The ontogeny of morphological defenses in Kemp's ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) sea turtles

Abstract

Marine turtles are large reptiles that compensate for high juvenile mortality by producing hundreds of hatchlings during a long reproductive lifespan. Most hatchlings are taken by predators during their migration to, and while resident in, the open ocean. Their survival depends upon crypticity, minimizing movement to avoid detection, and foraging efficiently to grow to a size too difficult for predators to either handle or swallow. While these behavioral antipredator tactics are known, changes in morphology accompanying growth may also improve survival prospects. These have been only superficially described in the literature. Here, we compare the similarities and differences in presumed morphological defenses of growing loggerhead (Caretta caretta) and Kemp's ridley (Lepidochelys kempii) posthatchlings, related species that differ in growth rate, timing of habitat shift (the return from oceanic to neritic locations), and size at maturity. In both species, vertebral spination and carapace widening increase disproportionally as small turtles grow, but later in ontogeny, the spines regress, sooner in ridley than in loggerhead turtles. Carapace widening occurs in both species but loggerheads are always longer than they are wide whereas in Kemp's ridley turtles, the carapace becomes as wide as long. Our analysis indicates that these changes are unrelated to when each species shifts habitat but are related to turtle size. We hypothesize that the spines function in small turtles as an early defense against gape-limited predators, but changes in body shape function throughout ontogeny-initially to make small turtles too wide to swallow and later by presenting an almost flat and hardened surface that large predators (such as a sharks) are unable to grasp. The extremely wide carapace of the Kemp's ridley may compensate for its smaller adult size (and presumed greater vulnerability) than the loggerhead.

Biphasic allometric growth in juvenile green turtles Chelonia mydas

“…An initial widening during early growth has also been documented in 4 other marine turtle species: loggerheads Caretta caretta and Kemp's ridley Lepidochelys kempii (Salmon et al 2015); flatbacks Natator depressus (Salmon et al 2016); leatherbacks Dermochelys coriacea (Pate & Salmon 2017). How-ever, all these measurements stemmed from laboratory-reared turtles.…”

“…Carapace widening is weakly (but significantly) developed in leatherbacks (Pate & Salmon 2017) and is more pronounced in young flatbacks than in young green turtles (Salmon et al 2016). It is most strongly expressed in juvenile Kemp's ridley turtles that, from an early age, possess shells that are as wide as they are long (Salmon et al 2015), a condition that uniquely persists even in adults (Pritchard 2007).…”

The early ontogeny of carapace armoring in hawksbill sea turtles (Eretmochelys imbricata), with comparisons to its close relatives (Loggerhead, Caretta caretta; Kemp's ridley, Lepidochelys kempii)

“…We hypothesized that given their divergence times, it was likely that form differences might more strongly characterize how hawksbills differ in morphology from loggerhead and ridley turtles, whereas loggerheads and ridleys were most likely to differ from one another in emphasis. There is preliminary evidence that supports this hypothesis (Palaniappan, ; Salmon et al, ). Here, we provide additional morphological data from all three species that appears to strengthen that hypothesis.…”

“…Loggerhead and ridley turtles used for this study were individuals reared at the National Oceanic and Atmospheric Administration (NOAA) Sea Turtle Facility located in Galveston, Texas, USA (Lat 29.30N, Long 94.79W). Husbandry procedures are described in Higgins () and Salmon et al (). Loggerheads were reared for 3 years and ridleys for 22 months before they were used in trials to test the efficacy of Turtle Excluder Devices (TEDs).…”

“…Early growth in marine turtles results in allometric (disproportionate; Huxley, ) changes in body dimensions (hereafter, “shape”). Those shape changes in some species are coupled with the development of spines that result in small juveniles that differ in appearance from both hatchlings and adults (Salmon, Higgins, Stewart, & Wyneken, ). These changes likely function in defense, especially against gape‐limited predators (Salmon & Scholl, ).…”