Sea urchins are marine invertebrates that look like spiny pin cushions. They live in coastal areas and eat large amounts of underwater plants, such as seagrasses. Since seagrass plants can differ in taste and texture, sea urchins face many decisions about which seagrass plants to munch on. In our research, we wanted to explore which seagrass traits are preferred by sea urchins. The results of this work will help researchers understand factors influencing grazing rates on seagrass, which could impact restoration activities.
The seagrass growing along the Nature Coast contains varying concentrations of phosphorus (P) in its leaves. Due to a natural gradient in P availability, the seagrass that grows near Crystal River, FL contains more P than its southern neighbors growing in Weeki Wachee, FL. In addition to differences in P content, this seagrass also differs in size and likely in texture, depending on the sampling location. Therefore, we wanted to test whether sea urchins preferred seagrass from a specific location along the Nature Coast and whether this preference was due to differences in leaf chemistry and nutrients (i.e. taste) or leaf structure (i.e. texture). In other words, which location produces the most appetizing seagrass and why?
We offered sea urchins (Lytechinus variegatus) a sample of turtle grass (Thalassia testudinum) collected from Crystal River, FL (highest P content), Homosassa, FL, and Weeki Wachee, FL (lowest P content) and found they preferred the higher P content turtle grass from Crystal River over the less nutritious turtle grass from Weeki Wachee, FL. To explore whether this preference was due to the leaf nutrients or leaf texture, we conducted the same experiment but with all options having the same texture. We offered the sea urchins a sample of artificial seagrass leaves containing ground up turtle grass from the same locations, again finding that the sea urchins preferred the turtle grass from Crystal River and Homosassa over the turtle grass from Weeki Wachee. These results suggest that sea urchins prefer the turtle grass that is enriched with P, regardless of plant texture.
How can sea urchins discriminate among plants?
Sea urchins have tube feet with sensory receptors, which can aid in the detection of chemicals in the water. This chemosensory ability plays a role in food selection, helping the urchins discriminate among multiple food options. Past studies have similarly found that sea urchins preferred nutrient-enriched seagrass, but these studies often focused on the impact of nitrogen or nitrogen and phosphorus together, rather than isolating the impact of phosphorus.
Why does this matter?
Seagrass is an invaluable resource. It provides habitat for commercially and recreationally valuable fishery species, habitat for endangered species, carbon storage, sediment stabilization, nitrogen removal via denitrification, and storm protection due to decreased wave energy. Therefore, seagrass supports local communities by reducing erosion, boosting fishery production, increasing recreational and cultural value, and minimizing the negative effects of storms.
Because high levels of herbivory can decimate seagrass meadows, understanding sea urchin feeding behavior and the plant traits that influence these feeding decisions is important for the restoration and management of seagrass habitats. Globally, nutrient levels in coastal waters are increasing, and this work will help us understand and predict patterns in herbivory and seagrass persistence while preserving the many valuable economic and cultural benefits that seagrasses provide both locally and globally.