P.S.Z.N. 1: Xlarinc Ecology. I1 (3): 263-275 (1990) Acccptcd: March 26, 1990 0 1990 Paul Parey Scicntific Publishers. Berlin and Hamburg ISSN 0173-9565 Observations on the Aggregative Behavior of Mysidium columbiae, the Mangrove Mysid RICHARDF. MODLIN Department of Biologiciil Scicnccs, ‘The University of Alabama in Huntsville. Huntsvillc, .Alab;imn 35899, U. S. A. \Vith 3 figures Kc? words: ,My.sitltrceu, My.sitliw~i col~rn1hc,shoal structure. diurnal dynamics. iii;in- groves. Cnribhcon Sca, light intensity. Abstract. The mangrove mysicl My.sitfiurn cdirnihitre (Zihihiti<) occt~rsin aggrcgations of tens to millions of indiviclual>. Few indcpth studies on the nggrcgativc bchavior of this spccics havc cwiiiincd \tructiiral composition, I-csponsc to light intensity changes. and its ability to rctlucc predation. Snorkel arid SCUBA observations, as well ;is aquarium cxperimeiitation on shoals/ schools of this spccics in sh:111ow waters around a mangrove nrchipcl;igo were made at irrcgulnr intervals to dctcrminc their structurc. dynamics. and influcncc on predation. In stimnicr, aggrcga- tions occurred as large cylindrical shoals. in winter mall ovoid schools. In daylight. shoals/schools wcrc very sensitive to changes in light intensity. At night the smallest juveniles formed compact swarms. while largcr individuals bccainc solitary. At sunrise a dcfinitc rc-shoaling behavior cxistcd. Shoals/schools were stratified by life stages. A hypothesis stating that conspecific schools in winter did not mix was tested. Mysitliitrn shoalskchools sccrncd to discouragc predation by fishes. Problem The formation of aggregations is a common behavior of many pelagic (MAUCH- LINE, 1980; HARGREAVES,1985), deep water hyperbenthic (FOSSA,1985), and Coastal mysid spccies (CLUTTER, 1969; WITTMANN, 1976; O’BRIEN,1988). Attempts to interpret the role of this behavior have been made (CLUITER,1969; O’BRIEN,1988). Additionally, various physiological and environmental factors that influence the formation and maintenance of mysid aggregations have been investigated (STEVEN,1061; CLUTTER,1969; WITTMANN,1976; O’BRIEN,1988). These studies have provided schemes to classify the aggregations based on sociality (CLUTTER,1969), geometry (WI-ITMANN,1976), and intrinsic biological processes (ZELICKMAN,1974; O’BRIEN,1988). These schemes seem applicable to the swarming, schooling, or shoaling patterns of most mysid species. Mysidium columbiae (ZIMMER)is a ubiquitous mysid in the tropical and subtropical western Atlantic (MAUCHLINE,1980). It occurs almost exclusively in U. S. Copyright Clearance Center Code Statcment: 0173-9565/90/1103-0263$02.50/O 264 MOIILIN large homotypic aggregations in the waters surrounding mangrove islands (SI'EVEN.1961; GootisuE, 1965; MODL~N,1987), but aggregations are also found near coral structurcs (EMERY,1968; MODLIN,1954, 1957) and just below the water surfacc in pclagic aggregations seaward of coral reefs (MODIJN,unpub- lished data). The only information on thc aggregative behavior of M. columbine is available in SEVEN(1961) and EMERY(1968). Mysiriium colunihiae aggregations range from tens to millions of individuals. Rarely are kl. colurnbiae individuals found solitary in the water column during daylight hours. The organization and response of these aggregations are very similar to those in fish species with obligatory schooling behavior (STEVEN, 1961). This report presents information obtained from field and laboratory observations on the structure, diurnal dynamics, and responses to light of M. coluinltitre aggregations in the western Caribbean Sea, specifically in the shoal waters in and around Twin Cays, Belize. Material and Methods In this papci-. aggregations :ire classified cithcr as shoals. school. or swill-nis. Definition?,of the tlircc Arc lll0d~i'i~~lti~)IlSOf tht>Sc USCd by WII-IhlANN (1976) Lllld O'!<Rli:N (1988) Xld dcfincci iiS lollows~dio:iIs arc aggregations > 2.0m in horizontal extent. 0.5 m or liirgcr in diameter. with iiidivicluals uniformly sp;iccil. swimming parnllcl and in the same direction (polnrizcd), schools arc pt-rl;iiiLccl aggrcg:itioiis <2.0 111 in length or diameter: and swarms :IIC cohesive groupings. < I .Om iii Ic ngr 11 or cI i:I iiic tc r, con1poscd of nori-par:il Iel swim miiig ind ividu:i Is. U iiclc irwat e r v i sihi I ity aro ti nil Twin Cays w :IS aboti t 2 .O m . Conscq ti cii t I y . snor kc I ing gca r was usccl t(1 niakc it! siric observations on il. columhioc aggregations. Snorkeling allowccl continuous monitoring of specific shoalslschools for cxtcnclcd periods cithcr on one day or divided over scvcral days. 4ncc their locations did not changc. Aggregations wcrc obscrvctl and sampled during December I'M. 1987 and 1088, and June 198s at Twin Cays and Carric Bow Cay. Bclizc. Shoals wcrc also observed during October 19,Jh. 1987 and Scptcmbcr 19x8 ol'f Grand Cayman Island, British West Indics, with the aid of SCL'RA. In siru observations were recorded on an underwater writing slotc. Samples were obtained by sweeping a small clip net, with ;I rectangular opening of 66..5cm2 and shallow bag (depth 4.0cri-1, mesh size 0.4 nim). through ;I shoal or school obliquely for a known. prcdctcrniiricd distancc. Larger dip nets to collcct un entire school were consiclcrcd, but offcrctl too much resistance to movement and allowcd many larger mysids to escape. Specimens wcrc fixed in the field and counted, sized, and sexed in the laboratory; density and gcncriil organization of schools and shoals were the,? estimated. To dctcrrninc organization more spccifically, i. c., stratification by life stages. the small dip net was passed through some shoals in one horizontal swccp. A minimuin of thrcc such sweep, each at a different depth, were taken in cvcry shoal or school samplctl for this purpose. These sampled aggregations were allowctl to reorganize for a minimum of 10 min bcforc additional samplcs wcrc taken. A dip net with an opening of435cm20nd;I 30cm dccp soft bag wiis tiscd to collect specimens for aquarium experiments. 'l'his nct minimized mysid damage and stress. Live specimens were inimcdintcly transferrcd to a plastic, 8 I bucket about half full of seawater of ambient lcmpcraturc and salinity and transported to the laboratory. Within one hour of collection thcsc specimens were placed in 401 aquaria. The behavior of M. columbiue during the night was observed with the aid of an Ikclite Mini-C flashlight. A red filter was placed over the lens to eliminate any photokinctic response by the mysids. Additional ;2.I. cnliirrthiue wcrc obtained in June 1988 from light-trap samplers sel for 30min about unc hour aftcr dark at various locations around Carrie Bow Cay and in the Twin Bays study site. Three light-traps wcrc usctl each night for five nights (n = 15). These were set primarily to snrriplc ichthyoplnnkton. Light intensities ncnr schools during December 1988 wcrc determined with a photographic exposure meter scaled in n plastic bag. This light meter was used to estimate the percent attenuation Aggrcgation in Mysitlium colitnzhiue 265 of surfacc light. A QSI-140 lntcgrating Quantum Scalar lrmdiancc Mctcr was used to mcasurc undcrwatcr light intcnsitics dircctly during Junc 19HH. Surface watcr salinity was mcasurcd with an hydromctcr. Salinity at dcpth was dctcrmincd from sigma-t values of a watcr samplc collected in a plastic bottlc at thc dcsircd depth. Watcr tcmpcraturc w3s mcasurcd simultancously in situ with a mcrcury thcrrnomctcr. M 250m South Point Fig. 1. 'Twin Cays mangrovc (TC) and Carric Bow Cay (CBC) barricr rccf systcms off central Bclize. Ccntral America. Thc Twin Bays study sitc in Twin Cays is stippled. Blackcncd circlcs at TC and CBC are additional collecting and obscrvation sites whcrc Mysidiurn colurnbitre was present. Open circles arc sites lacking M. colurnbiae. Stipplcd arrows indicatc pelagic M. columbiae shoals. Insets show thc relationship of the two study areas. thc location of CBC and TC in thc central portion of the Carribcan Barricr Kccf System, and thc location of thc barricr rccf in Central America. 266 MOVLIN Prior to observing diurnal aggrcgative behavior in the field, two to three schools of M.colurnbiae were simultaneously maintained in plcxiglas, rectangular 301 aquaria (0.25 in wide x 0.5 m long x 0.3 m deep). Aquaria werc placed on a rectangular table designed to temporarily $.rap ovcrtlow aquarium water (wet table); water depth on this table was about 0.15m. About 100 to 150 individuals comprised an aquarium school and :ill within an aquarium were collcctcd from the same shoal at Twin Cays. These schools were held for a maximum of 30 h and then released because thcy wcrc not fed while in captivity. Observations were niadc irregularly throughout thc day and night. Most intensive observations were madc during sunrise and sunset. Aquarium specimen? were used to tcst interactions between conspccific schools. Spccimcns from two widely separated. non-communicating M.colurnbiae schools were placed in the sainc aquarium. However, the individuals from one school wcre stained with neutral red dye (How~~<v,1985) to distinguish them from those of the other school. Spccimcns were stained for 60 to 90min in a plastic bucket that contained &5 I of seawater. The stain was inixccl at a concentration of 10mg. I-'. Stained specimens were transferred from tlic staining bucket to thc aquarium with a dip net immediately hcforc or after the introduction of the unstained group. The aquariiinj schools were kept scparatc by ;I removable transpal-cnt plcxiglns partition until each organized into a distinct school; the partition was then removed. Stained individuals could be distinguishcd for up to 30 h. No differences in mortality wcrc ohscrvcd bctwccn stained and unstained individuals. A continuous riinning watcr system fed the aquarium with a flow rate of about 21. 11-'. Siilinity, tciiipeiatiirc, light intensity, and photopcriod wcrc ambient in all experiments. Lca.it scluarcs regressions. corrclations xiid Student-t tests found in the ARS'I'AT Statisticill Software package wcrc ~iscdto rlctcrniillc the clcgrcc of rcl:itionship between a viit-icl y of paramc- tcl-5.