Molecules of keystone significance are relatively rare, yet mediate a variety of interactions between organisms. They influence the distribution and abundance of species, the transfer of energy across multiple trophic levels, and thus they play significant roles in structuring ecosystems. Despite their potential importance in facilitating our understanding of ecological systems, only three molecules thus far have been proposed as molecules of keystone significance: saxitoxin and dimethyl sulfide in marine communities and tetrodotoxin in riparian communities. In the course of studying the neuroecology of chemical defenses, we identified three mycosporine-like amino acids (MAAs)-N-ethanol palythine (= asterina-330), N-isopropanol palythine (= aplysiapalythine A), and N-ethyl palythine (= aplysiapalythine B)-as intraspecific alarm cues for sea hares (Aplysia californica). These alarm cues are released in the ink secretion of sea hares and cause avoidance behaviors in neighboring conspecifics. Further, we show that these three bioactive MAAs, two [aplysiapalythine A (APA) and -B (APB)] being previously unknown molecules, are present in the algal diet of sea hares and are concentrated in their defensive secretion as well as in their skin. MAAs are known to be produced by algae, fungi, and cyanobacteria and are acquired by many aquatic animals through trophic interactions. MAAs are widely used as sunscreens, among other uses, but sea hares modify their function to serve a previously undocumented role, as intraspecific chemical cues. Our findings highlight the multifunctionality of MAAs and their role in ecological connectivity, suggesting that they may function as molecules of keystone significance in marine ecosystems.

Three new mycosporine-like amino acids (MAAs), aplysiapalythines A, B, and C (1, 2, and 3, resp.) were isolated from opaline, a glandular component of the defensive ink secretion of sea hares (Aplysia californica) collected from waters off southern California. Here, we report the structure of these MAAs determined by mass spectrometry and NMR data. These new MAAs are structurally related to two known MAAs that are also present in sea hare opaline, i.e., asterina 330 (4) and palythine (5), and for which we also provide detailed data here for comparison. The fact that three of the five MAAs that we identified from sea hare opaline are novel molecules is interesting given that this represents a relatively large addition to the current list of known MAAs. This is likely because most researchers have identified MAAs through HPLC with UV detection, which is imprecise given similarities in UV spectra for different MAAs. Our findings suggest that there is much greater diversity of MAAs than is currently known. Results published elsewhere show that 1, 2, and 4 are alarm cues for conspecific sea hares at natural concentrations, but 3 and 5 are not.Introduction. -Mycosporine-like amino acids (MAAs) are widely distributed in marine organisms including bacteria, plants, invertebrates, and vertebrates [1]. Animals are generally thought to be incapable of synthesizing MAAs, but rather they are thought to sequester MAAs from their diet or from symbionts; however, this view has been questioned [2]. MAAs, of which ca. 30 are known, are traditionally considered to be screens against solar radiation, but other functions have also been proposed, including anti-oxidants, osmotic regulation, providing a source of amino acids in embryonic development, and induction of spawning [3 -10].A new function for MAAs was recently identified from our work on chemical defenses of sea hares (Aplysia californica) collected from the waters of southern California [11]. We found that MAAs are present in high concentrations in opaline, which is one of the glandular products of the sea hares ink secretion. Of the five major MAAs in opaline of these sea hares, three are novel, and two of these novel MAAs, together with nucleic acids and nucleosides in the ink component of the sea hares ink secretion, function as intraspecific alarm cues [11]. In this article, we describe the isolation and the structure elucidation of these novel MAAs.

We previously used bioassay-guided fractionation to identify phycoerythrobilin (1) and its monomethyl ester, aplysioviolin (2), as components in the ink secretion of a marine gastropod, the sea hare Aplysia californica, that act as chemical deterrents against predatory blue crabs. This was the first report of 1 as a natural product. Compound 2 was previously reported as a natural product from three species of Aplysia (A. fasciata, A. dactylomela, and A. parvula), but the reported structure and composition of stereoisomers of 2 are different among these species. Sea hares are thought to produce 2 from phycoerythrin, a photosynthetic pigment in their red-algal diet composed of a phycobiliprotein covalently linked to the chromophore 1, by cleavage of the covalent bond and methylation of 1, but neither the sequence nor the anatomical location of the cleavage and methylation is known. In this study, we clarify the structure of 1 and 2 in ink secretion of A. californica, and describe the distribution of 1 and 2 in the tissues of sea hares. We conclude that cleavage of the covalent bond in phycoerythrin occurs first, forming 1 in the digestive gland, followed by methylation of 1 to yield 2 in the ink gland.