The Digestive Diverticula in the Carnivorous Nudibranch, Melibe leonina, Do Not Contain Photosynthetic Symbionts

Abstract

A number of nudibranchs, including Melibe engeli and Melibe pilosa, harbor symbiotic photosynthetic zooxanthellae. Melibe leonina spends most of its adult life on seagrass or kelp, capturing planktonic organisms in the water column with a large, tentacle-lined oral hood that brings food to its mouth. M. leonina also has an extensive network of digestive diverticula, located just beneath its translucent integument, that are typically filled with pigmented material likely derived from ingested food. Therefore, the focus of this project was to test the hypothesis that M. leonina accumulate symbiotic photosynthetic dinoflagellates in these diverticula. First, we conducted experiments to determine if M. leonina exhibits a preference for light, which would allow chloroplasts that it might be harboring to carry out photosynthesis. We found that most M. leonina preferred shaded areas and spent less time in direct sunlight. Second, we examined the small green circular structures in cells lining the digestive diverticula. Like chlorophyll, they exhibited autofluorescence when illuminated at 480 nm, and they were also about the same size as chloroplasts and symbiotic zooxanthellae. However, subsequent electron microscopy found no evidence of chloroplasts in the digestive diverticula of M. leonina; the structures exhibiting autofluorescence at 480 nm were most likely heterolysosomes, consistent with normal molluscan digestion. Third, we did not find evidence of altered oxygen consumption or production in M. leonina housed in different light conditions, suggesting the lack of any significant photosynthetic activity in sunlight. Fourth, we examined the contents of the diverticula, using HPLC, thin layer chromatography, and spectroscopy. The results of these studies indicate that the diverticula did not contain any chlorophyll, but rather harbored other pigments, such as astaxanthin, which likely came from crustaceans in their diet. Together, all of these data suggest that M. leonina does sequester pigments from its diet, but not for the purpose of symbiosis with photosynthetic zooxanthellae. Considering the translucent skin of M. leonina, the pigmented diverticula may instead provide camouflage.

The distribution and evolutionary dynamics of dopaminergic neurons in molluscs

“…Here, we extend comparative aspects of the distribution of catecholaminergic neurons targeting two distinct ecological and less investigated groups of Euthyneura: the sea angel - obligate predatory pelagic mollusc Clione limacina (Pteropoda, Gymnosomata) and its prey - the sea devil - Limacina helicina (Pteropoda, Thecosomata) as well as the plankton eater Melibe leonina (Nudipleura, Nudibranchia, (Watson et al, 2021)), see Fig. 1 .…”

In the sea slug Melibe leonina the posterior nerves communicate stomach distention to inhibit feeding and modify oral hood movements

“…In recent years the sea slug Melibe leonina has emerged as a promising species in which to study the neuronal regulation of behavioral state. It feeds using rhythmic movements of its oral hood to capture both planktonic prey ( Watson and Trimarchi, 1992 ) and organisms on the kelp and sea grass on which it tends to reside ( Watson et al, 2021 ), and it can feed while either stationary or crawling. Both its rhythmic feeding movements ( Watson and Trimarchi, 1992 ) and locomotor activity ( Newcomb et al, 2014 ) can be easily quantified, and the expression of these movements changes with time of day ( Newcomb et al, 2014 ) or hunger state ( Lee and Watson, 2016 ).…”