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Simplified biology of the Shore Crab - facts about soft crabs When we see a crab, running along the shoreline, we think of the organism we are looking at, as being a creature of a certain size, i.e. the size we see. This however, as you will soon see is only part of the story. What we are looking at is the shell of the creature that lives inside and can best be thought of as a suit of armour. Crabs are invertebrates, i.e. animals lacking a backbone. The suit of armour acts not only as protection, but also as support for the creature living within. The suit of armour however is a fixed size and as the crab grows inside the armour plating, its home gets ever more restrictive. Eventually, the crab outgrows its shell and needs to grow a new one. When a crab needs a new shell, it enters what we know as the peeler stage. A new shell is grown under the old one; the new shell is soft and has the texture of skin. The crab also starts to draw back into its body, material out of the old shell, to use in the
The soft and totally vulnerable crab, crawls out of its old shell and ingests more water until it is about 30% bigger than it was previously. Over the next few days this new bigger shell hardens, until the crab can eventually support its own weight with the new shell and it is hard enough to start moving around. Once hard, the crab ejects the water it had previously ingested and shrinks back to its previous size. It now has a nice new shell about 30% bigger than the crab living inside, giving the crab plenty of room to grow, before it needs to moult again, one of the amazing facts about soft crabs.
The female by this time, if she is living in an estuary, will have moved down river towards the open sea, where the young hatchlings stand a better chance of survival. The eggs hatch at night at high tide and the young Protozoea drift out to sea in the upper layers of the ebbing tide, with metamorphosis into 1st stage Zoea larvae, taking place en-route. The Zoea live in the plankton for the next few months, the exact time depending on temperature and time of year. Because of the
At some point during this period, they reverse their pattern with regards the tide, now staying lower in the water column on the ebb tide and riding the surface currents toward the shore on the flood. As the megalops lowers in the water column, it samples the seabed by sitting on it. If the area is to the megalops liking it will settle and take up the benthic life of a crab, if not it will lift off and try a new area. The megalops continues to do this until it finds a substrate suitable to its needs, preferring to die in the water column, than settle in an undesirable area.
A bit like us, once adult, the only thing on the mind of a male crab, during summer is sex, so much so that he will resist moulting during this period, so that he can mate with as many females as possible. Once the summer is over and water temperatures fall, females stop moulting in numbers and the males will moult themselves if they need to. When a female crab reaches a certain size (in the order of 40mm) normally about 2 year old, she will move into the sub-tidal area and no longer be available to shore collection by anglers. The male however hangs around a bit longer, probably because this is where the majority of young fast moulting females are located. Males stay in this inter-tidal region for up to 4 years, until probably the stresses of the region with daily drying, wave action, temperature fluctuations, fighting and mating, tell their toll on an old body. Whereupon, they also migrate into the calmer and less stressful environment of the sub tidal regions, to live out the rest of their lives in more serene conditions. Most scientists believe that crabs need a water temperature of 10 degrees C or more to moult. This is not the case, some male shore crabs moult in early spring, prior to the female moult season starting, at temperatures as low as 6 or 7 degrees. This however is always below the low tide level, explaining why this has gone undetected by field scientific work. As the water warms to around 10 or 11 degrees the males in our estuaries and on our shoreline undergo a mass moult, often entering burrows excavated into the mud walls of the estuary banks at the upper level of the tidal range. Emerging, hungry for a mate, the summer temperatures soon bring them females coming into moult and the cycle continues. So ends our brief description of a crabs fascinating and stressful, short life. There are so many other things to know about a crab, such as the escape reflex and limb regeneration, which is deserving of a page of its own. We hope however, that this narrative has increased your knowledge of crabs and that you are sufficiently interested by the above information to research the topic further yourself. *Please click on any of the crab pictures to see a larger image. |
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| Biology
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