Effects of sea hare ink secretion and its escapin-generated components on a variety of predatory fishes

Flow cytometric characterization of the hemocytes of sea hares from tidal pools in Jeju Island off the south coast of Korea

The members in the family Aplysidae known as sea hares play a crucial role as a grazer in small tidal pools or shallow subtidal hard bottoms. Like other marine gastropods, hemocyte types and functions of sea hares are limitedly known. This study identified and characterized the hemocytes of common Aplysia species Aplysia kurodai, A. juliana, and A. oculifera in rocky tidal pools in Jeju Island off the south coast of Korea using flow cytometry and light microscopy. The flow cytometry identified three different hemocytes in the hemolymph of the three sea hare species: granulocytes, hyalinocytes, and blast-like cells. The granulocytes exhibited pseudopodia on the cell surface and granules in the cytoplasm. Morphology of the hyalinocyte was similar to that of the granulocytes, while they lack cytoplasmic granules. The blast-like cells were small and round, with very thin cytoplasm. The hyalinocytes were the most abundant in the hemolymph, accounting for 89.8-92.6% of the total hemocytes. Flow cytometry indicated that the granulocytes and blast-like cells were less than 5.6% and 5.4% of the total hemocyte populations. Flow cytometry also revealed that the granulocytes and hyalinocytes are engaged in cellular defensive activities such as intra-cellular lysosomal content, phagocytosis, and ROS production. The mean lysosomal contents of the granulocytes (0.4 × 10⁵-0.2 × 10⁵ A U.) were 2-3 times higher than that of hyalinocytes (0.2 × 10⁵-0.6 × 10⁵ A U.). In addition, the ROS production of the granulocytes (0.98 × 10⁶-1.95 × 10⁶ A U.) was about twice higher than that of the hyalinocytes (0.62 × 10⁶-1.14 × 10⁶ A U.). Of the three species of sea hares, the granulocytes showed comparatively higher phagocytosis capacity (70.4-92.3%) than that of the hyalinocytes (34.8-46.0%). Flow cytometry and microscopy indicated that the hemocyte types and their functions were identical, regardless of the species.

Bioactive Compounds from Marine Heterobranchs

The natural products of heterobranch molluscs display a huge variability both in structure and in their bioactivity. Despite the considerable lack of information, it can be observed from the recent literature that this group of animals possesses an astonishing arsenal of molecules from different origins that provide the molluscs with potent chemicals that are ecologically and pharmacologically relevant. In this review, we analyze the bioactivity of more than 450 compounds from ca. 400 species of heterobranch molluscs that are useful for the snails to protect themselves in different ways and/or that may be useful to us because of their pharmacological activities. Their ecological activities include predator avoidance, toxicity, antimicrobials, antifouling, trail-following and alarm pheromones, sunscreens and UV protection, tissue regeneration, and others. The most studied ecological activity is predation avoidance, followed by toxicity. Their pharmacological activities consist of cytotoxicity and antitumoral activity; antibiotic, antiparasitic, antiviral, and anti-inflammatory activity; and activity against neurodegenerative diseases and others. The most studied pharmacological activities are cytotoxicity and anticancer activities, followed by antibiotic activity. Overall, it can be observed that heterobranch molluscs are extremely interesting in regard to the study of marine natural products in terms of both chemical ecology and biotechnology studies, providing many leads for further detailed research in these fields in the near future.

Molecules and Mechanisms Underlying the Antimicrobial Activity of Escapin, an l-Amino Acid Oxidase from the Ink of Sea Hares

Many marine animals use chemicals to defend themselves and their eggs from predators. Beyond their ecologically relevant functions, these chemicals may also have properties that make them beneficial for humans, including biomedical and industrial applications. For example, some chemical defenses are also powerful antimicrobial or antitumor agents with relevance to human health and disease. One such chemical defense, escapin, an l-amino acid oxidase in the defensive ink of the sea hare Aplysia californica, and related proteins have been investigated for their biomedical properties. This review details our current understanding of escapin's antimicrobial activity, including the array of molecules generated by escapin's oxidation of its major substrates, l-lysine and l-arginine, and mechanisms underlying these molecules' bactericidal and bacteriostatic effects on planktonic cells and the prevention of formation and removal of bacterial biofilms. Models of escapin's effects are presented, and future directions are proposed.

Beach to Bench to Bedside: Marine Invertebrate Biochemical Adaptations and Their Applications in Biotechnology and Biomedicine

The ocean covers more than 70% of the surface of the planet and harbors very diverse ecosystems ranging from tropical coral reefs to the deepest ocean trenches, with some of the most extreme conditions of pressure, temperature, and light. Organisms living in these environments have been subjected to strong selective pressures through millions of years of evolution, resulting in a plethora of remarkable adaptations that serve a variety of vital functions. Some of these adaptations, including venomous secretions and light-emitting compounds or ink, represent biochemical innovations in which marine invertebrates have developed novel and unique bioactive compounds with enormous potential for basic and applied research. Marine biotechnology, defined as the application of science and technology to marine organisms for the production of knowledge, goods, and services, can harness the enormous possibilities of these unique bioactive compounds acting as a bridge between biological knowledge and applications. This chapter highlights some of the most exceptional biochemical adaptions found specifically in marine invertebrates and describes the biotechnological and biomedical applications derived from them to improve the quality of human life.

A Waterborne Pursuit-Deterrent Signal Deployed by a Sea Urchin

Selection by consumers has led to the evolution of a vast array of defenses in animals and plants. These defenses include physical structures, behaviors, and chemical signals that mediate interactions with predators. Some of the strangest defensive structures in nature are the globiferous pedicellariae of the echinoderms. These are small venomous appendages with jaws and teeth that cover the test of many sea urchins and sea stars. In this study, we report a unique use of these defensive structures by the collector sea urchin Tripneustes gratilla. In both the laboratory and the field, globiferous pedicellariae were unpalatable to fish consumers. When subject to simulated predator attack, sea urchins released a cloud of pedicellaria heads into the water column. Flume experiments established the presence of a waterborne cue associated with this release of pedicellariae that is deterrent to predatory fish. These novel results add to our understanding of how the ecosystem-shaping sea urchin T. gratilla is able to reach high densities in many reef habitats, with subsequent impacts on algal cover.

Finding food: how marine invertebrates use chemical cues to track and select food

This review covers recent research on how marine invertebrates use chemical cues to find and select food.

Ink from longfin inshore squid, Doryteuthis pealeii, as a chemical and visual defense against two predatory fishes, summer flounder, Paralichthys dentatus, and sea catfish, Ariopsis felis

Chemical and visual defenses are used by many organisms to avoid being approached or eaten by predators. An example is inking molluscs-including gastropods such as sea hares and cephalopods such as squid, cuttlefish, and octopus-which release a colored ink upon approach or attack. Previous work showed that ink can protect molluscs through a combination of chemical, visual, and other effects. In this study, we examined the effects of ink from longfin inshore squid, Doryteuthis pealeii, on the behavior of two species of predatory fishes, summer flounder, Paralichthys dentatus, and sea catfish, Ariopsis felis. Using a cloud assay, we found that ink from longfin inshore squid affected the approach phase of predation by summer flounder, primarily through its visual effects. Using a food assay, we found that the ink affected the consummatory and ingestive phase of predation of both sea catfish and summer flounder, through the ink's chemical properties. Fractionation of ink showed that most of its deterrent chemical activity is associated with melanin granules, suggesting that either compounds adhering to these granules or melanin itself are the most biologically active. This work provides the basis for a comparative approach to identify deterrent molecules from inking cephalopods and to examine neural mechanisms whereby these chemicals affect behavior of fish, using the sea catfish as a chemosensory model.

Defense through sensory inactivation: sea hare ink reduces sensory and motor responses of spiny lobsters to food odors

Antipredator defenses are ubiquitous and diverse. Ink secretion of sea hares (Aplysia) is an antipredator defense acting through the chemical senses of predators by different mechanisms. The most common mechanism is ink acting as an unpalatable repellent. Less common is ink secretion acting as a decoy (phagomimic) that misdirects predators' attacks. In this study, we tested another possible mechanism – sensory inactivation – in which ink inactivates the predator's reception of food odors associated with would-be prey. We tested this hypothesis using spiny lobsters, Panulirus argus, as model predators. Ink secretion is composed of two glandular products, one being opaline, a viscous substance containing concentrations of hundreds of millimolar of total free amino acids. Opaline sticks to antennules, mouthparts and other chemosensory appendages of lobsters, physically blocking access of food odors to the predator's chemosensors, or over-stimulating (short term) and adapting (long term) the chemosensors. We tested the sensory inactivation hypotheses by treating the antennules with opaline and mimics of its physical and/or chemical properties. We compared the effects of these treatments on responses to a food odor for chemoreceptor neurons in isolated antennules, as a measure of effect on chemosensory input, and for antennular motor responses of intact lobsters, as a measure of effect on chemically driven motor behavior. Our results indicate that opaline reduces the output of chemosensors by physically blocking reception of and response to food odors, and this has an impact on motor responses of lobsters. This is the first experimental demonstration of inactivation of peripheral sensors as an antipredatory defense.

Oesophageal chemoreceptors of blue crabs, Callinectes sapidus, sense chemical deterrents and can block ingestion of food

Decapod crustaceans such as blue crabs possess a variety of chemoreceptors that control different stages of the feeding process. All these chemoreceptors are putative targets for feeding deterrents that cause animals to avoid or reject otherwise palatable food. As a first step towards characterizing the chemoreceptors that mediate the effect of deterrents, we used a behavioral approach to investigate their precise location. Data presented here demonstrate that chemoreceptors located on the antennules, pereiopods and mouthparts do not mediate the food-rejection effects of a variety of deterrents, both natural and artificial to crabs. Crabs always searched for deterrent-laced food and took it to their oral region. The deterrent effect was manifested as either rejection or extensive manipulation, but in both cases crabs bit the food. The biting behavior is relevant because the introduction of food into the oral cavity ensured that the deterrents gained access to the oesophageal taste receptors, and so we conclude that they are the ones mediating rejection. Additional support comes from the fact that a variety of deterrent compounds evoked oesophageal dilatation, which is mediated by oesophageal receptors and has been linked to food rejection. Further, there is a positive correlation between a compound’s ability to elicit rejection and its ability to evoke oesophageal dilatation. The fact that deterrents do not act at a distance is in accordance with the limited solubility of most known feeding deterrents, and likely influences predator–prey interactions and their outcome: prey organisms will be attacked and bitten before deterrents become relevant.