology had previously recorded Gambierdiscus in the Balearic Islands, providing an earlier Mediterranean presence for the genus.

Every additional record enlarges the region in which fish may acquire toxins and challenges monitoring systems to keep up.

Examining archived samples

Freezers and archive boxes are important to marine research, as historic seawater samples can show when a species first appeared.

By re-examining preserved material, University of Alicante scientists were able to compare the modern coastline with earlier periods and identify genuine changes.

In 2011, an assessment of 30 local samples found no sign of Gambierdiscus, indicating that it arrived later.

Such a timeline enables agencies to organise focused surveillance rather than waiting for illness in people or fish deaths to draw attention to the issue.

Monitoring Gambierdiscus australes

Long-term surveillance now has an additional target, as this microalga can remain inconspicuous while still introducing toxins into the food chain.

Seasonal surveys can examine seabed habitats and water samples together, while University of Alicante teams can compare the findings with toxin testing in local fish.

Early warnings are especially important when concentrations begin to increase, as authorities can then revise fishing advice and laboratory screening.

Without regular monitoring, an algal group such as Gambierdiscus australes could spread for years before an outbreak reveals its presence.

The detection of a tropical toxin producer on Spain’s mainland coast demonstrates how rapidly small organisms can alter seafood safety maps.

Regular sampling, transparent communication with agencies and thorough testing should help keep risks low as Mediterranean waters continue to warm.