Pink Helmet

Aglantha digitale is a holoplanktonic hydromedusa with a vertical distribution that varies between epipelagic to bathypelagic due to reproduction phases and changes in temperature during each season. During spring and summer, A. digitale is found 25 meters below the surface. Throughout fall and winter, it is primarily found at 50 meters and deeper. This species is rarely found in extremely shallow or deep water.

Range depth: 25 to 700 m.

Average depth: 50 m.

Habitat Regions: temperate ; polar ; saltwater or marine

Aquatic Biomes: pelagic ; coastal

Some of the known predators of Aglantha digitale are the leatherback turtle, larger medusa jellyfish, and the bearded goby. Another predator of A. digitale is Flabellina goddardi, a sea slug able to consume this type of prey because the slug is unaffected by the hydroid's defensive nematocysts.

Aglantha digitale has few means of self defense from predators: it can contract its umbrella to perform jet propulsion and escape any danger; it can emit an array of vibrant light to distract the predator; and finally, if there are no other means of escaping, it defends itself with its tentacles. Its tentacles are lined with nematocysts that sting and stun its prey or predators, destroying cells of the tissues that the tentacles come into contact with. The tentacles also detach if the hydroid needs to free itself from entanglement.

Known Predators:

• Leatherback turtle, Dermochelys coriacea
• Bearded goby, Sufflogobius bibarbatus
• Nudibranch, Flabellina goddardi

Body structure consists of a thin, cylindrical, translucent umbrella that ranges from colorless to subtle shades of pink, orange, or red. The top of the umbrella forms a conical apical projection. The umbrella can grow 10 to 22 mm in height and 5 to 11 mm in width, forming its helmet-like shape. The average bell height of adults is 12 mm, but may reach up to 22 mm. Eight brightly-colored tubular gonads are suspended from underneath the top of the umbrella. The gastric peduncle is located in the central cavity and is nearly the same height as the umbrella. Attached to the gastric peduncle are eight radial canals along with the gastrovascular cavity. Usually, A. digitale has up to 80 wispy tentacles that extend from the base of the umbrella, but some are found with fewer due to encounters with prey and predators.

Range length: 10 to 22 mm.

Average length: 12 mm.

Other Physical Features: ectothermic ; heterothermic ; radial symmetry ; venomous

Sexual Dimorphism: female more colorful

The lifespan of this species varies among geographic populations. Individuals found in the subarctic Pacific, the North Sea, and the Northeast Atlantic have an annual life cycle. However, populations of A. digitale in Toyama Bay, Ogac Lake in Baffin Island, and the Strait of Georgia, have biennial life cycles, while up to four generations have been observed co-occurring in Norwegian fjords.

Range lifespan
Status: wild: 1 to 4 years.

Average lifespan
Status: wild: 1 years.

Typical lifespan
Status: wild: 1 to 4 years.

Average lifespan
Status: wild: 1 years.

Aglantha digitale, also known as the pink helmet, is found in northern seas, and northern parts of the Atlantic and Pacific oceans. One of the most widespread species of hydromedusae, it also inhabits the North sea of Skagerrak, the Arctic Ocean, Dvina Bay, Onega Bay, Hudson Bay, and Hudson Basin and Kandalaksha Bay in the White Sea.

Biogeographic Regions: arctic ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

Diet consists of zooplankton, such as small shrimps and other planktonic organisms including microalgae, copepods, and tintinnids.

Aglantha digitale is a sink-fisher like many of the other hydromedusae. It swims upward with its tentacles contracted, then sinks upside-down with its tentacles extended horizontally. Aglantha digitale captures prey that come across its extended tentacles. When feeding, A. digitale lifts its prey with tentacle flexions to the margin of its bell. Then it points its manubrium and grips the prey with its flared lips. If the prey hasn’t been stunned by the nematocysts on the tentacles, nematocysts lined at the end of the manubrium will immobilize it completely.

Animal Foods: aquatic crustaceans; zooplankton

Plant Foods: phytoplankton

Other Foods: microbes

Primary Diet: carnivore (Eats non-insect arthropods); planktivore

Aglantha digitale is an intermediate link between microplankton and megaplankton, tranferring energy from lower to higher trophic levels. This species feeds on small copepods, chaetognaths, ciliates and diatoms. In turn, A. digitale is preyed upon by Atlantic mackerel and Chum salmon .

This species has no current commercial value.

There are no known adverse effects of Aglantha digitale on humans.

The inside of its bell is lined with giant ring axons and circumferentially lined with myoepithelial cells. Near the apex, there are eight giant motor axons. This primitive nervous system acts to detect any disturbance within the water or upon the animal and allows sends electrical impulses to the bell muscle fibers so that the animal can respond quickly.

Other Communication Modes: photic/bioluminescent

Perception Channels: tactile ; vibrations ; electric

This species is not threatened, nor is it currently managed.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

The eggs of Aglantha digitale are unusual compared to other hydrozoans. The egg contains large and irregular sized yolk globules that give it a crater-like surface. The egg then becomes a planula that remains afloat. The planula then develops into a larva called an actinula, which bears nematocysts. The actinula later develops into a medusa; there is no polyp stage in this species.

Development - Life Cycle: indeterminate growth

There is no specific information on how Aglantha digitale spawns, but hydrozoans in general are broadcast spawners.

Mating System: polygynandrous (promiscuous)

Aglantha digitale spawns between June and September. Maturity is determined by comparing the length of the gonads to the height of the bell. Individuals with gonads longer than 10% of their bell height are considered mature, while those with gonads shorter than 10% of their bell height are considered immature.

Breeding interval: Aglantha digitale breeds once every year.

Breeding season: Winter

Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; sexual ; fertilization (External ); broadcast (group) spawning; oviparous

Because most hydrozoans are broadcast spawners, there is no parental investment in the young. Larvae are planktonic, and develop independently.

Parental Investment: no parental involvement

The most common surface-dwelling jellyfish in Arctic and sub-arctic waters

Diet is unknown

clear translucent, bell taller than breath, with a small conical apical projection; ~80 marginal tentacles generally destroyed during capture; 8 radial canals, 8 statocysts; long gastric peduncle with 4 small simple lips; 8 white, sausage shaped gonads often obvious in larger individuals

Holoplanktonic, without a polyp stage; Generation time 1 year; Life cycle 1 year, possibly 2 in the Arctic

Widely distributed Arctic/sub-arcitc species; Typically found above 200 m; Both on shelves (neritic) and over basins (oceanic), but more common over deeper waters than shallow areas

Swims by contracting the velum.

Gonads are yellowish in color, manubrium and tentacles are usually pink. The surface of the subumbrella strongly iridescent.

Oceanic, widely distributed species in Arctic and sub-Arctic seas. In the Arctic is found in the Barents Sea, White Sea, Kara Sea, Chukchi Sea, Bering Sea, Sea of Okhotsk

Umbrella with a protruding tip at the top. Walls very thin. Velum wide, with well-developed musculature. Short manubrium with 4 labia is located at the tip of a cylindrical gastral stem, which is almost as long as the inside of the umbrella. The edge of the umbrella carries numerous (up to 100) thin tentacles, which are often broken off during fixation and 8 statocysts. Gonads originate from the radial canals near the base of the gastral stem and hang freely in the atrium of the umbrella.

Height up to 20-30 (sometimes 40) mm, width up to 15 mm.

Aglantha digitale is a species of hydrozoan in the family Rhopalonematidae. It is found in the Arctic and sub-Arctic where it is one of the most common jellyfish. It is unusual in having both a slow swimming action, through pulsating its bell, and a rapid escape response.

only medusae, direct development

upper epipelagic and glacial