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Ancestor: Palaemon serratus (Common Prawn)

Size: Smallest species: 15cm long, Largest species: 25cm long

Dietary Needs: Detritus and dead things that have sunken to the seafloor. Larvae eat microalgae and more algal detritus until they grow larger and start venturing deeper into the ocean.

Life Cycle: Females keep the eggs carried by her swimmers on the underside of her abdomen after they have been fertilized by a male, sometimes with the tail curled so that it points forward and the whole tail is holding the eggs. There can be thousands of eggs carried by one female at a time. The larvae are on their own when they hatch, beginning life as near-microscopic plankton living in the open pelagic zone. They become benthic dwellers when they gain enough mass and density to comfortably do so. When they start living on the seafloor they usually begin in the shallows in sponge fields or amongst algae, similar to smaller types of prawn. They don't usually live in on the benthic zone of deeper waters until near-adult. However sometimes a juvenile brittle prawn will spend it's entire development in deeper waters, especially if it's geographically located far from the coast.

The rostrum is sexually selected by the female and also by competition between males. However it now serves a self-defence purpose also.

Other: They are brittle mainly due to their hardness compared to their ancestor prawns, but not compared to other crustaceans in general. The mineralisation is extremely light (thus does not require a diet that is very high in the required mineral ions) and protected from the external ocean acidity by a cuticle. They can survive attacks by predators similar-sized or slightly larger than themselves that would have ended their ancestors before. They are able to survive fractures to their exoskeleton, especially as they grow a fresh one beneath the old damaged one that will be shed. Some lost limbs can also regrow. Their chance of living to an "old" age increases as they get older. When first hatched they have an extremely high mortality rate, out of tens of thousands of eggs perhaps only one of those will grow into an adult that lays or fertilizes more eggs. Despite living for a few years longer than their ancestors, moults become less frequent with age and this eventually kills it.

One reason for their extended age is so they can live in the deeper, colder waters where there is less crowding and less light, so there is less chance of happening across a predator that will sense it by sight.

If they take longer to grow, they are using less energy when idle. This means they can survive on less food while idle. This is helpful when there is mostly detritus and sometimes plankton to eat at these depths, more or less depending on currents.

If they can get larger, they can defend themselves easier and produce more eggs. They have larger snippers which are used for attacking back when attacked as well as breaking apart food, and their serrated rostrum is also more formidable. Pushing upwards off the seafloor with their legs they can jab an enemy above them. Fighting back only works however when the threat is not too much larger than themselves.

Against larger predators the prawns employ an ancient and common behaviour amongst decapods, which is "lobstering". They bring their tail and scaphocerites together while curling their bodies into a tight crescent in an instantaneous motion that propels them  backwards out of the area at great speed. By repeating this motion they can keep propelling themselves.