We verified changes in the burrowing behavior of yellow-phase Japanese eels depending upon substrate type by observing burrowing by individuals in the following laboratory experiments: 1) flat sand (approximately 1 mm) substrate; 2) a small rock (approximately 60 mm in long diameter and 20 mm in short diameter) on flat sand substrate; and 3) five small rocks on flat sand substrate. Burrowing behaviors with rapid body undulations were observed least in sediment type 3 (five small rocks on a flat sand substrate) than in the others. Burrowing behavior tended to successful on sand-small rock substrate (type 2 and type 3) compared to sand alone (type 1). We found that eels often excavated the sand-small rock substrate when burrowing by the leveraging their bodies against the edges of small rocks. This behavior enabled eels to hide within the substrate without having to use the rapid body undulations to bury themselves. These results suggest that higher densities of yellow eels found distributed on gravel and rock substrates in rivers are probably because yellow eels can save time when burrowing and avoid the use of rapid body undulations when doing so.
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