Sacoglossa belongs to the Subclass of Opithsobrabchia (Class: Gastropoda). There are about 300 species in the world. The radula of Sacoglossa is composed of ascending limb, descending limb, and a ascus sac to store used teeth. This ascus sac is a unique character of sacoglossans in mollusus. In this study, I compared the morphology, and the composition of elements in sacoglossan radulae. A total of 16 species in 5 families, including 12 new records, has been observed. Their teeth morphology was triangular or blade-like. The radula sac is surrounded by a layer of epithelium cells and the arrangement of the teeth in the sac was linear, dense or fraction. Positive correlations between body length and the number of ascending radular teeth, descending radular teeth, teeth in ascus sac or the total number of teeth were found except for Volvatella vigourouxi (Family: Volvatellidae). The elemental compositions in various radula parts were examined by energy dispersive spectrometer. I found that sacoglossan teeth were composited by Fe, Na, Mg, Si, P, S and Ca, and no significant difference among parts but significantly different among species. In contrast, elemental compositions of sacoglossans were significantly different from Tambja sp. (Order: Nudibranchia), Aplysia parvula (Order: Anaspidea) and Cellana toreuma (Order: Archaeogastropoda). In the meantime, sacoglossans and Tambja sp. were closer than others. Additionally, hollow or broken teeth were found in some sacogloosan ascus indicates that tooth reabsorption may present. However, further study is necessary to elucidate the reabsorption mechanism.

Sacoglossan sea slugs are one of the best known examples of specialist herbivores in the marine environment and can be found strongly associated with their algal hosts and food sources. Perhaps the most intriguing characteristic of many sacoglossans is their ability to sequester functional chloroplasts from their algal food sources in a process called kleptoplasty. Despite this, there continues to be issues regarding taxonomic identification of species. In turn, the ecological characteristics of many of these slugs, such as algal host and food source preference, as well as their behavioral aspects, have received little attention. A prime example of these issues arises in one such kleptoplastic sacoglossan found at Sunset Beach, Tarpon Springs, USA. The slug had previously been identified as Elysia patina based on a recent description, but later evidence suggested this was incorrect. Furthermore, despite the evidence found for the slug’s photosynthetic capabilities, little was known of its ecological relationships and behavior. The purposes of this study were to: 1) correct the uncertain taxonomy of the Tarpon Springs slug previously identified as Elysia patina, and 2) explore the ecological and behavioral characteristics of the slug. First, a comparative study was performed between the Tarpon Springs slug and its original description, as well as descriptions for the superficially-similar congener slug, Elysia papillosa. The gross anatomy, dorsal surface vascular morphology, radular morphology, egg mass morphology, and developmental timeline of the Tarpon Springs slug were used as means of comparison with the previous descriptions. The results of the comparison show that the Tarpon Springs slug was in fact E. papillosa rather than E. patina, and that the most recent description used to identify the slug as E. patina was incorrect and should not be used. Second, a descriptive study of the ecological and behavioral aspects of E. papillosa at the Sunset Beach site were performed. From October 2014 to September 2015, bi-weekly algal collections were made to determine the seasonal abundance of the slug and a possible relationship between slug abundance and algal abundance. Next, a second collection study was performed bi-monthly from April to July 2016 to identify the preferred algal host of E. papillosa between the three most abundant rhizophytic algae at the site, Penicillus capitatus, Penicillus lamourouxii, and Halimeda incrassata, all of which have been previously reported as being hosts of E. papillosa. The results of these studies showed no relationship between slug abundance and algal abundance, however E. papillosa was found to have a seasonal population fluctuation, with the fewest slugs found in winter and summer months and the most slugs found in the spring and fall months, especially in April and May. Elysia papillosa was also found in significantly higher numbers on the alga P. capitatus compared to the other two algal species, although some slugs were found on P. lamourouxii. Only one slug was found on H. incrassata, indicating it is not being used as a host despite previous reports. Further exploration into the genetics of sequestered chloroplasts would provide critical details into E. papillosa’s feeding behavior in situ. Lastly, because of E. papillosa’s photosynthetic abilities, an experiment was performed to determine if E. papillosa exhibited phototactic behavior. Fully-fed slugs were found to have no preference for either light or dark conditions, indicating their behavior was not being influenced by their photosynthetic abilities. This information provides a means of comparison with future studies of the phototactic behavior of kleptoplastic slugs, and could provide insight into how the longevity of functional chloroplasts in each species of slug could influence their behavior.