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The Effects of Extended Photoperiods on the Drift of the Mayfly Ephemerella Subvaria Mcdunnough (Ephemeroptera : Ephemerellidae)

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The Effects of Extended Photoperiods on the Drift of the Mayfly Ephemerella Subvaria Mcdunnough (Ephemeroptera : Ephemerellidae) Hydrobiologia vol . 62, 3, pag . 209-214, 1979 THE EFFECTS OF EXTENDED PHOTOPERIODS ON THE DRIFT OF THE MAYFLY EPHEMERELLA SUBVARIA MCDUNNOUGH (EPHEMEROPTERA : EPHEMERELLIDAE) Jan J . H . CIBOROWSKI* Department of Zoology, Erindale College, University of Toronto, Mississauga, Ontario, Canada . L5L iC6 * Present address : Department of Zoology, University of Alberta, Edmonton, Alberta, Canada . T6G 2E9 Received March 20, 1978 Keywords : Drift, photoperiod, Ephemeroptera, development . Abstract of the organism . MUller (1965) showed that the nocturnal drift pattern of Baetis rhodani was directly related to the Field drift studies indicated that the nocturnal drift density of E . length of the dark period, however the effect of daylength subvaria nymphs was greater in early May than in early Novem- on subsequent nocturnal drift has not been elaborated . ber . This study examines the effect of extended photoperiods Laboratory studies showed that the number of individuals ap- pearing in the drift was a linear function of the duration of the on the drift of the mayfly Ephemerella subvaria Mc- preceding photoperiod . Nymphs had a greater propensity to Dunnough. drift when they were not in a state of active growth than when they were growing . The tendency of individuals in a single laboratory population to drift was observed to change under conditions of constant temperature and randomized photoperiod . This sug- Methods and materials gests that the shift was due to some internal physiological change rather than to an external cue . Ephemerella subvaria nymphs were collected for both life It is suggested that drift in E. subvaria functions as a method history analyses and experimental purposes from a riffle of relocation from fast-water areas to slow-water pools and stream margins . Redistribution to these areas may reduce on the East branch of the Credit River, near the Forks of mortality incurred during spring run-off and during emergence . the Credit, Peel County, Ontario, Canada . Specimens were collected from mid-October, 1974 to June, 1975 and from September 1975 until the beginning Introduction of November, 1975 . Samples were taken at weekly inter- vals during the months of September, October and No- The importance of light intensity in controlling the diel vember and biweekly during other months . All indivi- periodicity of invertebrate behavioural drift is well duals were preserved in Kahle's fluid . Head width mea- established (MUller, 1966 ; Holt & Waters, 1967 ; Elliott, surements were made using a Wild dissecting microscope 1967). Drift normally remains at a relatively low, constant at 25 x magnification with an inset ocular micrometer . level during daylight hours (Chutter, 1975) and increases Field investigations were conducted on May 2-3 and on following a reduction in light intensity below a certain November 1-2, 1975 to determine nocturnal patterns and critical level (Bishop, 1969; Chaston, 1969 ; Ciborowski, seasonal variation in the drift of E . subvaria . A square- 1976). mouth drift sampler (Anderson, 1967) with an opening The intensity and pattern of drift during darkness is of 625 cm 2 and 256-micron mesh was secured in fast dependent largely on the particular behavioural response water approximately 35 cm deep and 1 .5 m from the stream bank . Current velocities at the net mouth were determined with a Gurley Pygmy current meter at the Dr. W. Junk b .v. Publishers - The Hague, The Netherlands beginning and end of each study . 209 Studies began at least 30 minutes before sunset and the photoperiod experiment data to study the relation- continued for a period of 6 hours after sunset . Catchment ship between numbers of individuals drifting during dark- jars were changed at 3o-minute intervals throughout ness and the duration of the preceding photoperiod . each experiment and the contents preserved in Kahle's Analysis of covariance (Snedecor & Cochran, 1961) was fluid . All samples were completely sorted in the laborato- used to test for differences in regression coefficient and ry at 12 x magnification using a Wild dissecting micro- intercept between replicates . scope . Kolmogorov-Smirnov tests of goodness-of-fit (Sokal Laboratory experiments were carried out in a con- & Rohlf, 1969) were used to establish whether field and trolled-environment room using an artificial stream laboratory drift patterns were random and whether they driven by a plexiglass paddlewheel . A depth of io cm of significantly differed from one another . dechlorinated tap water was used throughout . Mean cur- rent velocity in the stream was 1o cm/sec over a 20 x 30 cm area of gravel substrate (pebbles 16-23 mm diameter) . Results The rest of the stream channel was bare . A removeable drift net was located 2 cm downstream from the sub- Head width measurements indicated that E. subvaria strate . nymphs in the Credit River grew rapidly throughout the One hundred and fifty live E. subvaria nymphs, trans- autumn until the end of November (Fig. 1) . Little growth ported from the field in wide-mouth vacuum bottles was evident during the winter, although individuals did containing river water, were introduced into the stream not become dormant . Growth resumed towards the on October 30, 1974 and maintained under simulated beginning of March and subimagoes were first observed natural photoperiods for 3 days. The nymphs were then emerging from the river on May 10 . Further details on subjected to a randomized series of continuous light ex- the life history of E. subvaria are given by Ciborowski posures ranging from 3 to 14o hours in length . Photo- (1976). periods used were of the following durations : 3 hours, 5 Field drift densities of E. subvaria nymphs (numbers hours, 6 hours and one-hour increments up to 16 hours, per m' of water sampled) were much higher in May than and 20 hours, 4o hours and 2o-hour increments up to 140 in November (Fig. 2b) . A large increase occurred after hours. Following each light period, the stream was sunset for the May study but not for that in November . darkened and the drift net placed in the stream. There- The pattern of drift was random in November (p > .05) after, the net was changed at 3o-minute intervals over a 6 and significantly non-random in May (p < .05), maxi- hour period . Trapped individuals were stored in enam- mum density being reached approximately 2 hours after elled trays containing aerated water, and returned to the sunset. stream at the conclusion of each trial. Mortality in the two photoperiod experiments was The experiment was repeated using nymphs collected comparable ; 121 nymphs were recovered after the first March 17, 1975 . A new order of trials was selected for the series of trials and 118 after the second . It is assumed that replicate, although photoperiods were of the same dura- most of the mortality occurred during the beginning of tion as in the first study . each experiment as a result of handling . Very few dead A third group of nymphs collected December 13, 1975 nymphs were recovered in the drift net, and these were was placed in the stream and maintained under conti- predominantly found during the first few trials of each nuous light for a total of 1215 hours. Drift was then series . monitored during 6 hours of darkness . Linear regression accounted for 62 percent of the varia- Water temperature in the first experiment was 5°C and tion on the November photoperiod experiment . Closer in the second and third experiments was 2°C. In each examination of the data, however, indicated that a rela- case, laboratory temperatures were within 2 degrees of tively low number of individuals drifted in trials con- that in the field at the time of collection . ducted before November 26 whereas far more drifted at An ubiquitous growth of an aquatic hyphomycete in later dates . Regressions calculated for the early and late the artificial stream provided a food supply in all trials . portions of the data separately, explained 85 and 93 per- This was adequate to maintain the last group of nymphs cent of the experimental variation, respectively (Fig. 3) . through the winter to maturity . Slopes of the two regressions were significantly different Least-squares regression analysis was performed on (p < .005) . 210 2.6 E . subvaria EMERGENCE 2.4 2 .2 E E 20 I I f 1 .8 I ~J~ ~ff~j I F- 1 I .6 0 I 1 .4 1 .2 I ' 0 Q 1 .0 w f 2 0 .8 o • 1974 z 0 .6 Q ∎ 1975 w 0 .4 0 .2 0.0 SE PT . OCT NOV_ DEC JAN . FEB . MAR APR MAY Fig . i . Mean head width of E. subvaria nymphs collected from sample site during 1974 and 1975 ± i standard devia- tion . The March photoperiod experiment was terminated at trials was 96, 117 and 267 individuals during early Novem- its midpoint following a power failure which disrupted ber, late November and March, respectively . Clearly, at light, water current and temperature controls . Regression least some individuals drifted on more than one occasion of the available data explained 99 percent of the variation . in each experiment . Analysis of covariance indicated that the regression of the The 3o-minute values for drift of trials in each of these March data was significantly different from the early data groups were pooled to give an idea of the general November data in slope (p < . o1) and from the late No- distributions of nymphs drifting with respect to time vember data in intercept (p < . 005). (Fig. 2a) . Drift pattern was random for the early Novem- Nymphs which entered the drift were not returned to ber data (p > . 05) and significantly non-random (p < .oi) the stream until the end of that trial, and as a result were for the other two series. When laboratory and field pat- unavailable to re-enter the drift later on during the same terns were compared, the only significant differences were trial .
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