Barnacles-All together now

by Matthew Norton

It has been a long several months of limiting our interactions with each other to slow the spread of coronavirus. No doubt it has given many of us a renewed appreciation of the sociable lifestyle. But for some animals, social distancing is not an option, unless they disregard the primary objectives in life, survive, grow and reproduce. Even moving further away, or closer to each other on a temporary basis is impossible for those animals in the sea who can barely move for the majority of their lives.

Barnacles are one such group of animals. As young larvae they are free to move around and explore the oceans, but as adults they are firmly cemented to a single spot. Like other crustaceans (e.g. crabs, lobsters), barnacles have jointed legs which they have repurposed for catching food. They also reproduce by directly fertilising each other, rather than releasing clouds of eggs and sperm into the water. A task for which they are equipped with the longest male genitalia, relative to body size, in the animal kingdom.

barnacle article image 1 — Rock barnacles out of water with their ‘shields’ closed to the outside world (left) and goose barnacles in water with their feeding cirri for catching food (right). Both types of these types of barnacle possess these feeding structures.

But as well endowed as barnacles are, their prospects for mating partners are severely limited by their lack of mobility. Their inability to relocate to fresh feeding grounds is also hugely restrictive and leaves little room for error when looking for property in the vast ocean. Judging by the many factors they consider, including water flow, food availability and their closest neighbours, they seem to be well aware of the importance of this one decision.

Barnacles also approach the decision making process itself very cautiously. Should a larva find a promising neighbourhood they will likely anchor themselves with a temporary sticky glue and make follow-up inquiries. As well as ‘walking’ around to investigate multiple sites, they can also make use of the thin layer of slime that covers virtually every hard surface in the ocean. A layer that contains a vast network of microorganisms, stray proteins and other nuggets of biological information that the barnacle larvae can tap into. Depending on what evidence they uncover, the barnacle larvae will either settle and transform into an adult, or detach and look elsewhere.

barnacle article image 2 — A barnacle larva looking for their first and only new home (left) and a splattering of young barnacles who have made their decision and are beginning to grow (right).

Even if they do find that perfect spot, there is a good chance that it has already been claimed by barnacles from another species who are not keen to share. These conflicts of interest can get bloody as demonstrated in one dramatic example in 1961 with two seashore species, Chthamalus stellatus and Semibalanus balanoides (called Balanus balanoides at the time). Chthamalus stellatus almost always settle on the higher shore (further from the sea), even though their young could try and settle further down to the water’s edge. But anyone foolish enough to try would be smothered, uplifted, crushed or otherwise dispatched by the larger and faster growing Semibalanus balanoides barnacles.

barnacle article image 3 — The bullied, *Chthamalus stellatus* (left), and the bully, *Semibalanus balanoides* (right). Both are driven by competition for the limited space on the rocky shore. *Chthamalus stellatus* has the advantage of being able to thrive in the harsher conditions of the upper shore, where they are further from the sea. But *Semibalanus balanoides* can grow faster and bigger in the easier conditions of the lower shore and overpower *Chthamalus stellatus* with brute force and no mercy. In nature there is often another species to take you down a peg long before you reach your physical survival limit.

As careful as barnacles are when considering where to live for the rest of their lives, there are still those who may be prepared to take a leap of faith. Whether they are choosing to be bold or driven by desperation as their reserves dwindle, death or celibacy will be the likely outcomes for those barnacles who venture into unknown territory. But should these pioneers somehow succeed the rewards for their gene line could be enormous. With species of barnacle already living on whales and floating around on their own buoyancy aids, who knows what new way of living the bold barnacles will find next.

From a human perspective

As much as barnacles would prefer to settle into ‘conventional habitats’, if push comes to shove they will settle anywhere they can. These unusual homes include boats, piers, buoys and probably any hard surface we submerge in water, causing us a fair amount of grief in the process. Boat owners in particular will find their presence to be particularly annoying and expensive from the significant drag they can cause when attached to the hull in large numbers. To maintain the same speed as if the hull was clean, they would have to burn through more fuel and incur heightened running costs as a result. The extra input of greenhouse gases is also far from ideal, though some of our methods for dealing with this problem have not done the environment any favors either.

barnacle article image 4 — These are just a few examples of how barnacles can find their way onto anything we build, dump, or use in the sea.

Removing barnacles with brute force is difficult and time consuming work thanks to their ‘cement’, one of the strongest natural adhesives we have ever come across. The best approach is to prevent them from getting that foothold in the first place and for a long time this meant coating boat hulls with tributyl-tin (TBT). After the use of TBT was banned, other toxic compounds took its place, but there is always the issue of collateral damage when using harmful chemicals to solve our problems in the water. Fortunately, we are making progress in developing effective, but non-toxic methods such as designing boat hulls to mimic the surface of animals and plants who have found success in keeping these hitchhikers away.

barnacle article image 5 — A selection of animals and plants who have provided inspiration for designing anti-fouling surfaces on boat hull. Including (clockwise from top left) sharks, pilot whales, geckos, lotus flower, brown algae and pea pods.

Meanwhile, barnacles can sometimes be useful when they are attaching to surfaces in the ocean. For instance, by attaching to the body and clothing of a floating corpse they could provide useful clues for determining a cause of death. The age of the barnacles on discovery of their newfound floating home can help us to estimate how long the victim has been in the water. If this estimated floating time matches the estimated time of death then it is likely the victim died in the water due to accidental drowning. But if the evidence suggests that the victim was already dead when they entered the water then it is more likely that foul play was involved.

Regular readers of this blog may remember that I covered a similar idea in an article about diatoms that I published last summer. The tendency of these microscopic algae to settle on floating corpses and which organs they have reached inside the body can also provide clues that might lead to establishing a cause of death. Both methods do come with flaws that need to be accounted for, particularly regarding the exact species and variation in the environment. But combining the two would surely give us a more accurate picture of what happened to the unfortunate soul they now call home.

Barnacles have managed to intrigue and infuriate us for a very long time. They even caught the attention of Charles Darwin who devoted a lot of time to studying barnacles intensively as he developed his theory of natural selection. Today, they still hold a lot of importance in advancing human technology with considerable interest in how to adapt their formidable ‘cement’ for our own ends. The secrets of this natural superglue are only just beginning to reveal themselves and who knows how big the treasure trove of possibilities is when it comes to barnacle biology.

barnacle article image 6 — Part of Darwin’s collection of barnacles along with a handwritten list of the different specimens on display at the Zoological Museum of Copenhagen.

barnacle article image 7 — Thanks for reading

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