A record number of manatees have been killed in Florida waters this year following a bloom of harmful algae, known popularly as a 'red tide.' According to state wildlife officials, the bloom – in Gulf of Mexico waters off southwest Florida – has killed 174 manatees through Monday, surpassing the previous record of 151 manatee death-by-red-tide cases in a calendar year, in 1996.

The red tide is caused by a species of phytoplankton called Karenia brevis, which produces a suite of toxins known as brevetoxins. In humans, brevetoxins are responsible for a syndrome known as neurotoxic shellfish poisoning or NSP. Symptoms of NSP, which follows the consumption of shellfish that have ingested the toxic algae, include muscle pain, lack of coordination, abdominal pain, nausea, diarrhea and headaches, and a strange phenomenon in which cold objects feel hot and vice-versa. Death is rare, and recovery normally takes place over two to three days.

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For marine animals, however, the consequences can be more severe, not least because of the environment in which they live.

“They're basically paralyzed, and they're comatose," Virginia Edmonds, animal care manager of Florida mammals for the Lowry Park Zoo in Tampa, told the Tampa Bay Times. “They could drown in 2 inches of water."

Signs that a manatee has come into contact with the red tide include a lack of coordination and stability in the water, muscle twitches, seizures, and difficulty lifting its head to breathe.

Manatees are especially susceptible, because they congregate in the warm offshore waters where the blooms occur, and because fragments of Karenia brevis become attached to the seagrass that the animals eat, which means they are likely to be exposed for some time after the bloom dies down.

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According to NOAA, brevetoxin has been implicated in numerous previous manatee mass mortalities off Florida, including in 1963, 1982, 1996, 2002, and 2003; it was also fingered as the likely culprit in the deaths of more than 740 bottlenose dolphins along the US east coast in 1987-88.

Karenia brevis is not the only species of algae to cause red tide blooms; not all are harmful, and those that are may affect wildlife and humans in different ways. (Despite the name, not all of them turn the water red, hence the preferred scientific term of Harmful Algal Blooms, or HABs.) In general terms, HABs are on the rise worldwide, with a variety of factors contributing in different circumstances: the transportation of algal species to 'new' environments via ships' ballast water; increased nutrient pollution in coastal waters, which stimulates phytoplankton growth; and increasing sea surface temperatures, among others.

Blooms of Karenia brevis have been recorded off Florida's Gulf coast since the 1840s, and have been associated with fish kills as well as manatee mortalities and NSP. Because they generally occur 10-40 miles offshore, they are far less likely to be stimulated by nutrient pollution than some other phytoplankton blooms, although once they are carried inshore, increased nutrient loads may enable them to persist and grow.

Florida has an estimated 4,000 to 5,000 manatees. The most common cause of mortality is collisions with boats, which killed almost 800 manatees from 1995 to 2005.

IMAGE: Florida manatee. (Joe Raedle/Getty Images)