Whitish film covering a newborn white shark was not intrauterine material but embryonic epithelium

We provide an alternative interpretation for the “whitish film” that covered the young white shark observed with an aerial drone in a paper published recently (Gauna and Sternes, Environ Biol Fish 107:249–254, 2024). The paper claimed that this whitish film was a leftover intrauterine substance (e.g., uterine milk, which is a maternal secretion to nourish embryos), thus suggesting that the individual was a newborn. However, we are skeptical of this interpretation for the following reasons: (1) our previous studies showed that the secretion of intrauterine substances in white sharks ceases in mid-gestation and, (2) even if the secretion continues until birth, the substance is unlikely to be retained on the skin for a long time after birth. Based on data from Lamna ditropis, a close relative of the white shark, we hypothesize that the whitish film is an embryonic epithelium that covers the surface of skin denticles. This reinterpretation does not alter but reinforces the major conclusion of (Gauna and Sternes, Environ Biol Fish 107:249–254, 2024) that the individual sighted is a newborn.