Angels and Butterflies of the North

Team Aquatilis has studied the sea for a number of years. Before we set sail in 2015 towards new discoveries, we will tell you about the various aquatic creatures we have already encountered during our dives in different parts of the world. We’ll show them to you and tell you about their lives.

Let’s start with Arctic pteropods which inhabit icy polar waters. Science currently knows two of them: Sea Angels (Clione limacina) and Sea Butterflies (Limacina helicina). These two creatures are inseparable. Indeed, the Angels feed on the Butterflies. More so, it is their only source of food. The Sea Angel is a small translucent orange monster, about 3-5 centimetres in length. It floats through the water column, slowly flapping its muscular wings. The Angel does this much like a garden butterfly, making a figure eight with its wings underwater, but a lot slower. You can watch a floating Sea Angel for a while and enjoy every second of it. They’re incredibly beautiful swimmers.

“Flying” Sea angel. Size: 3cm. The White sea, Russia.

What can be perceived as wings is actually an evolved grastropod’s foot. This ancient gastropod is the ancestor of all the snails and molluscs you might encounter both at sea and in your back garden alike. The Angel is therefore an unusual type of evolved snail which, at some point during the evolutionary process, decided that floating around with plankton is a better idea than crawling on the sea floor. This transformation created a brand new niche for the mollusc, which still maintains many snail characteristics. During the embryonic stage, the Sea Angel even grows a spiral shell, which then disintegrates rather quickly.

Limacina helicina — Sea butterfly. Size: 8mm. The Sea of Japan, Russia.

Sea Butterflies are close relatives of the Angels. They are smaller, dark in colour and preserve their shells. If you described them in one sentence, that sentence would be “small swimming snails with big ears”. They are tiny in size, reaching only about 4-5 millimetres, their wings included. They form massive groups which are so dense that they make the sea black. Sea Butterflies feed on nanoplankton which is unseen to the naked eye. To feed, the Sea Butterfly releases a ball of slime which is larger than its own body. Organic substances, such as single-cell creatures, tiny crustaceans, larva and other minuscule animals stick to this ball of slime. Then, the Sea Butterfly eats it, together with everything that’s stuck to it.

That’s what you see when Sea angel “opens” the head

When the Sea Angel feels hungry, you can witness a terrifying scene straight out of sci-fi films. The Angel’s head conceals six large hooked tentacles covered in tiny spikes. When an Angel approaches a Butterfly, it senses the latter’s presence with its chemical receptors. The Angel’s head then splits open, releasing the six tentacles which inflate and grow to half the creature’s size. The Angel starts flapping its wings with tremendous speed, circling the place where it sensed the Butterfly. If the Angel is lucky enough to immediately catch its prey, the hunt lasts for mere seconds. The Butterfly gets stuck between two hooks, which close upon it like two fingers.

Rare shot. Successfully captured moment of hunting.

To begin its meal, the Angel needs to turn the Butterfly’s shell with the opening towards its mouth. To do this, it needs to release its grip for a split second. Yearning to escape, the Butterfly attempts to move, but the Angel captures it again. This is repeated until the shell is in the correct position. After this, the Angel’s head releases another implement — the creature’s jaw, which is formed of even more hooked spikes. The Angel methodically catches on the Butterfly’s soft curled body, extracting it from its shell. The Angel’s mouth, just like the mouth of all other molluscs, contains a radula, which is a special grater made out of chitin. This allows molluscs to grind even the toughest food. The soft-bodied Butterfly is therefore immediately pureed and consumed.

Clione with the crashed Sea butterfly in the mouth. Size: 4cm. The White sea, Russia.

In one season, a single Sea Angel can eat up to 500 Butterflies, accumulating nutrients in fat droplets under its skin. Afterwards, it can survive without food for several months. Sea Angels and Sea Butterflies are active for only a few weeks in the end of May and the beginning of June. First, the Butterflies suddenly turn the waters black. Then, after a few days, the Sea Angels appear. During this time, the sea looks like a thick snail soup, filled with tiny big-eared snails and orange Sea Angels, happily hunting and releasing their buccal cones.

Limacina helicina in the water column. Size: 2mm. The White sea, Russia.

After three weeks, regardless of their number at the start of the season, the Sea Butterflies disappear from the waters entirely. Nobody knows where they go. After a week, the Angels follow suit. It is thought that they go to the sea floor and spend a whole year plotting something or other down there. Nobody knows what though, or even how to find it out. During the rest of the year, you may encounter single specimens in the open waters from time to time, but that’s a rarity. Where this tremendous mass of molluscs disappears to remains a great mystery.

Pteropods are beautiful.

    Aquatilis Expedition

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    Aquatilis Expedition is an epic three-year underwater journey through the unknown, set to explore the world’s most extraordinary creatures.

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