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A Long-Term Mesocosm Study on the Settlement and Survival of Juvenile European Lobster Homarus Gammarus L. In Four Natural Substrata

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A Long-Term Mesocosm Study on the Settlement and Survival of Juvenile European Lobster Homarus Gammarus L. In Four Natural Substrata
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  L  Journal of Experimental Marine Biology and Ecology249 (2000) 51–64www.elsevier.nl/locate/jembe A long-term mesocosm study on the settlement and survivalof juvenile European lobster  Homarus gammarus  L. in fournatural substrata a,b, c a *Adrian Linnane , David Mazzoni , John P. Mercer a  National University of Ireland  ,  Galway ,  Shellfish Research Laboratory ,  Carna ,  Galway ,  Ireland  b  National University of Ireland  ,  Galway ,  Martin Ryan Marine Science Institute ,  Galway ,  Ireland  c  Dipartimento di Protezione e Valorizzazione Agro -  Alimentare  (   DIPROVAL  ), Universita ’   degli Studi di Bologna ,  via del Guasto  5   /  b , 40126   Bologna ,  Italy Received 2 April 1998; received in revised form 12 January 2000; accepted 22 February 2000 Abstract To date, the natural substratum preferences of early benthic phase (EBP) European lobsters(  Homarus gammarus ) remain largely unknown. This study utilised a large scale mesocosmexperiment to determine if the animal favours cobble ground, similar to its American counterpart(  Homarus americanus ), or has other substratum preferences. Postlarvae were provided with thechoice of settling on four natural substrata: sand, coralline algae, mussel shell and cobble. Over anine month period, the number and size of juveniles on each substratum was recorded, with loss of chelipeds used as an indication of social interaction. After a 30 day period, a non-randomdistribution of lobsters was observed on the four substrata. Juveniles were more abundant insubstrata which provided pre-existing shelter in the form of interstitial spaces, i.e. cobble andmussel shell, than in sand or coralline algae. The survival of individuals from postlarvae to 30 dayold juveniles ranged from 5 to 14% with surviving benthic recruits showing a clear mode at 6–8 2 mm carapace length (CL) in size distribution. The density of lobsters per m of cobble remained 2 relatively constant (18/m ) throughout the study period while the density of juveniles on mussel 2 shell decreased significantly (35 to 5/m ). The size distribution of lobsters on each substratumalso varied 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 cobble had a mode of 10–12 mm CLwithin a range of 8–24 mm CL. Overall, the results underline the importance of shelter-providinghabitat such as cobble or crevice-type substrata to EBP European lobsters. They also confirm thatfor a shelter-dwelling animal such as a lobster, the physical structure of the habitat is a key factorin determining both the size and number of its inhabitants.  ©  2000 Elsevier Science B.V. Allrights reserved. Keywords :   Homarus gammarus ; Settlement; Survival; Preference; Substratum*Corresponding author. Tel.:  1 353-91-524-411; fax:  1 353-91-525-005.  E  - mail address :   adrian.linnane@nuigalway.ie (A. Linnane)0022-0981/00/$ – see front matter  ©  2000 Elsevier Science B.V. All rights reserved.PII: S0022-0981(00)00190-8  52  A .  Linnane et al .  /   J  .  Exp .  Mar  .  Biol .  Ecol . 249 (2000) 51 – 64  1. Introduction Most benthic marine invertebrates have a complex life cycle involving a pelagicdispersal phase followed by settlement onto a preferred substratum associated with theirearly benthic phase (EBP) habitat (reviewed by Chia and Rice, 1978; Burke, 1983;Crisp, 1984; Morse, 1985; Svane and Young, 1989). The duration of this phase canextend from a few minutes to several months, depending on the species (Mileikovsky,1971; Scheltema, 1986).Where a suitable site is absent or under conditions unfavourableto subsequent survival and growth, many species have the ability to prolong planktoniclife, thereby demonstrating an important degree of selectivity for the settlementsubstratum (Grosberg, 1981; Botero and Atema, 1982; Cobb et al., 1983; Petersen, 1984;Young, 1989; O’Conner, 1991). Whereas the larval form of the European lobster(  Homarus gammarus ) has been identified in nature (Tully and O’Ceidigh, 1987), thebenthic habitat to which it recruits to remains largely unknown.Several laboratory studies have examined the substratum preferences of EBPlobsters and many have revealed the animals’ ability to utilise a wide range of habitats of varying structural complexity ranging from unsieved mud to macroalgalcovered rocks (Berrill, 1974; Howard and Bennett, 1979; Botero and Atema, 1982;Pottle and Elner, 1982; Barshaw and Bryant-Rich, 1988; Boudreau et al., 1993).However, up to the end of the 1980s there were few quantitative descriptions of newly recruited lobsters in nature (Hudon, 1987; Able et al., 1988) as conventionalbenthic sampling methods such as cores and grabs were unsuitable for gravel andcobble substrata, areas believed to be prime nursery areas for EBP lobsters. Thisproblem was addressed with the development of the airlift suction sampler (Incze andWahle, 1991; Wahle and Steneck, 1991). The device was first tested in the UnitedStates and proved considerably successful in identifying benthic recruitment habitatsand nursery grounds of EBP American lobster (  Homarus americanus ) within thecoastal Gulf of Maine. Here, juveniles were seen to be restricted to the shelterproviding habitat of cobble substratum (Wahle and Steneck, 1991) where average 2 population densities were as high as 6.9 individuals/m .In 1994, a sampling study was undertaken in the west and southwest coasts of Ireland and the Channel Islands, UK, in order to gain some insight into the habitatrequirements of EBP European lobsters. To date, there are no published reports of newly settled European lobsters in the wild. Despite being unable to locate juvenilelobsters, the study revealed that the species diversity within cobble habitats in Irelandand the Channel Islands is high, with reptant decapods such as squat lobsters(galatheidae), porcelain crabs (porcellanidae), mud crabs (xanthidae) and snappingshrimp (alpheidae) appearing to dominate (Wahle, 1998). This is in direct contrast tothe American situation, where in the Gulf of Maine the diversity is relatively lowwith  Homarus americanus  and the rock crab  Cancer irroratus  being by far the twomost abundant species.This study aimed to elucidate where juvenile European lobsters may spend their earlybenthic life by quantifying the settlement of postlarvae onto four types of substratum andmonitoring their substratum preference over a 9 month period.   A .  Linnane et al .  /   J  .  Exp .  Mar  .  Biol .  Ecol . 249 (2000) 51 – 64   53 2. Materials and methods 2.1.  Experimental animals All lobsters used in the experiment were hatched at the National University of Ireland,Galway, Shellfish Research Laboratory, Carna, County Galway, Ireland, using themethods described by Mercer and Brown (1994). Within 24 h of reaching stage IV,postlarvae were transferred directly from the hatchery to the experimental unit. 2.2.  Experimental unit  The trials were undertaken in an outdoor rectangular (10 3 4 3 1 m) concrete pondwhich was lined with black polythene (Fig. 1). The bottom of the unit was covered with 2 a layer of coarse sand approximately 50 mm deep. The pond was divided into 40 3 1 mplots, each of which was filled with one of the following substrata: coarse sand (1–1.5mm) as classified by Wentworth (1936), coralline algae (  Lithothamnion sp . ), musselshell (  Mytilus edulis ) or cobble (64–256 mm; Wentworth, (1936)). Thus, each sub-stratum was represented by ten plots. The sand, coralline algae and mussel shell plotswere approximately 3–4 cm in depth while each cobble plot contained two layers of cobble stone.By positioning the plots as shown in Fig. 1, the experiment endeavoured to make eachsubstratum equally available to every lobster.While realising that choice is dependent on Fig. 1. Plan view of experimental pond showing the exact location of each natural substratum. C: Cobble, L:Lithothamnion (coralline algae), M: Mussel Shell, S: Sand.  54  A .  Linnane et al .  /   J  .  Exp .  Mar  .  Biol .  Ecol . 249 (2000) 51 – 64  the sequence in which substrata are encountered, the large sample size and the longduration of the study contributed to counteract effects associated with biased settlementpattern related to plot distribution. In addition, research has shown that postlarvallobsters are capable of rapid, directional swimming (Ennis, 1986; Hudon et al., 1986;Cobb et al., 1989) with the function of allowing animals to move into areas suitable forsettling. Combined with this capability, the experimental design endeavoured to allowindividuals to behaviourally determine their distribution and final settling location.Seawater entered the pond via an inflow pipe positioned at one corner of the unit.Throughout the experiment the inflow was maintained at approximately 10 l/min. Waterexited the system via a central standpipe (70 mm diameter) which was covered with a 2mm mesh screen to prevent juveniles escaping from the pond. 2.3.  Releases Stage IV juveniles were removed from the rearing unit using a hand net andtransferred into a 10 l plastic container filled with seawater. Lobsters were released intothe pond from the central standpipe at a rate of 250 individuals every 15 min. For eachtrial, a total of 4000 postlarvae were used thereby giving an initial density of 100 2 animals/m . Prior to release, the water volume was lowered to approximately 5 cmbelow the outflow level. 2.4.  Sampling protocol Lobster distribution in different substrata was examined on both a short- andlong-term basis. ‘Short-term’ consisted of three separate trials in which postlarvae werereleased into the pond, allowed to settle and then sampled 30 days later. Samplingconsisted of draining the system and manually searching each plot for lobsters.Discovered lobsters were removed from the unit and the number of individuals on the 2 respective plots was recorded. A  x   test was used to test the null hypothesis that theobserved distribution of settled lobsters would not differ significantly from equalproportions among substrata. The carapace length ((CL), measured using a verniercalipers) and cheliped number of each individual was also recorded. Between trials, thesystem was drained for a period of 5 days to ensure that undiscovered lobsters did notinvalidate the results.The ‘long-term’ trial consisted of returning the juveniles to their srcinal substratumfollowing the third, short-term census. The pond was refilled and periodic sampling wasundertaken at 30 day intervals for the next consecutive five months. A final census wastaken a further three months later, nine months after the postlarvae had been introducedinto the system. When sampling, care was taken to minimise disturbance. Recovered juveniles were maintained in seawater and on return to their selected habitat were placedunder cover of the substratum to avoid desiccation. The surface water temperature withinthe system was recorded daily.   A .  Linnane et al .  /   J  .  Exp .  Mar  .  Biol .  Ecol . 249 (2000) 51 – 64   55 2.5.  Feeding Prior to the introduction of postlarvae into the experimental system, the unit wasseeded with brine shrimp (  Artemia  sp.). Initially, 100 g of hatched cysts were released,with 50 g added every 2 days subsequently. As the temperatures within the systemdropped during the long-term study, the feeding ration was reduced to 50 g every 4 days.The  Artemia  were hatched by placing the cysts into a 50 l conical vessel containingaerated, 1  m m U.V. filtered seawater at a temperature of 18 8 C for a period of 12 h. 2.6.  Data analyses The results were analysed using Chi-square,  t   and ANOVA tests. Where multiplecomparisons were made over the 9 month study, the probability of Type 1 error wasadjusted via the Bonferroni procedure and significance was accepted at  P  , 0.01.Wherethe ANOVA analysis indicated significant differences, a Tukey test (Zar, 1996) was usedto examine differences between pairs of treatments. The calculations were performed ® with MINITAB (MINITAB Inc., U.S.A) and SYSTAT (Wilkinson et al., 1992)statistical software. 3. Results 3.1.  Survival and distribution within substrata Survival of lobsters from postlarvae to 1 month old ranged from 5% to 14% (Table 1). 2 As postlarvae did not settle in equal proportions as hypothesised ( x   5 49.31, df  5 3, P , 0.05) the null hypothesis concerning lobster distribution was rejected. In caseswhere mussel shell and coralline algae were chosen, animals were found sheltering in Table 1The number of lobsters found in test substrata and the end of each 1 month trial and overall mean densities.Significance tests between substrata are also included. Means were calculated from 30 quadrats of eachsubstratum during 3 replicate trials. Each trial had an initial release of 4000 postlarvaeTrial No. of lobsters per substratumnumberMussel Cobble Coralline Sand Total Survivalshell algae (%)1 213 124 47 0 384 9.62 78 113 26 0 217 53 350 187 9 0 546 14 F  * df   P 2 Mean n/m 21.3 14.1 2.7 28.8 2  , 0.05S.E. 2.53 1.44 0.5 * Single-factor ANOVA.
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