Grooming behavior by elongated third maxillipeds of phyllosoma larvae of the smooth fan lobster riding on jellyfishes

• The 3rd maxillipeds of the phyllosoma of the smooth fan lobster are long enough to groom the whole body.
• The phyllosoma spend ca. 50% of their time grooming using the 3rd maxillipeds when they are swimming.
• Inhibition of this behavior enhances growth of microbes on the body surface.
• A major free amino acid in jellyfish tissue, glycine, induces the grooming behavior.
• Adaptation of this grooming behavior and morphology of the larvae is discussed.

Animals groom their body surface to prevent or remove parasites and non-biological fouling that can negatively impact their mobility, appearance, and health. The jellyfish riders, phyllosoma larvae of the smooth fan lobster Ibacus novemdentatus Gibbes which spend their planktonic larval period riding on and eating live jellyfish, have a characteristic body shape, a leaf-like flat body without gills and a pair of elongated third maxillipeds. Although the third maxillipeds lack exopods and are very long compared with that of other types of decapod crustacean adults and larvae, their function is not known. In the present study, we hypothesized that the elongated third maxillipeds of I. novemdentatus phyllosoma larvae constitute a grooming organ specialized to remove fouling from their body surface and/or an organ to manipulate their food, and tested the hypothesis through morphological and behavioral experiments. Our morphological observations show that the third maxillipeds are longer than the larva's body and that the tip of the maxillipeds has a comb-like structure. Our behavioral experiments show that the larvae spent about 50% of their time grooming with their third maxillipeds but did not use these maxillipeds to manipulate jellyfish for eating. The maxillipeds removed an experimentally-applied foulant from most of their body parts, suggesting that this artificial foulant stimulated grooming on that location. Thus, the grooming has characteristics of both programmed and stimulus-driven behaviors. Inhibition of this behavior enhanced growth of microbes on the body surface of that phyllosoma fed with jellyfish. Glycine was identified as major free amino acid in jellyfish tissue using amino acid analysis and as a major metabolite by nuclear magnetic resonance (NMR) spectroscopy. Glycine induced appetitive and grooming behaviors in the larvae. Thus, we conclude that the phyllosoma larvae use the elongated third maxillipeds for cleaning their entire body surface and that the maxillipeds are functionally specialized for this cleaning behavior. This frequent whole body cleaning behavior with specialized appendages might be adaptive to their lifestyle of riding on jellyfish, where they are exposed to large amounts of mucus released from the jellyfish which could otherwise cause growth of microbes on their body surface.