Published On: Mon, Jan 14th, 2019

Influence of substrate orientation on tadpoles’ feeding efficiency [RESEARCH ARTICLE]

INTRODUCTION

The external oral apparatus of most anuran larvae is comprised of a soft, marginally papillated oral disc that surrounds keratinized jaw sheaths and rows of keratinized labial teeth (McDiarmid and Altig, 1999). The function of the marginal papillae is not well established, but some studies suggest that they can facilitate certain tadpoles’ ability to adhere to substrates in lotic environments (e.g. Altig and Johnston, 1989). However, the keratinized structures of tadpoles are well described because of their use in anuran systematics (e.g. Orton, 1953; Starrett, 1973; Vera Candioti, 2007). The keratinized jaw sheaths and labial tooth rows lie anterior and posterior to the oral opening. Those keratinized structures are used by tadpoles to scrape or bite organic material off the substrate as food (Wassersug and Yamashita, 2001). The keratinized and soft structures vary substantially in complexity among species as both labial tooth rows and marginal papillae can vary in size, arrangement, and configuration (McDiarmid and Altig, 1999; Altig, 2007).

Morphological variation in oral structures of vertebrates usually reflect the resources that are consumed [e.g. bird beaks reflect the type of food they eat (Darwin, 1859; MacArthur, 1958)]. Consequently, morphological variation in oral structures typically correlates with dietary niche (Begon et al., 2006). However, this is not the case for many tadpoles. Tadpoles of different species, each with diverse oral structures, can coexist in a single pond during the same season, yet individuals of each species have similar gut contents (Rossa-Feres et al., 2004; Prado et al., 2009). This finding suggests that tadpoles are dietary generalists, and it raises questions related to resource partitioning in diverse community assemblages of tadpoles. Are tadpoles of many species able to use the same resources in the same place and at the same time without one species outcompeting the other?

Our working hypothesis is that anuran larvae divide up the environment not necessarily in terms of the food that they feed on, but in their efficiency for grazing upon various surfaces, each of which have different intrinsic physical properties. These substrate properties could include orientation, firmness and texture. Here we explore the first of these properties and ask, ‘how does the orientation of substrates affect tadpoles’ feeding efficiency?’ When anuran larvae differ in their efficiency to feed on a particular surface, those efficiency differences might affect growth and development. If so, less efficient species may explore other microhabitats to avoid competition with more efficient species (Alford, 1986).

Morphological characteristics related to locomotion and feeding are key to a species’ ability to exploit the physical dimensions of a microhabitat (Higham, 2007). For example, the morphology of lizard digits is associated with climbing ability and thereby determines where they are able to feed [e.g. on the side of rocks or the underside of branches while upside down (Irschick et al., 1996; Higham and Jayne, 2004)]. This relationship between locomotor morphology and substrate utilization can apply to aquatic vertebrates as well. Aquatic salamanders are able to adjust the elevation of their heads to capture prey in the water in different orientations (Shaffer and Lauder, 1985). Similarly, cichlid fish have the ability to swim in different positions and to adjust the orientation of their bodies and oral apparatus to acquire food from substrates oriented at different angles (Rupp and Hulsey, 2014). Among tadpoles, differences in feeding efficiency exist even for species that have similar feeding behavior. When feeding upon suspended particles, species differ in rates of particle capture and also in efficiency at gathering particles of different sizes (Seale and Wassersug, 1979; Seale et al., 1982). Thus, it is possible that variation in the keratinized oral structures of tadpoles either limits or facilitates their ability to remove food from substrates at different orientations. If so, differences in feeding efficiencies on various substrates may thus both force and enable tadpoles to partition the environment even when the food matter growing on the surfaces may be abundant and the same.

We manipulated the angle at which food was offered to tadpoles and tested how this angle (i.e. orientation) affected the feeding efficiency of tadpoles from several species that differ in their oral morphology. We hypothesized that species (1) differ in their ability to graze in surfaces at different orientations and (2) partition their habitat use based on the orientation of the surfaces on which they graze most efficiently. We predicted that bottom-dwelling species would be more efficient removing food from horizontal surfaces, as these tadpoles are usually negatively buoyant and typically forage on the bottom of ponds (Altig and Johnston, 1989; McDiarmid and Altig, 1999). Conversely, we expected that tadpoles that are more commonly found in the water column would be better able to acquire food from vertical and sloping surfaces, as they usually graze upon stems and leaves above the bottom surface. We also predicted that tadpoles that share the same microhabitat, but have different oral morphologies, would differ in their feeding efficiency in relation to substrate orientation. Species, for example, with smaller oral discs and fewer keratinized structures may have more flexible oral discs (Altig, 2006). These more flexible disks may permit them to feed more efficiently upon more contoured surfaces, such as the vertical stems of aquatic plants. Conversely, species with a higher number of keratinized structures usually have a larger oral apparatus, which is in general ventrally oriented, so they may be more efficient feeding on horizontally oriented substrates. Finally, because species that occur at the same depth of the water column and have similar oral morphologies usually share the same resources, we predicted that these species would feed on the same orientations with a similar efficiency.

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