Japetella diaphana

Brief diagnosis:

A bolitaenin octopus that has...

• Mantle reaching a maximum known length of about 145 mm, becoming more darkly pigmented toward maturity
• Eyes (relatively large) on short optic stalks
• A large photophore developing around the beak in mature females, appearing as a ring of bright yellow tissue (studied and reported on in detail by Herring et al., 1987) 
• Arms in males without a hectocotylus (no ligula on either Arm III) but with abruptly enlarged suckers (starting around sucker #5) on Arm IIIR in males approaching maturity. 

Live animal

A small, gelatinous octopus generally found below the photic zone. The arms are very short compared to the bulbous mantle and have suckers in a single series.  The eyes are large and iridescent, and the reflective digestive gland can be seen inside the mantle. Many small iridophores within the head and arm tissue strongly reflect any light sources and may appear golden, silvery or blue-green.   

Many more live images can be seen on MBARI's Midwater octopus species page.

Genetics

Several gene regions have been sequenced for Japetella diaphana (including whole mitogenomes) and are available on GenBank; COI barcode sequences can also be found on BOLD.  

Beaks

3D (anaglyph) photos of several Japetella beaks can be found here: Lower, Upper 

Japetella diaphana is found throughout the tropical and subtropical regions of the world's oceans but extends into boreal waters in the North Pacific (Thiele, 1949, Nesis, 1982).

Vertical distribution 

In Hawaiian waters, Young (1978) found the distribution pattern shown below. Small octopods (ca. 5-20 mm ML) were found near 170-270 m depth or mostly between 500 and 800 m. At a ML of over 20 mm, most were captured at depths of 700-950 m. Two gravid females were captured at about 1050 m (red circles with green rings). No mature males were taken. Three brooding females were taken between 725 and 800 m.

© Richard E. Young

The distribution seen here is similar to that found by Lu and Clarke (1975) for J. diaphana in the tropical Eastern North Atlantic, except that they recorded several daytime captures of large octopods at shallow depths. Two of these captures were extreme: One was in the upper 100 m (60 mm ML) and the other in the 100-200 m range (70 mm ML). Neither octopus was mature. At present, we have no explanation for this peculiar aspect of their vertical distribution. 

Birk et al. (2019) studied the metabolism of J. diaphana in relation to oxygen minimum zones and reported several interesting observations. This species can inhabit regions with lower metabolic indices than most marine animals, but cannot support aerobic metabolism in the OMZ core (unlike the 'vampire squid' Vampyroteuthis infernalis). Even more unusually, J. diaphana is more hypoxia tolerant in warmer water than colder water (the opposite is usually true in ectotherms).

 

Further to the diagnostic characters under Identification above, Japetella diaphana has the following features.  

Arms III exhibit sexual dimorphism; in males, suckers are slightly enlarged on the distal 2/3rds of the arms, and as males approach maturity, the 5th and subsequent suckers on Arm IIIR become abruptly enlarged compared to the others (aucker 5 ~2x height of sucker 4). Hectocotylization (i.e., modification of the arm tip) is unknown.

© Richard E. Young

 

The eyes are adjacent to brain in young animals but become positioned slightly further apart in older animals (optic stalks short). They are relatively large compared to Bolitaena of comparable size.

Descriptions of the beaks can be found here: Lower beak; upper beak.

The skin has iridophores, overlying the dorsal arms and head that lie in a plane transverse, rather than parallel, to the body axis. The function of these unusually positioned iridophores is unknown.

© Danté Fenolio

Near sexual maturity, the strong iridescence of the digestive gland and eyes are lost (Young, 1972) and pigmentation increases but not as greatly as in Bolitaena pygmaea.

The increase in salivary gland size found in male Bolitaena pygmaea has not been formally documented in this species, but we have seen a nearly mature male with a salivary gland over twice as large as was seen in a female of the same body size. The male salivary gland in the fresh octopus was also a pearly, opaque white, unlike the normal translucent appearance of the gland of subadults.

© Monterey Bay Aquarium Research Institute (MBARI)

Young are found between about 200 and 300 m or at much greater depths. Hatchlings have a ML of about 3 mm and eggs are about the same size (pers. obs.; Schwarz et al., 2019). The young paralarvae have a different chromatophore pattern than that of Bolitaena pygmaea young, but the chromatophores are often lost during capture. Indeed, the smaller Japetella hatchling on the family page shows two posterior mantle chromatophores that apparently were lost from the paralarva illustrated here.

Gravid females are found at the lower end of the vertical distribution range near 1000 m (nothing is known of the vertical distribution of near-mature and mature males). Brooding females have been observed near 800 m off Hawaii, and one individual collected 1352 m in the Gulf of California was carrying over 1400 eggs (Schwarz et al., 2019).

The life history appears to be very similar to that of Bolitaena pygmaea. Present data suggest the following scenario:

Spawned females float at a depth of around 800m and brood clusters of interconnected eggs between their arms. At this depth downwelling sunlight is too low to reveal their presence; this is about as close as they can get to the surface under this visibility constraint. Females probably do not feed while brooding and die after the young hatch. At a temperature of 4-5°C, brooding may take up to two years. Hatchlings either swim to the upper 200-300 m where food presumably is more plentiful, or are carried there by the female at the time of hatching. Young Japetella descend into mesopelagic depths as they grow but the size at which they descend is variable (ca. 7-20mm ML). At maturity the female, and presumably the male, descends to depths of just over 1000 m. Males may first attract females, using secretions from the posterior salivary gland that act as pheromones; then, presumably as the female draws close and when she considers conditions safe, she signals the male with the oral light organ and mating occurs. 

Schwarz et al. (2019) studied a large dataset of J. diaphana to investigate its growth and life history.  They concluded that the growth increments observed in its beaks are likely formed on something longer than a daily basis, perhaps representing feeding events. The largest mature female's beak had 207 growth increments, while the estimated egg brooding period is around two years (Schwarz et al., 2018), strongly suggesting that this species has a greater lifespan than other pelagic octopods.     

Diaphanous octopus

Midwater octopus