Okavango Delta, Botswana

PREDATOR-INDUCEDCYCLOMORPHOSIS OF DAPHNIALAEVIS (BRANCHIOPODA,CLADOCERA) INATROPICALFLOODPLAIN (OKAVANGODELTA,BOTSWANA)

M. LINDHOLM Departmentof Biology,University of Oslo, P .O.Box 1027, Blindern, N-0316 Oslo, Norway

ABSTRACT

Thisstudy reports on cyclomorphosisof Daphnialaevis ina seasonal oodplainin the Okavango Delta,Botswana. Data were sampled during the oodingseason 1998, with laboratory experiments supplementingthe eldobservations. As newborn shfryentered the oodplain,the relative helmet sizeand tail spine length in D. laevis graduallyincreased, whereas total body length decreased. No relationshipbetween temperature and cyclomorphosis was detected. Gut content analysis of 170 sh fry(predominantly Cichlidae) showed extensive planktivory, but D. laevis wasnegatively selected amongthe zooplankton. In general, the species of Cladocerawere concentrated in the littoral, which wasdominated by densemats of Algaeand macrophytes. Distribution of D. laevis,however,showed 1 anoppositepattern, with highest densities (max. 114 ind. l ¡ / intheopen pool area, where refugia frompredation were scarce. This strongly suggests that the extensive cyclomorphosis observed is essentialfor the co-existence of shfryand D. laevis.Toourknowledge, this is the rststudy of cyclomorphosisin D. laevis.

ZUSAMMENFASSUNG

Diesist ein Bericht ü berCyclomorphose bei Daphnialaevis ineiner saisonal ü beruteten Flä che imOkavangoDelta, Botswana. Freilanddaten wurden in derÜ berutungsperiode des Jahres 1998 gesammeltund durch Laborexperimente ergä nzt. Nach dem Erscheinen frisch geschlü pfter Jung - schenahm die Lä nge von Helm und Dorsocaudalstachel bei D. laevis allmählich zu, wä hrend sich dieKö rperlä nge verringerte. Ein Zusammenhang zwischen Cyclomorphose und T emperaturwar nichterkennbar. Mageninhaltsanalysen von 170 Jung schen (vorwiegend Cichliden) ergaben, daß Zooplanktoneine wesentliche Nahrungskomponente ist, wobei D. laevis allerdingskaum betroffen war.Im Allgemeinen waren Cladoceren im Litoral konzentriert, wo dichte Algenmatten und 1 Makrophythendominierten. Im Gegensatzdazu zeigte D. laevis höchste Dichten (max. 114 Ind. l ¡ / imzentralen,offenen Bereich der Ü berutungs ä che,wo Refugien vor Rä ubern Mangelware sind. Cyclomorphosewird als unabdingbare V oraussetzungfü r dasgemeinsame V orkommenvon Jung - schen und D. laevis erachtet.Unseres Wissens ist dies die erste Untersuchung von Cyclomorphose bei D. laevis.

INTRODUCTION

Several planktonicorganisms showthe ability to seasonally changetheir shape orundergoallometric shifts, afeature termed cyclomorphosis(Hutchinson, 1967; Black &Slobodkin,1987). Typical attributes are enlargedspines, as observed bothamong dinophytes (Dinophyta), diatoms (Bacillariophyceae),rotifers (Ro- tifera) andcladocerans (Cladocera), or helmets, foundonly among cladocerans, notably Daphnia spp.The causes forthis phenomenonmay be complex,and both abiotic (turbulence,photoperiod, light, temperature) as well as biotic (predation) explanationshave been considered (Havel, 1987; see fora rewiewJacobs, 1987). Amongcladocerans, an extensive literature documentscyclomorphosis in the gen- era Daphnia and Bosmina,apparentlyprimarily inducedby invertebrate predators (Dodson& Havel,1988; Pijanowska, 1991; Larson & Dodson,1993; Hellsten & Stenson,1995; Lysebo, 1995). Less is knownabout morphological responses to sh predation.Machà cekÏ (1993), however, recorded a reducedmean size in Daph- nia galeata Sars, 1864exposed to its natural shpredators ( Rutilus rutilus (Lin- naeus,1758) or Perca uviatilis Linnaeus,1758), and Tollrian (1994)identi ed kairomonesfrom sh as the cause ofenlarged helmets in Daphnialumholtzi Sars, 1885. Reports onthe phenomenonfrom warmer climate zonesare scarce. Allanson et al. (1990)commented on the paucityof recordsconcerning cyclomorphosis in southernAfrican zooplankters. W etzel (2001)considered it tobe nearly absent in tropical andsub-tropical fresh waters, apparentlydue to limited annualtempera- ture uctuations.Green (1967), however, observed enlarged helmets in Daphnia lumholtzi livingnear shore in LakeAlbert (Uganda),whereas the mid-lake popu- lation producedhelmets ofordinary size. Green(1967) suggested sh predationas the cause ofthe pattern found.Zaret (1972)observed horned morphs of Ceriodaph- nia cornuta Sars, 1867near shore in the tropical LakeGatun (Panama), exhibiting a distributional pattern similar to that ofthe planktivorousteleost Melaniris chagresi (Meek& Hildebrand,1914). The pelagial zone,however, devoid of this predator, was inhabitedby unhorned morphs. Correspondingly, Rayner (1981) recorded the presenceof enlargedhelmets of Diaphanosomaexcisum Sars, 1887during summer andautumn in LakeMidmar (SouthAfrica). Daphnialaevis Birge,1879 inhabits bothpermanent and temporary waterbod- ies, butappears to prefer shallow ponds (Infante, 1984; Kraus, 1986; T aylor& Mahoney,1990).Its geographicalrange includes the Nearctic, Neotropical,and Ethiopianregions (T ayloret al., 1998).The species is widespreadthroughout Africa (Mavuti &Litterick, 1981;Green & Kling,1988), and it is probablythe most commondaphniid on the southernsub-continent (Green, 1990). Although the plasticity ofits headshape has occasionally beencommented upon (Rayner,