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Vitreledonella richardi
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	Octopoda
Family:	Amphitretidae
Subfamily:	Vitreledonellinae
Genus:	Vitreledonella Joubin, 1918
Type species
Vitreledonella richardi Joubin, 1918
Species
Vitreledonella alberti Joubin, 1924 Vitreledonella richardi Joubin, 1918

Vitreledonella is a genus of mesopelagic octopods from the family Amphitretidae which contains two species, one of which is the glass octopus.

These octopods have the sucker on their arms arranged in a single series with the suckers widely separated from each other. The third left arm is hectocotylised with a spherical vesicle at the distal end and in males the other arms have suckers which are enlarged beyond the web. The eye has strong lateral compression with a near rectangular shape in lateral view and with the width equal to the diameter of the lens. There is a ventral, blunt rostrum-like extension on the eye which contains iridescent tissue above the eye. The opening to the mantle is broad, the radula is multicuspid and linear in form with the first and second lateral tooth each being unicuspid, which means that this species has a heteroglossan radula. The long and slender digestive gland is spindle-shaped and the stomach is positioned dorsally to the digestive gland.

Amphitretus and Bolitaena are two other transparent, gelatinous pelagic incirrate octopods. Both of these genera differ from Vitreledonella in that the right third arm is hectocotylized and the radula is ctenodont (with comb-like individual teeth).

List of species

According to the EOL:

Vitreledonella	Vitreledonella alberti Joubin, 1924 Vitreledonella richardi Joubin, 1918

Description

Adult

Vitreledonella, like other Amphitretidae, are externally characterized by their gelatinous, transparent, and colourless bodies growing to an adult total length of 45 centimetres with a mantle length of 11 centimetres. The suckers are widely separated from one another and are found in a single row on each arm. The radula is heteroglossan in shape with unicuspid first and second lateral teeth, a multicuspid rhachidian, and a smooth rostrum. The eye of the Vitreledonella is laterally compressed into a cylindrical shape with a central lens and is relatively small as compared to the traditionally much larger, extruding eyes seen on other Octopods. The rectangular shape of the eye likely causes a reduced field of vision on the horizontal plane, but allows Vitreledonella to minimize the silhouette of the eye visible to predators from below in an attempt to reduce the size of opaque structures on or within their transparent bodies as to aid in camouflage. Internally, the digestive gland is large and opaque and is oriented vertically as octopus swims to also attempt to reduce its silhouette.

Paralarvae

An Octopod paralarva showing similar characteristics to Vitreledonella paralarvae

The paralarvae of Vitreledonella are much smaller than the adult stage at a size of approximately 10 millimetres. They are characterized by gelatinous and transparent bodies with an elongated oval and a small, narrow head. Like the adults, the arms have a singular row of suckers. The beak is poorly chitinised and includes a lower jaw which contains approximately 30 teeth-like structures. The function of the teeth-like structures of the paralarvae is not however, to be used in the biting or tearing of flesh as Vitreledonella paralarvae feed primarily through external digestion and ingest the softened or liquified digested material. The teeth-like structures are instead used in the suctioning of internal fluids of prey as well as the removal of partially digested flesh from the exoskeletons of crustacean prey. As the Vitreledonella develop, the teeth-like structures disappear and the now-smooth radula are used to crush the exoskeletons of crustacean prey for internal digestion.

Ecology

Diet

Vitreledonella preys upon various types of crustaceans. This could include crabs, shrimp, etc. They also feed on various types of small fishes, clams, and snails. Studies have found that the glass octopus has teeth, during the paralarvae stage, to help function in digestion. However, after a period of maturation, a faint development of a beak forms. Due to the beak of the glass octopus being less developed than other octopus species, it could indicate consumption of soft-bodied prey as well.

Behavior

Throughout the period of this species' evolution, cephalopods have become intelligent individuals. One of the largest advances in its evolution that the octopus has seen, in general, has been its large nervous system, including large brain. Just to provide a sense of how big their nervous systems are, the common octopus, Octopus vulgaris, has an approximate 500 million neurons in its body. Due to their expansive nervous systems and connective neurons throughout the body, all octopus species have the ability to navigate the best possible route for themselves. They are notorious for being able to escape simple and challenging mazes - especially in the laboratory. Octopus, like the glass octopus itself, utilizes visual cues to help distinguish between two environments, then take the best route. Like many other species of octopus, Vitreledonella richardi, uses its eight long arms to sense and control its surrounding environment. With their arms, they also have the ability to sense chemicals (smell or taste). This species of octopus is especially sensitive to its external environment due to the fact that it resides within deep-sea areas, which causes a lack of light for vision purposes. It is always ready to quickly propel itself through the water, using its arms to create jet-stream propulsions of water. Many aspects of the glass octopus' behavior, as well as the behavior of other octopus species, is solely driven by different strategies to evade predators.

Predators

Scavengers and other organisms often attempt to prey on octopus eggs, even when the female is present to protect them. However, studies have shown that remains of Vitreledonella have been found within the stomach contents of northern bottlenose whales (Hyperoodon ampullatus) within the northeast Atlantic ocean. This was found by sampling stranded northern bottlenose whales. In all four samples, the food remains consisted mainly of all cephalopod beaks. Through various testing methods, the species, Vitreledonella richardi, was identified. This proves that this type of octopus is preyed upon by other larger marine animals. Furthermore, studies performed within the Ecuadorian Pacific area have indicated the presence of the glass octopus within the stomach contents of three different shark species. Multiple sharks from the species groups of Carcharhinus falciformis, Carcharhinus limbatus and Nasolamia velox were sampled from. Amongst the presence of the glass octopus, their prey included bony fishes, elasmobranchs, molluscs, crustaceans, and turtles. Moreover, it is important to note that because the glass octopus is practically transparent, this feature works to its advantage, when avoiding predation by other marine organisms.

Habitat

World map indicating tropics and subtropics, the known distribution of Vitreledonella

Vitreledonella is a meso-bathypelagic octopus found in tropical and sub-tropical waters. While rarely captured by scientists or fishing operations, specimens have been retrieved from the Arabian Sea, the Canary Islands, and Ecuador. Most specimens are retrieved from the first 300 metres of the water column however specimens have been retrieved from as deep as 1000 metres below sea level. The low-light environment of the mesopelagic and bathypelagic zone attribute for the transparent and colourless nature of the body of Vitreledonella, a characteristic shared with many other deep-sea invertebrates. Vitreledonella undergo ontogenetic vertical migrations with different life-stages being found at different depths. The adults are found primarily in the meso-bathypelagic zone below 1000 meters while the paralarvae are often found within the first 400 meters of water with larger paralarvae preferring shallower depths before descending as they reach later developmental stages.

Threats

As like other meso-bathypelagic invertebrates Vitreledonella is threatened by anthropogenic hazards. Deep-sea invertebrates are particularly affected by microplastic pollution and are shown to be consistently ingesting microplastics in addition to their natural food sources.

Vitreledonella is also threatened by heavy fishing in the tropical and sub-tropical oceans where it may be directly killed as a result of being caught as bycatch, or through the overfishing of its predators such as the sharks of the Arabian Sea.

Reproduction

As in other pelagic octopuses, Vitreledonella must overcome many challenges to mate. Species of Vitreledonella must first locate one another in the water column to perform internal sexual fertilization. In males, the hectocotylus is a modified arm that is used to transfer sperm into the female. Unlike in many other cephalopods, Vitreledonella males do not detach their hectocotylus during mating and instead mate together with one octopus being within the web of another Externally, in Vitreledonella males the distal third left arm contains the spermatophore groove with an elongated arm tip. Internally, the reproductive ampulla is relatively large and the accessory gland is elongated. After fertilization within the female, the eggs are stored within the mantle cavity until hatching where they are then released and the hatchlings migrate towards the surface in the meso-bathypelagic zone.