Abstract.During the culture of large numbers of lobsters,Homarus gammarus(L.), it was noticed that more than half the animals lacked a distinct crusher claw. The majority bore two morphologically similar cutter claws even though their ages ranged up to 3 years (92mm carapace length).The proportion of 3‐month‐old juveniles (11–13 mm carapace length) bearing clearly differentiated crusher claws was increased from approximately 30 to over 80 by adding live oyster spat to the rearing chambers. The differentiation of the crusher claws was completed within approximately 4 moults (71–83 days) and was morphologically more pronounced in lobsters grown in the presence of spat than in the 30 of control lobsters which developed crusher claws sponta
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