Phagomimicry is a

defensive behaviour Deimatic behaviour or startle display means any pattern of bluffing behaviour in an animal that lacks strong defences, such as suddenly displaying conspicuous eyespots, to scare off or momentarily distract a predator, thus giving the prey anima ...

of sea hares, in which the animal ejects a mixture of chemicals, which

mimic MIMIC, known in capitalized form only, is a former simulation computer language developed 1964 by H. E. Petersen, F. J. Sansom and L. M. Warshawsky of Systems Engineering Group within the Air Force Materiel Command at the Wright-Patterson AFB in ...

food, and overwhelm the senses of their predator, giving the sea hare a chance to escape. The typical defence response of the sea hare to a predator is to release two chemicals -

ink Ink is a gel, sol, or solution that contains at least one colorant, such as a dye or pigment, and is used to color a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, reed pen, or quill. Thi ...

from the ink gland and

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

ine from the opaline gland. While ink creates a dark, diffuse cloud in the water which disrupts the sensory perception of the predator by acting as a smokescreen and as a decoy, the opaline, which affects the senses dealing with feeding, causes the predator to instinctively attack the cloud of chemicals as if it were indeed food. This ink is able to mimic food by having a high concentration of amino acids and other compounds that are normally found in food, and the attack behaviour of the predator allows the sea-hares the opportunity to escape.

Inking behaviour

The inking behaviour exhibited in phagomimicry is in response to predator threat. Sea hares have many natural predators such as

starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfis ...

lobster Lobsters are a family (Nephropidae, synonym Homaridae) of marine crustaceans. They have long bodies with muscular tails and live in crevices or burrows on the sea floor. Three of their five pairs of legs have claws, including the first pair ...

s, and other

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s. When threatened by a predator, phagomimicry behaviour begins. An ink solution is released from both the opaline and ink glands individually, then the compounds mix in the mantle of the sea hare to form the ink mixture. When ink is released it creates a smoke-screen like

defense mechanism In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to internal conflicts and ...

allowing the sea hares time to escape while also affecting the

olfactory The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, ...

and gustation senses of their predator. Predators are tricked into thinking that they have captured their prey due to the specific chemical composition of the ink released. This induces feeding behaviours in the predator, and again gives the sea hares a better chance of escaping predation.

Opaline gland

The opaline gland is a structure resembling a bundle of grapes attached to a central canal which is composed of

epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

cells. Synthesis of the opaline substance happens in the opaline vesicles themselves, as there are only opaline vesicles and muscle cells in the opaline gland. The gland is innervated by three separate

motor neuron A motor neuron (or motoneuron or efferent neuron) is a neuron whose cell body is located in the motor cortex, brainstem or the spinal cord, and whose axon (fiber) projects to the spinal cord or outside of the spinal cord to directly or indirectly ...

s, and is composed of single large cells and vesicle cells, all of which have enlarged nucleus. These cells are inclosed in an external layer of muscle. When a

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. Th ...

detects a predator threat, dopamine is released onto one of the three motor neurons. The dopamine release causes a gland contraction, which then causes the expulsion of the opaline substance.

Ink gland

The ink gland is smaller in size than the opaline gland, and is composed of two cell types: rough endoplasmic recticlum (RER) and granulate cells. The cells are surrounded by a layer of muscle, to contraction and expelling their contents. The RER is the formation site of the anti-predator protein, the granulate cells are for extra pigment storage. Pigment is dependent on the amount of red algae available to the sea hares, the higher the red algal consumption, the darker the colour of their ink. This mixing can cause another chain of reactions between the compounds that have further implications on the effect that the ink secretion has on predators.

Ink chemical composition

Both the opaline and ink gland secrete different substances that when mixed together form the ink released during phagomimicry. The secretion is very acidic (ink having a pH of 4.9 and opaline having a pH of 5.8) and contains high levels of bioactive molecules that can serve as feeding stimulants, feeding deterrents, and aversive compounds. Feeding stimulants can be found in both the ink and opaline secretions in the form of

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...

s (such as

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated − ...

and

arginine Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the a ...

), and serve to trick predators into thinking that the ink secretion is a food source. To induce the aversive feeding effects on predators the ink contains a compound from the opaline gland produced from the oxidation of L-lysine, which is then mixed in the mantle with the L-amino acid oxidase from the ink gland. Together this compound, called escapin is secreted in the ink, and is a feeding deterrent. The ink secretion can have a long-lasting effect on predators as chemical phagomimics can cause chemo-mechanosensory stimulation which overwhelms the sensory system and leads to confusion and eventually the cession of the attack.

Physical properties of Ink

The ink released from the ink gland is dark purple in colour, the colour depends on the type of algae consumed by the sea-hare. The opaline ink is white in colour, and when it mixes with the ink gland ink they form a compound that suspends itself in the water ( polidisperse suspension), creating a smoke-screen defence mechanism. The particle density of the ink is similar to that of species such as

cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control ...

, the particles range in size from 80–150 nm, with a density of 1.27 cm-3, which allows for the inks suspension in water .

References