Our intertidal zone, where the land meets the sea, is an area that has always intrigued and fascinated me. The thing that never ceases to amaze me is that the more you know about life in the rockpools the more intriguing and incredible it is. Even the most ordinary looking creature is extraordinary through its habits, feeding mechanisms and reproduction. They live in such a harsh environment – what we experience seasonally in a year – they endure in 12 hours. In one tidal cycle they have to survive extreme conditions. They go from being below the waves in a fairly constant cool temperature and salinity to having the tide dropping and, on a Summers day, the water in the pools heating up rapidly. The salinity increases as water evaporates and the amount of oxygen in the water drops as does the amount of food around them. Conversely, they have an increased pressure of predation in this now smaller environment. The tide rises and they have the fear of the waves smashing their bodies against the rocks. It is this unique and stressful environment which leads to some of the incredible habits and body shapes and structures of our intertidal creatures.
I love dolphins as much as the next person and if anybody asks what their favourite marine creature is many will say – dolphins, whales and seals. These are our iconic sea mammals and we feel a sort of spiritual connection with these intelligent and expressive animals. It’s a shame that there is not more coverage of some of the smaller creatures that may not have the expressive eyes and acrobatic skills but are none the less amazing animals. Let’s take a look at the barnacle for example. Barnacles are really quite incredible creatures that many people may curse when they walk over the rocks because of their sharp volcanic homes that they create.
Barnacles have an incredible life cycle - relatives of the crabs, lobsters and prawns they are classified as “Cirripedia” and Darwin did much to clarify the classification, different body shapes and various life cycles of the lowly and numerous barnacle species. He was the one to point out that they were not molluscs like mussels, periwinkles and other bivalves but actually crustaceans.
They start off life as much of our marine intertidal animals do as a larval stage – but not just as one but two stages. It starts off as a “naupilus” a free swimming, one eyed planktonic creature. After several moults (just like a crab moults) it develops into the next more complex stage which is called a “cyprid” and here they search out a place to call home. They use structures called antennules, like antenna, to find a good spot to settle for the rest of its sedentary life. Once it’s found the perfect spot it cements itself to the rock with its head...very strange behaviour. Once attached the cyprid then creates a sturdy, rocky, volcanic looking home around its delicate body – this takes about 12 hours. Now, it looks like the barnacle that we all know.
Once settled, it starts its cyclical feeding on the high tide. Cirripedes means “curl footed” in Latin and it is this apparatus that it uses to scoop out tiny bits of food from the water. On the low tide to protect itself from predators it closes a trapdoor and rests in its own cool pool of water as the tide drops. But if it’s stationary how does it reproduce with its neighbour? Well, this is another fascinating area of the barnacle life cycle – with a long (largest relative to body size in the animal kingdom) extendable phallus the barnacle searches for a neighbouring female and fertilizes her externally. Once this has taken place the phallus then falls off! Once the fertilized eggs are “hatched” into the naupilus larvae they are released into the water column and so the life cycle begins again...amazing stuff! You might have thought the different life stages of a frog were amazing but what about that for a life cycle! So, this Summer next time you look at a barnacle take a minute to think about the extraordinary lengths it has gone to to get where it is!
So, there we have it - a seemingly insignificant creature that has got to be respected for its incredible life cycle and fight against the odds for survival in this harsh and intertidal zone.