Surface Sensilla on the Predaceous Fresh-Water Leech Erpobdella montezuma: Possible Importance in Feeding

Sensilla of the predaceous fresh-water leech Erpobdella montezuma were examined by scanning and transmission electron microscopy. Densities of sensilla and cilia within each sensillum were highest in the anterior-most annulus and were less in more posterior annuli. The most dramatic decrease was between annulus 1 and 3; there was little difference in densities of each from midbody to the posterior region. The highest densities of sensilla were found on the posterior sucker (x = 48.0 x 103/cm2 ) and annulus 1 (x = 36.1 x 103/cm2). In our preparations, cilia near the mouth and on the sucker were flush with the epidermal surface, but those in other regions protruded. Arrangement of G (grouped) and S (single) cilia along the longitudinal axis of the leech are described; the G cilia typically comprise 70-90% of the total, except on the dorsal posterior end where G cilia are ca. 46% of the total. The ultrastructure of G and S cilia was examined. Feeding experiments comparing response to prey species that swim in the water column with response to a more benthic congeneric prey species suggests that movement by prey is important in their detection by this leech. We postulate that the sensilla are important in detecting these movements. Leeches have a variety of sensory receptors in the body wall including mechanoreceptors, eyes, and segmental sensilla (Blackshaw & Nicholls, 1979; Derosa & Friesen, 1981; Mann, 1962; Nicholls & Baylor, 1968). Ciliated sensory cells have been observed on the cuticle of leeches in two genera: Batracobdella (see Desser & Weller, 1977) studied by transmission electron microscopy, and Hirudo medicinalis (see Derosa & Friesen, 1981) by scanning electron microscopy. The importance of these ciliated sensory cells in responding to low amplitude waves was suggested by Derosa & Friesen (1981). Friesen (1981) demonstrated experimentally that the filiform structures within sensilla of Hirudo medicinalis are "likely candidates" for transducing structures. Receptor organs with many cilia also are present on the surface of the earthworm Lumbricus terrestris (Aros et al., 1978), and the oligochaete Branchiobdella pentodonta (Farnesi et al., 1982). The present paper examines the surface sensilla of the recently described predaceous fresh-water leech, Erpobdella montezuma (Davies et al., 1985) and explores the potential relationship of these structures to feeding. An earlier paper by Blinn & Dehdashti (1984) identified this species as Erpobdella punctata. Erpobdella montezuma feeds at night in Montezuma Well, Arizona, on the immature stages (<4 mm) of the amphipod, Hyalella montezuma. This unique amphipod (Cole & Watkins, 1977) swims in the open water column and feeds on the nannoplankton (Blinn & Johnson, 1982). Our study of the 1 We thank Behrooz Dehdashti for his assistance in the feeding studies. 2Publication costs, in part, are being met by a grant from the Spencer-Tolles Fund of the American Microscopical Society. TRANS. AM. MICROSC. SOC., 105(1): 21-30. 1986. ? Copyright, 1986, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:20 UTC All use subject to http://about.jstor.org/terms 22 TRANS. AM. MICROSC. SOC. This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:20 UTC All use subject to http://about.jstor.org/terms VOL. 105, NO. 1, JANUARY 1986 distribution of cuticular sensilla, and some aspects of their ultrastructure, is presented with a view toward elucidating the ability of E. montezuma to detect the presence of these immature amphipods at night. MATERIALS AND METHODS Specimens of Erpobdella montezuma were collected after sunset from within the top 1 m of the water column in Montezuma Well, Arizona. Montezuma Well is a unique solar-regulated chemostat in northern Arizona with an annual mean temperature of 21.1?C ? 4?C (Boucher et al., 1984). Leeches were fixed in 4% glutaraldehyde and 0.05 M phosphate buffer (pH 7.2) at room temperature for 1 h. Specimens were washed three times in 0.05 M phosphate buffer at room temperature over a 30-min period and cut into 0.5-1.0-cm lengths. Post-fixation was in 1% OsO4 in 0.05 M phosphate buffer for 30 min. Entire dissected leeches were dehydrated in an ethanol series. Tissues for scanning electron microscopy (SEM) were dried from CO2 by the critical-point method (Anderson, 1951). The specimens were coated with ca. 20 nm of gold-palladium and examined with an AMRay 1000 SEM. Specimens prepared for transmission electron microscopy (TEM) were subjected to the same fixation and dehydration procedures as described for SEM. Minute sections were embedded in BOJAX epoxy mixture (Grim, 1967), sectioned with glass knives, double stained, and examined with a JEM 7A TEM. Laboratory experiments were conducted to determine the ability of E. montezuma to detect prey with different swimming behaviors. Two congeneric amphipod species, the benthic, Hyalella azteca and the endemic swimming species, H. montezuma, were used as food in attempt to explore the possible importance of mechanoreception in feeding. Ten amphipods of similar size (2-3 mm) of each species were placed in separate containers with 500 ml of water from Montezuma Well. Each container had a 15-cm column. All feeding experiments were conducted in the dark for a 6-h period with one leech (0.51.0 g wet wt.) introduced into each experimental container. Numbers of amphipods remaining in each container were counted at the end of each feeding experiment. A total of 12 experimental paired feeding trials were conducted. FIGS. 1-6. Scanning electron micrographs of the surface of Erpobdella montezuma. Fig. 1. Anterior end showing a high concentration of sensilla above (arrows) and dorsal to the mouth. Scale bar represents 300 ,m. Fig. 2. Higher magnification of annulus 1 reveals the ciliary components of sensilla, their location in a concavity of the epidermis, the high concentration of sensilla, and a wrinkled surface which may be an artifact of critical-point drying. Scale bar represents 20 ,um. Fig. 3. Sensilla in annulus 1 with some cilia apparently adherent along their length and a moderate number of single cilia (arrow) of varying lengths. Scale bar represents 3 Am. Fig. 4. Sensilla in annulus 3 are in rows and much less concentrated than in the anterior regions. Scale bar represents 30 Am. Fig. 5. Higher magnification of a sensillum from midbody. Most cilia are in groups (G) of two or more and a few single cilia are present; the single cilia often are longer (arrow). Scale bar represents 2 ,m. Fig. 6. In the midbody, most sensilla are in rows and on distinct convexities (arrow) of the surface. Scale bar represents 300 um. 23 This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:20 UTC All use subject to http://about.jstor.org/terms TRANS. AM. MICROSC. SOC. t^ -^??sr^a .. . . 9 ,.,' r tl_ FIGS. 7, 8. Scanning electron micrographs of the surface of Erpobdella montezuma. Fig. 7. Higher magnification of a single convexity and sensillum. Fig. 8. A sensillum from the dorsal posterior (annuli, 110-120) region of the body. Scale bar for Figs. 7, 8 represents 5 gim. FIGS. 9, 10. Transmission electron micrographs of cilia within a sensillum of Erpobdella montezuma. Fig. 9. Each single cilium appears to contain a "9+2" microtubule axoneme. The 10 surrounding microvilli are interconnected with a node-filament complex (arrow). More peripheral microvilli contain distinct, centrally located, material (C). Fig. 10. Grouped cilia, or G-cilia contain nine doublet peripheral microtubules near the cell surface (*). More distally, one of these nine doublet microtubules (arrow) appears to be dislocated centrally. G-cilia also are surrounded by some microvilli containing a core (C). In this region, G-cilia do not appear to be fused. Scale bar for Figs. 9, 10 represents 0.25 inm. 24 This content downloaded from 157.55.39.231 on Wed, 05 Oct 2016 04:16:20 UTC All use subject to http://about.jstor.org/terms VOL. 105, NO. 1, JANUARY 1986