Aglantha digitalepink helmet

Geographic Range

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. (Pertsova, et al., 2006; Schuchert, 2012; Takahashi and Ikeda, 2006)


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. (Hosia and Ulf, 2007; Ikeda and Imamura, 1996; Mackie, et al., 2003; Pertsova, et al., 2006)

  • Range depth
    25 to 700 m
    82.02 to 2296.59 ft
  • Average depth
    50 m
    164.04 ft

Physical Description

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. (Colin, et al., 2003; Meech, 2004; Pertsova, et al., 2006)

  • Sexual Dimorphism
  • female more colorful
  • Range length
    10 to 22 mm
    0.39 to 0.87 in
  • Average length
    12 mm
    0.47 in


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. (von Dassow, 1998)


There is no specific information on how Aglantha digitale spawns, but hydrozoans in general are broadcast spawners. (Hammond, 2009)

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. (Takahashi and Ikeda, 2006)

  • Breeding interval
    Aglantha digitale breeds once every year.
  • Breeding season

Because most hydrozoans are broadcast spawners, there is no parental investment in the young. Larvae are planktonic, and develop independently. (Hammond, 2009)

  • Parental Investment
  • no parental involvement


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. (Takahashi and Ikeda, 2006)

  • 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 is a planktonic species with two types of swimming behaviors. One is a rapid escape response with the bell margin contracting every 100 milliseconds. The other swimming behavior is a rhythmically slow swimming pattern characteristic of most types of medusae. In both types of movements, the subumbrellar muscle sheet is responsible for bell contractions. The muscle sheet contracts and propels the animal through the water by expelling water from its bell. While swimming, it withdraws its tentacles into its bell, reducing frictional drag. (Colin, et al., 2003; Roberts and Mackie, 1980)

Home Range

This planktonic species has neither home range nor territory.

Communication and Perception

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. (Roberts and Mackie, 1980)

Food Habits

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. (Colin, et al., 2003; Mackie, et al., 2003)

  • Other Foods
  • microbes


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. (Bickell-Page and Mackie, 1991; Mackie, et al., 2003; Melville, 2010)

Ecosystem Roles

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 . (Saito, et al., 2012)

Economic Importance for Humans: Positive

This species has no current commercial value.

Economic Importance for Humans: Negative

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

Conservation Status

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


Angelabelle Abarientos (author), San Diego Mesa College, Adolfo Nunez (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Renee Mulcrone (editor), Special Projects.


Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


an animal that mainly eats meat


the nearshore aquatic habitats near a coast, or shoreline.

  1. active during the day, 2. lasting for one day.

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


uses electric signals to communicate

external fertilization

fertilization takes place outside the female's body


union of egg and spermatozoan


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


generates and uses light to communicate


photosynthetic or plant constituent of plankton; mainly unicellular algae. (Compare to zooplankton.)


an animal that mainly eats plankton


the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


an animal which has an organ capable of injecting a poisonous substance into a wound (for example, scorpions, jellyfish, and rattlesnakes).


movements of a hard surface that are produced by animals as signals to others

year-round breeding

breeding takes place throughout the year


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Bickell-Page, L., G. Mackie. 1991. Tentacle autonomy in the hydromedusa Aglantha digitale (Cnidaria): an ultrastructural and neurophysiological analysis. Philosophical Transactions: Biological Sciences, 331/1260: 155-170.

Cartwright, P., A. Nawrocki. 2010. Character evolution in hydrozoa (phylum Cnidaria). Integrative and Comparative Biology, 50/3: 456-472.

Colin, S., J. Costello, E. Klos. 2003. In situ swimming and feeding behavior of eight co-occurring hydromedusae. Marine Ecology Progress Series, 253: 305-309.

Hammond, G. 2009. "Hydrozoa" (On-line). Animal Diversity Web. Accessed January 30, 2013 at

Hosia, A., B. Ulf. 2007. Seasonal changes in the gelatinous zooplankton community and hydromedusa abundances in Korsfjord and Fanafjord, western Norway. Marine Ecology Progress Series, 351: 113-127.

Ikeda, T., A. Imamura. 1996. Abundance, vertical distribution and life cycle of a hydromedusa Aglantha digitale in Toyama Bay, southern Japan Sea. Bulletin of Plankton Society of Japan, 43/1: 31-43.

Mackie, G., R. Marx, R. Meech. 2003. Central circuitry in the jellyfish Aglantha digitale IV. Pathways coordinating feeding behaviour. The Journal of Experimental Biology, 206: 2487-2505.

Meech, R. 2004. Impulse conduction in the jellyfish Aglantha digitale. Hydrobiologia, 1/3: 81-89.

Melville, K. 2010. "Finally! Something that eats jellyfish" (On-line). Science A Go Go. Accessed January 30, 2013 at

Pertsova, N., K. Kosobokoba, A. Prudkovsky. 2006. Population size structure, spatial distribution, and life cycle of the hydromedusa Aglantha digitale (O.F. Müller, 1766) in the White Sea. Oceanology, 46/2: 228-237.

Roberts, A., G. Mackie. 1980. The giant axon escape system of a hydrozoan medusa, Aglantha digitale. The Journal of Experimental Biology, 84: 303-318.

Saito, R., A. Yamaguchi, S. Saitoh, K. Kuma, I. Imai. 2012. Abundance, biomass and body size of the hydromedusa Aglantha digitale in the western and eastern subarctic Pacific during the summers of 2003–2006. Plankton and Benthos Research, 7/2: 96-99.

Schuchert, P. 2012. "Aglantha digitale (O.F. Müller, 1776)" (On-line). World Hydrozoa Database. Accessed January 30, 2013 at

Takahashi, D., T. Ikeda. 2006. Abundance, vertical distribution and life cycle patterns of the hydromedusa Aglantha digitale in the Oyashio region, western subarctic Pacific. Plankton and Benthos Research, 1/2: 91-96.

von Dassow, G. 1998. "Development of the jellyfish Aglantha" (On-line). A Comparative Embryology Gallery. Accessed January 30, 2013 at