By Dr. Jeff Schmid | Conservancy Environmental Research Manager
Some marine turtles have a “diet of glass”, but this is not the eating disorder of hyalophagia or a circus sideshow performance. Rather, this phrase was coined by Dr. Anne Meylan regarding her research on the consumption of marine sponges by hawksbill turtles. Green turtles and, to a lesser degree, loggerhead turtles may also prey on sponges but they are not an important food item in their respective diets. Kemp’s ridley turtles are primarily crab-eaters but other interesting dietary habits have been reported in the literature. For example, Kemp’s ridleys in the Ten Thousand Islands of southwest Florida ate sandy-skinned tunicates (i.e., sea squirts) like popcorn with some diet samples containing over one hundred individual tunicates. A few of the turtles had also consumed an unidentified sponge and neither of these sessile (i.e., permanently attached and immobile) organisms have been reported in any previous diet studies for Kemp’s ridleys.
Marine sponges represent some of the oldest animals inhabiting earth, with fossil records dating back to the Cambrian explosion some 530 million years ago. These primitive organisms lack nervous, digestive, and circulatory systems, relying instead on a number of specialized cell types to perform their life functions. Most sponges have a skeleton of sorts comprised of tough protein fibers (spongin) that are typically embedded with tiny glass-like structures called spicules. Among other characters, the size and shape of the spicules can be used in identifying different types of sponges. The spiculated spongin skeleton (try saying that five times fast!) provides a formidable defense but there are animals such as the hawksbill turtle that specialize in eating sponges, hence a diet of glass.
During recent studies, Kemp’s ridley diet samples collected in the Ten Thousand Islands contained numerous fragments of a yellowish-green material with a fibrous consistency. These fragments were the dominant food item in the samples. Microscopic examination of the material revealed the presence of sponge spicules and their rod-like shape with pointy ends, technically referred to as “oxeas”, was characteristic of sponges in the genus Halichondria. Dr. Klaus Rützler, renowned spongiologist with the Smithsonian National Museum of Natural History, confirmed the identification and further identified a sponge fragment collected in the study area as Halichondria cf. melanadocia (unfortunately, no common name available). The “cf.” means that Dr. Rützler was confident with the genus designation of the scientific name but the species identification was problematic and the specimen resembled that of another known sponge species. Morphological features other than spicules are used for sponge identifications and these features were not apparent in the digested or frozen samples.
Subsequent diet samples for Kemp’s ridleys captured in the study area contained fragments of yet another type of sponge. Unlike the fibrous material of the Halichondria samples, this other sponge had a hard outer crust that looked like tiny glass spheres under the microscope. These spherical spicules are referred to as “sterrasters” and are characteristic of the sponge genus Geodia. Inside the sponge fragments were densely packed needlelike spicules (oxeas) of different sizes, some of which had multiple points radiating on one end. Based on these morphologies, Dr. Rützler identified these ingested fragments as Geodia cf. neptuni. This species is referred to as leathery barrel sponge and grows in many different forms. Interestingly, one of the Kemp’s ridleys from this round of sampling was a recapture that had shifted its consumption of Halichondria to that of Geodia after 324 days at large.
With this newfound knowledge, archived Kemp’s ridley diet samples collected in the Ten Thousand Islands during 1999–2000 were re-examined and the unidentified sponge consumed by turtles in this earlier study was identified as Geodia. Additionally, a diet sample that had been classified as containing an unidentified food item was identified as the sponge Halichondria. Hawksbill turtles throughout the Caribbean Basin feed upon the sponge taxa identified herein but we provided the first documentation of their consumption by Kemp’s ridley turtles. Furthermore, it appears this spongivorous feeding habit is unique to Kemp’s ridleys inhabiting the Ten Thousand Islands. Questions remain whether Kemp’s ridleys derive any nutritional benefit from consuming sponges or if consumption of novel food items such as tunicates and sponges is in response to limited availability of other prey such as crabs.
This blog was based on a natural history note co-authored with Dr. Rützler and published in the September 2021 issue of Herpetological Review. We thank David Shindle, Greg Curry, Leif Johnson, and the many volunteers who assisted with turtle capture and sample collection. Research activities were conducted in the Rookery Bay National Estuarine Research Reserve and performed under NMFS permit #18069 and FFWCC permit #136. Our study was funded by private contributions to the Conservancy of Southwest Florida and by research grants awarded from the Sea Turtle Grants Program, which is supported by proceeds from the sale of the Florida Sea Turtle License Plate. Learn more at www.helpingseaturtles.org.