Water's Least Wanted

Liz Thompson   Aug 16, 2019


Most Scuba Divers find the underwater world fascinating – and in fact the desire to see so many different species of fish, corals and other invertebrates as well as other sea life is often ‘why we dive’. While it’s fun to see living things beneath the surface - sometimes you find species that might not belong. In this blog post we will discuss four invasive species living in borrowed homes – and not at all welcome!

 Lionfish (pterois volitans)

 First spotted off the coast of Florida in 1985, the lionfish quickly rose to become one of the best-known invasive   species by divers in the Western hemisphere. It is theorized that this Indo-pacific native came to be found   throughout the waters of the Eastern United States, Bahamas and the Caribbean by either transportation in ships’   ballast tanks or through the illegal release by aquarium owners. One theory supports the release of these animals   comes from 1992 when the natural disaster of Hurricane Andrew  smashed an aquarium tank in Florida. The theory holds that about a half-dozen spiny, venomous lionfish washed into the Atlantic Ocean, contributing to the invasion.


Lionfish are prolific in part because of the rate at which they reproduce. Did you know that a single female lionfish is able to release around two million eggs per year, or roughly 17,000 eggs every three days? They can reproduce year-round and juveniles reach maturity after only one year. Another reason that the spread of these fish has happened so rapidly in this area of the world is due in part to the fact that until recently they have had no known predators, something that can be attributed to the 18 poisonous spines that can be found around their body. Able to consume over 100 different species of fish, the only thing that is currently helping to keep this invader in check in the fact that humans are now hunting them continuously, which helps to cull their population. It is also helpful that Lionfish are a VERY tasty – next time you’re in the Islands – or at your local fish monger, ask about Lionfish for a change of pace.

Killer Algae (Caulerpa taxiflia)

This tropical water native is supposed to be found in waters such as the Caribbean, Gulf of Guinea, and the Red Sea, however because of ships’ ballast dumps it is now found in the Mediterranean Sea as well. This problematic invasive species can survive up to 10 days outside of water, as long as it is in a moist environment. The algae smothers other algae species, seagrasses, and sessile, or immobile, invertebrate communities. Able to withstand low temperatures, Killer Algae can greatly reduce species diversity in an area, and when eaten by fish it contaminates them with toxins that are unsafe for human consumption.


There is some hope for removing this invasive species from waters it doesn’t belong in. At one-point, Killer algae was found off the coast of California- a state that then spent six years and $7 million to eradicate it. The result?  No more Killer Algae can be found in those waters.

Zebra Mussel (Dreissena Polymorpha)

Native to the Black, Caspian, and Azov seas, this small stripped shellfish has made its way into 32 of the 50 United States with freshwater sightings dating as early as 1988.  It is believed that this animal entered North America through the release of ballast water from shipping vessels traveling from Europe. Able to lay more than 40,000 eggs in a reproductive cycle and over 1 million in a spawning cycle, Zebra Mussels can be found attached to objects, surfaces, and even other mussels- whom they harm as they can interfere with feeding, growth, movement, respiration, and reproduction. The economic impact of zebra mussels can be extreme, due to their need to attach to hard surfaces and the ability to layer upon themselves. They can cause damage to industrial objects dues to their attaching to those items They can also damage, cause costly repairs, and require frequent maintenance to boat and recreational motors.  Even when they die, their sharp shells wash up on beaches; creating foul odors and cutting the feet of swimmers, which can harm local recreation.



Crown of Thorns (Acanthaster planci)

Another Indo-pacific native who found itself quite happy far from home is the Crown of Thorns. This species is believed to have migrated naturally, via larval traveling on currents and adult migration, to the areas it now invades- The Red Sea, the Great Barrier Reef, Hawai’i, and Palau to name a few. The Crown of Thorns is devastating to the areas it is now found due to the fact that it feeds on corals that have already been decimated by other factors, further increasing the decay of the coral colony.  Did you know that they feed by extruding their stomach out of their bodies and onto the coral reef, and then using enzymes to digest the coral polyps? Luckily, predators of crown-of-thorns starfish (mostly of small/young starfish) include the giant triton snail, humphead Maori wrasse, starry pufferfish and titan trigger fish.