Feeding Behavior of Rhithrogena Pellucida (Ephemer0ptera:Heptageniidae)
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J. N. Am. Benthol. Soc., 1988, 7(2):87-99 O 1988 by The North American Benthological Society Feeding behavior of Rhithrogena pellucida (Ephemer0ptera:Heptageniidae) D. MCSHAFFREYAND W. P. MCCAFFERTY Department of Entomology, Purdue University, West Lafayette, lndiana 47907 USA Abstract. Larvae of Rhithrogena pellucida (Ephemer0ptera:Heptageniidae) were observed feeding in observation flow cells using macroscopic video techniques. The stereotypic feeding behavior is described as cycles delineated by specific movements of the labial palps and consisting of stages of coordinated movement of the entire suite of mandibulate mouthparts. When employing the Labial Brushing Cycle, larvae used the labial palps to brush loosely accreted material from the substrate. When employing the Maxillary Scraping Cycle, larvae used special pectinate setae of the maxillary palps to remove material more tightly bound to the substrate. In both cycles, the legs of the larvae also removed material from the substrate. Larvae did not perform gathering cycles or feed on deposits of detritus. Food processing by successive fields of setae on various mouthparts to the point of ingestion is described in detail. Feeding observations combined with field and laboratory data on gut content and microhabitat distribution suggest that R. pellucida feeds primarily on periphyton. Comparisons are drawn between the primary scraping function of R. pellucida and the primary brushing function of Stenacron interpunctatum. A spectrum of adaptational strategies for obtaining food materials along a gradient of substrate association from seston to the most tightly attached materials is suggested and may be used to describe the feeding roles of many macroinvertebrates. Key words: Ephemeroptera, Heptageniidae, Rhithrogena pellucida, Stenacron interpunctatum, feeding behavior, morphology, functional feeding groups, ecology. The feeding behavior and functional mor- sides the comparative data that may have rel- phology of the heptageniid mayfly Stenacron in- evance to the study of adaptation and evolution, ferpuncfatum, and new methods for studying there are also obvious ecological applications. them, were reported by McShaffrey and Mc- One that we have pursued involves the com- Cafferty (1986).To allow these data to be viewed mon practice of assigning functional feeding within a larger comparative context we subse- groups (FFG) (Cummins 1973) to the various quently studied another midwestern mayfly, members of aquatic communities. Rhithrogena pellucida. This mayfly provides an Strenger (1953) studied the functional mor- interesting comparison because it too is a stream- phology of European species of Rhithrogena and dwelling member of the family Heptageniidae included observations of behavior and inter- but lives in different habitats and possesses pretation of morphology. Strenger's work, al- somewhat different mouthpart morphology. As though important, was limited by her inability with S. interpunctatum, our objectives with R, to film behavior for detailed analysis and the pellucida were to document both the feeding fact that mouthpart morphology was not stud- behavior and feeding structures. We did not ied at the ultrastructural level. We are fortunate attempt to document the entire range of be- to have been able to extend such research by havior of the species, or to determine the nu- having SEM and videomacroscopic techniques tritional significance of the material ingested. available to us. Rhithrogena mayflies have pre- Our functional morphology/behavioral ap- viously been regarded as both collector-gath- proach to studying feeding incorporates field erers and scrapers (Cummins et al. 1984), with observations, gut content analysis, scanning the primary designation being collectors. electron microscopy (SEM), and videomacros- copy into a comprehensive research protocol. Methods This allows critical examination of questions concerning diet and microhabitat and the re- Organisms were collected in April 1986 from lated adaptations of feeding behavior and cobble substrate in the Tippecanoe River in mouthpart morphology. An important feature north-central Indiana. They were maintained of this system is its redundancy; all questions in the laboratory at room temperature (20-25°C) are investigated using multiple techniques. Be- in 75 x 30 x 30-cm aquaria filled with substrate 87 D. MCSHAFFREYAND W. P. MCCAFFERTY [Volume 7 TOP VlEW FRONT VlEW 19881 FEEDING OF RHITHROGENAPELLUClDA 89 and water from the collection site; circulation This enclosure measured 1cm long, 2 mm wide, was provided by airstones. Only mature larvae, with a water depth up to 3 cm. A hemicylinder as judged by relative wingpad development in of black plastic (radius 1.5 cm), with 30 2.5-mm- Ephemeroptera larvae (McCafferty and Huff diameter holes, was placed around the enclo- 1978), were used for observation and videotap- sure on the inside of the tank. This plastic served ing. to isolate the organism visually and reduce un- Observations were carried out using the tech- wanted reflections during filming. niques outlined in McShaffrey and ~c~affert~Water flow was provided as in the shallow (1986), with some modifications. When study- flow cell; the water entered the tank cell through ing R. pellucida, all observations were made us- a tube attached with its outlet below the water- ing visible light supplied by a fiber optic illu- line inside the plastic hemicylinder directly be- minator to maximize resolution. Our experience hind the enclosure, and exited through another showed that S. interpunctatum feeding move- tube attached to one side of the tank cell. Water ments were the same in visible and infrared level in the cell was controlled by regulating light (McShaffrey and McCafferty 1986). inflow and outflow with valves. There was no Most observations were made in an obser- noticeable water current in the enclosure. The vation flow cell as described and illustrated in tank cell was mounted in place of the shallow McShaffrey and McCafferty (1986). With R. pel- flow cell on the stage of the observational the- Iucida, onl) the shallow (1 mm deep) flow cell ater. The final position of the organism, vertical was used. Water at room temperature (20-25OC) with the mouthparts facing the glass slide and was fed into the cell by gravity from an aquar- the videocamera, was the same as in the flow ium mounted on the roof of the observational cell. theater. Water leaving the cell collected in Food for the experiments was provided as another aquarium; a pump controlled automat- described in McShaffrey and McCafferty (1986). ically by a depth-sensing switch periodically Diatoms and other periphyton collected in the returned water to the upper tank. Water in both Tippecanoe River were cultured on glass slides aquaria was aerated. The flow of water was reg- in the laboratory. The composition of the pe- ulated by a valve; current speed was adjusted riphyton community on the slides was the same as described in the study of S, interpunctatum, as the previous study-a 95% covering of the with the same inherent difficulties in deter- diatoms Cocconeis and Achnanthes, numerous mining precise current speed (McShaffrey and bacteria between the diatoms, and the remain- McCafferty 1986). In most cases the flow was ing 5% consisting of scattered fungal hyphae, increased until the individuals began to orient Oscillatoria, Cladophora, Melosira, Fragilaria, Chlo- to the current, this usually occurred at low cur- rella, rotifers, and protozoa. Scanning electron rent velocities (1-5 cm/s). micrographs of typical growth on a slide surface In addition to the shallow flow cell, a tank are shown in Figure 2. Detritus taken from the cell was also employed. The tank cell (Fig. 1) holding tanks was introduced into the water was constructed of 2-mm-thick acrylic, 10.5 cm flow in the shallow flow cell at both high (>5 long, 4 cm wide, and 4 cm deep, and held a cm/s) and low (<5 cm/s) flow rates; detritus volume of 150 ml of water when in use. The was placed directly into the enclosure of the front face had a 1 x 3-cm rectangular section tank cell. Detritus settled to the bottom of the removed from the center; a portion of a stan- tank cell and the shallow flow cell at low flow dard glass microscope slide was attached over rates; at high flow rates detritus moved through the outside of this hole, and a 1-mm2mesh screen the shallow flow cell in suspension. We consid- was attached over the inside to form an enclo- ered feeding effective if ingestion of the ma- sure into which the individuals were placed. terial was observed. FIG.1. Tank cell used in feeding experiments with R. pellucida, drawn to scale. Unlabelled arrows show direction of water flow. Abbreviations: BP = black plastic, EO = experimental organism, FP = front plate, MS = microscope slide (viewing port), SC = screen (1mmz),WI = water inlet, WL = water line, WO = water outlet, WR = water reservoir. 90 D. MCSHAFFREYAND W.P. MCCAFFERTY [Volume 7 FIG. 2. SEM micrographs of the surface of a glass microscope slide used in feeding experiments. a. Plan view, bar = 100 pm. b. Oblique view showing vertical distribution, bar = 10 pm. Motion analyses were carried out by exami- tortion of mouthparts and is much faster than nation of the videotape recordings of the ob- critical point drying. servation sessions as in McShaffrey and McCaf- Gut contents of living larvae were placed on ferty (1986). Owing to sparseness of specimens glass microscope slides and examined imme- in the field, the need to sacrifice