A long-term mesocosm study on the settlement and survival of juvenile European lobster Homarus gammarus L. in four natural substrata

To date, the natural substratum preferences of early benthic phase (EBP) Eu ropean lobsters (Homarus gammarus) remain largely unknown. This study utili sed a large scale mesocosm experiment to determine if the animal favours co bble ground, similar to its American counterpart (Homarus americanus), or h as other substratum preferences. Postlarvae were provided with the choice o f settling on four natural substrata: sand, coralline algae, mussel shell a nd cobble. Over a nine month period, the number and size of juveniles on ea ch substratum was recorded, with loss of chelipeds used as an indication of social interaction. After a 30 day period, a non-random distribution of lo bsters was observed on the four substrata. Juveniles were more abundant in substrata which provided pre-existing shelter in the form of interstitial s paces, i.e. cobble and mussel shell, than in sand or coralline algae. The s urvival of individuals from postlarvae to 30 day old juveniles ranged from 5 to 14% with surviving benthic recruits showing a clear mode at 6-8 mm car apace length (CL) in size distribution. The density of lobsters per m(2) of cobble remained relatively constant (18/m(2)) throughout the study period while the density of juveniles on mussel shell decreased significantly (35 to 5/m(2)). The size distribution of lobsters on each substratum also varie d with time. By the conclusion of the trial, lobsters found in mussel shell had a mode of 8-10 mm CL within a range of 6-14 mm CL while those in cobbl e had a mode of 10-12 mm CL within a range of 8-24 mm CL. Overall, the resu lts underline the importance of shelter-providing habitat such as cobble or crevice-type substrata to EBP European lobsters. They also confirm that fo r a shelter-dwelling animal such as a lobster, the physical structure of th e habitat is a key factor in determining both the size and number of its in habitants. (C) 2000 Elsevier Science B.V. All rights reserved.