Gut morphology and relative gut length do not reliably reflect trophic level in gobiids: a comparison of four species from a tropical Indo‐Pacific seagrass bed

Trophic level and gut morphology are closely related in fish, and relative gut length (RGL) is a commonly accepted correlate of fish diet (Al-Hussaini, 1947; Geevarghese, 1983; Elliott and Bellwood, 2003; Karachle and Stergiou, 2010). Generally, gut length reflects diet as predators commonly have short and simple guts, while in omnivores and herbivores they are long and complex, reflecting digestibility and nutrient content of a species’ preferred food item (e.g. Al-Hussaini, 1947; Elliott and Bellwood, 2003; Karachle and Stergiou, 2010). Most species of Gobiidae are zoobenthivores, but their diet may also contain algae and organic matter (Stergiou and Karpouzi, 2002; Nakamura et al., 2003; Hernaman et al., 2009; Zander, 2011). This complicates assigning species explicitly to a trophic guild. Nevertheless, species have been attributed to trophic guilds based on food preferences (e.g. Horinouichi and Sano, 2000; Nakamura et al., 2003; Nanjo et al., 2008). Cryptocentrus spp., for example, are microcarnivores (Karplus, 1987; Thacker and Roje, 2011), Amblygobius and Valenciennea spp. feed on detritus and interstitial sand fauna, with A. phalaena additionally feeding on filamentous algae (Nakamura et al., 2003; Nanjo et al., 2008). Austrolethops wardi is the only known gobiid nearly exclusively relying on seagrass (Kneer et al., 2008; Liu et al., 2008). Accordingly, we tentatively expected long guts in the omnivorous A. phalaena and V. sexguttata and herbivorous A. wardi, and short guts in the zoobenthivore C. cinctus. We tried to identify trophic levels in four sympatric Indo-Pacific gobiids (Amblygobius phalaena, Austrolethops wardi, Cryptocentrus cinctus, and Valenciennea sexguttata) by comparing gut morphology and relative gut length sensu Al-Hussaini (RGL; 1947).