Important morphological and ecological modifications occur during the transition between pelagic and demersal phases in marine fish. However, it is still unknown how fast these shape changes may occur. We studied the shape changes of a common cryptobenthic fish, the triplefin Helcogrammoides chilensis (Cancino, 1960) during the shift from pelagic larvae to recently settled individuals, along rocky shores in central Chile during the austral summers of 2020 and 2021. The working hypothesis was that larval stages would show more allometry and faster shape changes in the head and the paired fins insertion than benthic juveniles, in preparation for their new environment. Shape changes were analyzed utilizing landmark-based geometric morphometrics, while age was estimated using sagittal otolith microstructure analysis. There was an important overlap in the size (length and weight) between older larvae and recently settled individuals (between 20 and 25 mm SL, and 0.08–0.17 g), nonetheless, the head shape and paired fins were clearly different between stages. Pelagic larvae (46–88 days post hatch) had a shorter pectoral fin base, a frontal mouth opening, and eyes located at the level of the tip of the upper jaw. Meanwhile, recently settled individuals (80–112 days post hatch) had wider, vertically positioned pectoral fins, mouths displaced to a vertical position, and eyes located upper and forward the head. Larvae experienced faster growth rates than settlers (0.24 vs. 0.02 mm day−1, respectively), and the pattern of ontogenetic shape changes decreased two orders of magnitude after settlement. It is plausible that after the pelagic–demersal shift most of the fish’s energy was used in body structure rearrangement and incrementing body pigmentation, as an adaptation of cryptobenthic juvenile to the rocky reef.
- Pelagic–demersal shift