PREY SELECTION BY CHAOBORUS PUNCTIPENNIS UNDER LABORATORY CONDITIONS

Robert W. WINNER & Jack S. GREBER

Department of Zoology, Miami University, Ohio, U.S.A .

Received February 28, 1979

Keywords : Chaoboruspunctipennis, predation, prey selection

Abstract

Equal numbers of Diaphanosoma leuchienbergianum, Daphniaparvula, and Diaptomus pallidus were offered to individual,fourth-instar larvae of Chaoborus punctipennis . When the preyspecies were presented separately to the larvae, coo% of the Dia-phanosoma, 67% of the Daphnia, and 57% of the Diaptomuswere consumed . However, when the three species were offeredto the predator concurrently, 9o% of the Diaphanosoma, to% ofthe Daphnia and < 1% of the Diaptomus were eaten . This strongselection for Diaphanosoma was consistent at three prey densi-ties .

Introduction

Most published studies of the food habits of late-instarlarvae of various species of Chaoborus indicate that thelarvae selectively prey on copepods in preference to clado-cerans (Swift & Fedorenko, 1975) . Lewis (1975, 1977) isapparently the only author to suggest that chaoborid lar-vae prefer cladocerans as food . His conclusion is based ona study of the food habits of a Chaoborus population ina tropical lake. Laboratory studies (Anderson & Raas-veldt, 1974 ; Swuste et al., 1973) also suggest that larvalchaoborids preferentially prey on copepods . Swift &Fedorenko (1975) suggest that the greater vulnerability ofcopepods is due to their more cylindrical body whichmakes them easier to ingest than cladocerans . All of theliterature which indicates a preference for copepods isbased on studies in which the cladocerans have bodieswhich are deeper dorsoventrally than laterally .

The objective of our research was to compare the feed-

Dr. W. Junk b . v . Publishers - The Hague, The Netherlands

Hydrobiologia vol . 68, 3, pag . 231-233, 1980

ing response of fourth-instar larvae of Chaoborus puncti-pennis to copepods and cladocerans when one of the cla-docerans shares certain characteristics with copepods .Diaphanosoma leuchtenbergianum has a body which isroughly circular in cross-section and its swimming move-ments and response to a tactile stimulus are more likethose of a copepod than those of a daphniid (Swift &Fedorenko, 1975) . Chaoboruslarvae were offered a choiceof D. leuchtenbergianum, Daphnia parvula, and Diapto-mus pallidus, all of which are potential prey for a popula-tion of C. punctipennis in a local reservoir, Acton Lake .

Methods

Fourth-instar larvae of C. punctipennis were collectedfrom Acton Lake as needed and starved for twenty-fourhours prior to being tested . D. parvula and D. leuchten-bergianum were reared in the laboratory and were offeredto the predators when less than twenty-four hours old .Late copepodid or adult D. pallidus were collected fromActon Lake as needed . The cladocerans used in the expe-riments ranged from 0.3 to o .6 mm in body length andthe copepods ranged from o .6 to 1 .o mm in body length .According to Roth (1971) C. punctipennis can ingest preyof a body length up to 1 .8 mm. However, Robert Debelak(unpubl. data) has shown that fourth-instar larvae of C .punctipennis select daphniids which are o .9 mm in lengthover those o.5 mm in length when the two sizes are avail-able in equal numbers . The larger diaptomids offered toC. punctipennis in our experiments, therefore, shouldhave been as, or more, acceptable on a size basis, than thetwo smaller cladoceran species . Predators were tested in-dividually in 20o ml of filtered lake water in 250-ml

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beakers. All experiments were conducted in the dark at20 ± 1°C for twelve hours. Prey species were always of-fered to the predator in equal densities . At the end of anexperiment, the number of each prey species remainingin beakers was determined and missing prey were as-sumed to have been eaten by the predator .

Results and discussion

An initial experiment was conducted to assure that allthree potential prey would be eaten by the larvae . Elevenpredators were tested for each of the three prey species .Each predator was offered five prey items . All three spe-cies were readily consumed when provided separately .However, the per cent consumed did differ; althoughloo% of the Diaphanosoma were eaten, only 67% of theDaphnia and 58% of the Diaptomus were consumed .

Table 1 . Number of prey consumed by individual Chaobo-rus when presented twelve of each of three prey species .

Prey consumed per 12-hrs

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Table 2 . Prey consumed by individual chaoborid larvae when presented equaldensities of three prey species .

Mean no. prey eaten per predator

In the following experiment, the three species were con-currently offered, at a density of twelve each, to the preda-tors. The eleven larvae tested were remarkably consistentin their behavior (Table i) . Of the ninety eight prey con-sumed, eighty eight (9o%) were Diaphanosoma, only ten(io%) were Daphnia and o were Diaptomus. Table 2shows the feeding response of chaoborid larvae at threeprey densities . Prey were provided to individual larvae atdensities of six, twelve, and twenty four of each species ;total prey densities were, therefore, eighteen, thirty sixand seventy two per larva . Diaphanosoma was consis-tently selected, comprising approximately ninety per centof all prey consumed at each of the three prey densities .

It is well documented that invertebrate predators mayselect between prey species according to size and shapedifferences of the prey (see Dodson, 1974 for a recent re-view) . Dodson (1974) concludes that much of the selec-tion occurs after the predator has captured the prey, i .e .,prey items of the wrong size or shape are rejected . He re-fers to this relationship between the behavior of the pred-ator and the size and shapes of the prey as a'Lock and Key'relationship . It is unlikely that such a mechanism wasoperating in our experiments . Such capture and rejectionshould have resulted in mangled or dead prey individualsbut such were never present in our experimental beakers .The fact that these experiments were carried out in dark-ness rules out the possibility of visual discrimination bythe predators . The high percentage of Daphnia and Diap-tomus consumed by Chaoborus when no other prey wasavailable makes it unlikely that the strong selection ofDiaphanosoma in the presence of the other two specieswas due to spatial differences in availability of the threeprey species . Therefore, we conclude that the chaoboridlarvae were discrimihating between items before striking,perhaps by detecting differences in vibrations induced inthe water by the swimming motions of the prey . Horridge(1966) has shown that these larvae can be induced to strikeat vibrating rods .

Total preyper predator

No.PredatorsTested

Diaptomus Diaphanosoma Daphnia Total

18 3 0 .3 3 .4 92% 0 .0 3 .736 11 0 .0 8 .0 90% 0.9 8 .972 3 0.0 9 .7 91% 1 .0 10 .7

Predator Diaptomusno .

Diaphanosoma Daphnia Total

1 0 10 0 102 0 9 0 93 0 9 0 94 0 11 0 115 0 5 2 76 0 8 2 107 0 7 3 108 0 4 2 69 0 8 0 8

10 0 9 0 911 0 8 1 9

Total 0 88 10 98

Our data suggest that Diaphanosoma should be parti-

cularly vulnerable to Chaoborus predation and circum-

stantial evidence from Acton Lake supports this . Winner

& Haney (1967) reported that D. leuchtenbergianum islargely confined to those areas of Acton Lake having a

depth of three meters or less . Chaoborus is abundant on-

ly in those areas of the lake having a depth greater than

three meters (Daniel, 1972 ; Winner, unpubl . data). Lewis

(1977) has concluded that a population of Chaoborus sp .

i n Lake Lanao, Phillipines exhibits a greater selectivity

for Diaphanosoma than for copepods .

References

Anderson, Stewart R . & Raasveldt, Linda G . 1974. Gammarusand Chaoborus predation . Occas . Paper 18, Canadian Wildl .Serv ., Ottawa . 24 pp .

Daniel, Paul M . 1972 . Acton Lake : Biology of its benthos andnotes on its physical limnology 1959-1970 . Ohio J . Sci . 72 :241-253 .

Dodson, Stanley I . 1974 . Adaptive changes in plankton mor-phology in response to size-selective predation : A new hypo-thesis of cyclomorphosis . Limnol . Oceanogr . 19 : 721-729 .

Horridge, G. A. 1966 . Some recently discovered underwatervibration receptors in invertebrates . In : H . Barnes (Ed .) Somecontempory studies in marine science . George Allen andUnwin, Ltd . pp . 399-405 .

Lewis, William M ., Jr . 1975 . Distribution and feeding habitsof a tropical Chaoborus population . Verh . Internat . Verein .Limnol . 19 : 3106-3119 .

Lewis, William M ., Jr . 1977 . Feeding selectivity of a tropicalChaoborus population . Freshwater Biol . 7 : 311-325 .

Roth, J . C. 1971 . The food of Chaoborus, a plankton predator,in a southern Michigan Lake . Ph . D . Thesis, Univ . Mich ., AnnArbor . 94 PP.

Swuste, H . F . J ., Cremer, R . & Parma, S . 1973 . Selective preda-tion by larvae of Chaoborus flavicans (Diptera, Chaoboridae) .Verh . Internat . Verein . Limnol . 18 : 1559 - 1563 .

Swift, Michael C . & Fedorenko, Alice Y . 1975 . Some aspects ofprey capture by Chaoborus larvae . Limnol . Oceanogr. 20 :418 -425

Winner, Robert W . & Haney, James F. 1967 . Spatial and seasonaldistribution of planktonic Cladocera in a small reservoir . OhioJ . Sci . 67 : 274-288 .

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