In many fish and amphibian species, vast numbers of embryos may hatch at the same time. In such situations, the hatchlings can be exposed to intensive cannibalistic interactions from conspecifics (members of the same species). How do hatchlings spend this vulnerable life stage?
Cannibalism success of the Japanese Ezo or Hokkaido salamander species (Hynobius retardatus) is highly dependent on the balance between the gape width of the cannibal (how wide it can open its mouth) and the head width of its prey, so fast growth in the pre-feeding stage is expected to contribute strongly to the survivorship of the salamander hatchlings in conspecific interactions. In this study, the authors report experimental evidence showing adaptive acceleration of growth and development in the pre-feeding hatchling stage. Ezo salamander hatchlings reared with conspecifics became larger and developed faster than those reared alone, the time to the start of feeding was shorter, and the burst swimming speed for hatchlings reared with conspecifics was faster.
The predation trials revealed the advantages of growth and developmental acceleration in cannibalistic interactions. The hatchlings reared with conspecifics were more successful at cannibalizing small hatchlings and were also highly resistant to being cannibalized themselves by large conspecifics, compared to hatchlings reared alone. Because salamander larvae that cannibalize other individuals in their early developmental period exhibit rapid growth and metamorphose early with larger size, growth and developmental accelerations are likely key mechanisms for their life history success.