BY ANN HAIGH

MARINE pleustonic animals, by definition, are those that live on the ocean surface due to their own buoyancy.

They are usually pelagic, meaning that they float out of our sight on the surface of the open sea away from the coastline. However, most animals in this category are not capable of independent movement and are at the mercy of the wind and ocean currents and, therefore, sometimes end up stranded on our shorelines. It is a wonderful thing that we occasionally get the opportunity to see these creatures of the deep when, due to storms or wind direction, they arrive on our shores in West Cork.

Portuguese man o’ war

Over the past few weeks, we have seen the arrival of fascinating creatures such as the Portuguese man o’ war (Physalia physalis) and by-the-wind sailors (Velella velella). Many mistake the venomous Portuguese man o’ war for a jellyfish and though it is closely related, it is in fact a species of colonial hydrozoan known as a siphonophore. Siphonophores are animals that are made up of colonies of organisms working together. The Portuguese man o’ war has four specialised parts, where each part is comprised of colonies of individuals called zooids. The four functional parts of this fearsome organism are the reproductive apparatus, the digestive system, the float or pneumatophore (a gas-filled bladder) and those infamous stinging tentacles. It is, of course, the pneumatophore that gives buoyancy and it is also the part that is most recognisable when washed up. Intriguingly, this buoyancy aid can be temporarily deflated to allow the creature to sink below the waves should a threat be perceived. However, it is certainly those tentacles that generate the most news stories.

Watch out

The tentacles of the Portuguese man o’ war are long, thin and blue and can be up to 50m in length. Venom-filled packets called nematocysts cover the tentacles and these can discharge upon contact. The tentacles are used to paralyze and kill fish and other small creatures but, unfortunately, they also deliver an excruciatingly painful sting to humans. In very rare cases this has been known to be fatal. Caution is certainly called for because, even when dead and washed up on the shore, the tentacles can still deliver a sting. If you spot them, it is best to keep yourself, children and pets away.

Predator and prey

While the Portuguese man o’ war is a predator, it is also predated upon. The sunfish, Mola mola, is known to feed on them as do turtles, such as the leatherback. Sadly, as it is easy to see, because the air bladder so closely resembles plastic rubbish like bottles or plastic bags, turtles often mistake ocean litter for prey and ingest it.[HF1] This can cause digestive blockages which can be fatal. A further predator of the Portuguese man o’ war is an unlikely one, a small carnivorous animal called the violet sea snail (Janthina janthina).

The violet sea snail

This species of sea snail is also pleustonic and pelagic. Even though we spotted one washed up at Toe Head earlier in the year, they are more likely to be found closer to the equator where waters are warmer. Again, it was a storm and some persistent wind that had caused the snail to drift our way. It is hard to imagine how a tiny sea snail can spend their life floating on the ocean. This curious species produces mucus which gets infiltrated by air pockets and the mucus hardens to produce a ‘bubble raft.’ They then sail the ocean upside down, suspended by this raft. In addition to the Portuguese man o’ war, they also feed on by-the-wind-sailors. Perhaps we might spot one of these violet coloured snails again soon as we have been seeing lots of by-the-wind sailors recently.

By-the-wind sailors

A by-the-wind sailor seen on a beach in West Cork after the windy weather.

Also relatives of jellyfish, these colonial hydrozoans are like miniature sailboats. They reach 8-10cm in length and have an oval base, on which they float, and an upright ‘sail.’ Their ‘sail’ allows them to catch the wind and travel along the ocean surface. Like the Portuguese man o’ war, they have tentacles which descend into the water from the bottom of their float which sting and catch their prey. Luckily, these tentacles are not as powerful and only deliver a very mild sting, if any. Often, by the time we find them on the beach, their blue ‘jelly’ and tentacles have dried up and disappeared and they have become bleached. Nearly every year Inchydoney beach becomes littered with their remains and it often makes it look like there are many bits of strangely shaped plastic littered across the beach.

Hopefully one day

The beautiful blue sea dragon is rare and yet to be recorded on Irish shores. (Photo: Shutterstock)

From the list of creatures that live and travel on the ocean’s surface we have spotted the Portuguese man o’ war, by-the-wind sailors and the violet sea snail. However, one creature that still eludes us, and is definitely on the bucket list, is a truly mesmerising animal, Glaucus atlanticus.

They are a sea slug, also known as the blue sea dragon, that floats upside down by using the water’s surface tension. This carnivorous slug feeds on other pelagic creatures, including the Portuguese man o’ war, and while incapable of stinging themselves, they store stinging nematocysts from their prey within their body and still can pack a real punch.

It is unlikely that we will spot them because they are relatively rare and usually live in warmer waters. They are also small, a maximum of 3cm, but the real issue is the camouflage that their blue and white colour provides, both in the water and while on their prey.

We will certainly keep trying, however, and a perfect time to head to our beaches and shorelines to see what mysterious creatures might have washed up is during the calm that follows a storm. Just remember to avoid touching any creatures that might sting.