Aliens on the Beach

The crab – an exceptional creature with eyes on stalks and teeth in its stomach. A life cycle being and feeding on plankton and then life finally starts….

Crab on beach
Crab on Beach. Photo by Ulrike Bohr from Pixabay.

The only guys with spare legs numbering 10 in total, two adapted with claws that differ in size according to the species. They use these claws for feeding and fighting. and some crabs fold the back two under their bodies when they walk. Some crabs fold their last pair of legs under their bodies when walking, so whats the deal with those legs? They are used for transferring sperm or tending eggs. Some reference has also been drawn to these legs being used for swimming.

Crabs have teeth as do lobsters !

They are not in the mouth but in the stomach and look like molar teeth surfaces. this arrangement is termed a ‘gastric mill’. These ‘teeth’ are used to grind the food in the stomach rather than in the mouth. Another interesting piece of information on these teeth is that ghost crabs actually use them to make a growling or grinding noise in an attempt to intimidate other crabs.

ghost crab
Ghost crab. Photo by djhixson from Pixabay.

What about breathing? Crabs have gills, right? but how come they can spend so much time out of water?

Crabs have articulating plates that close the gills off to reduce evaporation. As long as they keep the gills moist they can breathe, as the air is replenished in the layer of moisture. Crabs also store moisture in their bladders, blood and in specialized pockets scattered in their bodies. It is thought that the flow of air over the gills cause crabs to ‘blow bubbles’ from their mouths.

Crab blood is copper based (hemocyanin) rather than iron based (hemoglobin) which humans have. This is sufficient for the crab and the blood color is blue/green due to the copper. Iron on the other hand is much more efficient at carrying oxygen and humans have a bigger need for oxygen by the nature of our bodies.

Without getting too technical on this subject, our crab does not have a closed circulatory system with a heart as humans do. Instead, they have an open circulatory system where the blood is pushed through to the organs by the vessels and then it is filtered back to be pushed through again.

And food, crabs are omnivores eating both plant and meat. They play an important part in keeping our beaches clean. They are opportunistic feeders and will take on prey that they can manage using their powerful claws but predominantly eat algae, fungi, plankton, worms and carrion.

Crab eyes are positioned on stalks which are great for a view of the upper areas where most of their danger and opportunities exist. They have thousands of light sensitive facets on each stalk which enables them to see extremely well being able to detect ultra violet light as well. this is especially helpful in deep water where visibility is limited.

crab eyes
close up of crabs eyes. Photo by Nico Franz from Pixabay.

The female crab is able to store the males sperm for future use and will lay between 1000 and 2000 eggs in about 3 to 4 weeks after fertilization. the eggs are stored under her abominable flap. Once they hatch, the babies are called zoea larva and are transparent, have jointed limbs, a crusty tail for swimming and they drift away from their mothers. They are now part of the plankton community and are a food source for many ocean dwellers.

Life is a case of plankton eat plankton with the bigger ones consuming the smaller ones. As the survivors grow, they molt, each time a little bigger and the odds of survival possibly a little better. after 4 or 5 molts our baby crab has transformed itself into a megalopa larva.

This stage resemble their parents with small legs and claws but still use the little tail to move around. Within 5 to 6 days our megalopa larva will settle to the bottom and become a juvenile crab. Our baby now becomes a scavenger eating plant material and any bits and pieces it can find. also at this stage its coloring has changed and it is more suited to crawling on the rocks.

From this point our juvenile crab will struggle to survive and if successful will molt repeatedly until becoming an adult. they can expect to live for between 3 and 4 years.

gull with crab
Crab predator in action. Photo by Susanne Jutzeler, suju-foto from Pixabay.

Crabs have a host of predators such as birds, fish and humans, not to mention other crabs. Its truly a crab eats crab world out there !

3 replies on “Aliens on the Beach”

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