What is difference between Lobster, Crayfish and Rock Lobster ?!

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It all comes down to phylogenetics.

Let's start with "rock lobsters" first and clarify what this is. First off, rock lobster refers to a species of spiny lobsters whose scientific name is Jasus edwardsii. So, you are basically asking what the difference is between crayfish, lobsters, and spiny lobsters.

Let's look at the science behind this question: Lobsters, crayfish, and spiny lobsters are all decapods (members of the phylum arthropoda and subphylum crustacea). Even more specifically, they are in the infraorder Pleocyemata. This is far as they are related. To give you something to compare to, the order taxa is the level to which humans are related to lemurs. After that, we diverge.

Now let's look at the animals on their own:

Lobsters are in the family Nephropidae. An important characteristic of this family is that they mostly have claws. There are about 50 species in this family. The most common species are in the Homarus. Lobsters are found in salt water.

Spiny Lobsters are in the family Palinuridae. This family includes about 45 species, including your rock lobster. Spiny lobsters are found in freshwater and generally do not have the large claws of other lobsters.

Crayfish are in the families Astacoidea and Parastacidae. Crayfish are all found in freshwater and are generally smaller than spiny lobsters or lobsters (though there are enormous exceptions).

That was the complicated answer, here is the simple answer: Lobsters generally have large claws and live in salt water, spiny lobsters (rock lobster included) generally have no claws and also live in salt water, and crayfish live in freshwater and are smaller (but look similar to clawed lobsters).

Hope this helps!

Crustacean biologist at the University of Utah

If you have more questions, come ask me at crayfishmates.com.

They are all different breeds of shellfish. If by "Lobster" you mean like a maine lobster:
http://www.maine.gov/dmr/rm/lobster/mult…

And rock lobsters don't have large pinching claws like those lobsters:

http://www.science-in-salamanca.tas.csir…

they are harvested simply for their tail meat.

and Crayfish are tiny versions:

http://img.dailymail.co.uk/i/pix/2007/06…

And there are many more.

Slightly different gene pool within the same family.
Like most residents of Tennessee

~The Fat Man