Overview

Brief Summary

Overview

If you heard about a tiny, funny-looking animal that spends its life floating upside-down on the surface (1,2,3) of the Pacific, Atlantic, or Indian Ocean(2,3,4,5) thanks to an air bubble which it swallows and keeps inside its belly,(1,2,3,5) going wherever the currents and the wind take it,(3) you would probably think it was just a harmless creature that likes to relax in the water. But this slender,(5) up-to-3-centimeter-long(4,5,6) animal, which is called the blue glaucus,(3) blue sea slug,(1) or blue ocean slug,(2) is not nearly as innocent as it seems. The first trick it’s got up its sleeve is a form of camouflage called countershading that protects it from both flying and swimming predators while it floats.(2) The underside of the blue glaucus, which faces upward, is blue, helping it blend into the water’s surface when seen from above, while its back, which faces downward, is a more grayish color, helping it blend into the ocean when seen from below.(2,4) The second tricky feature of the blue glaucus is even more amazing. It feeds on hydrozoans (a group of animals in the same phylum as jellyfish), especially the highly poisonous Portuguese Man-O’-War.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(3) the blue glaucus, like some other sea slugs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may keep itself safe from the poison by releasing protective mucus and by hard barrier-like discs inside its skin.(7,8) But the blue glaucus does more than simply protect itself against these stings. It stores the swallowed poison inside the up to 84 finger-like structures or cerata(4,6) sticking out of its body, and then uses this poison to defend itself against other predators!(1,2,3,4,6)

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General Description

The blue glaucus (Glaucus atlanticus), sometimes called the blue sea slug(1) or blue ocean slug,(2) is a bizarre-looking marine creature in the group of sea slugs known as nudibranchs.(2,3) Found in the temperate and tropical waters of the Pacific, Atlantic, and Indian Oceans,(2,3,4,5) this slender,(5) up-to-3-centimeter-long(3,5,6) slug lives its life floating upside-down on the surface of the ocean thanks to an air bubble which it swallows and keeps inside its stomach.(1,2,4,5) Its color pattern, an example of a phenomenon known as countershading,(2) helps it avoid both flying and swimming predators while floating wherever the wind and the currents take it(4): its underside, which faces upward, is blue, helping it blend into the water’s surface when seen from above, while its back, which faces downward, is a more grayish color, helping it blend into the ocean when seen from below.(2,3) This camouflage, however, is not the blue glaucus’ only form of self-defense. It feeds on animals known as hydrozoans (in the same phylum as jellyfish), especially the highly venomous Portuguese Man-O’-War.(1,2,3,4) Although a sting by a Portuguese Man-O’-War is very painful to a human,(4) the blue glaucus, like some other nudibranchs, can swallow its prey’s stinging cells (known as nematocysts) without hurting itself.(1,2,7,8) It may be able to protect itself from the sting both by secreting mucus and by hard discs in its skin.(7,8) Far from being harmed by the poison, the blue glaucus stores it in the up to 84 finger-like structures or cerata(3,6) sticking out of its body, and uses it to defend itself against predators.(1,2,3,4,6)

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Taxonomy

The family Glaucidae has just two genera, Glaucus and Glaucilla and each of these genera have just one species: Glaucus atlanticus and Glaucilla marginata.Glaucus atlanticus:
  • can be up to 3cm in size
  • the species has up to 84 cerata - outgrowths on the upper surfaces of the body
  • the cerata hold the stinging cells (nematocysts) which are taken from the jellyfish they eat and are stored in special sacs called cnidosacs
This species closely resembles the other species in the family, Glaucilla marginata. The main differences between these two species are:
  • Glaucilla marginata has more cerata than Glaucus atlanticus, up to 137 in total
  • the tail (metapodium) of Glaucus atlanticus is much longer than that of Glaucilla marginata
  • Glaucus atlanticus is the larger of the two species - Glaucilla marginata only reaches 12mm in length
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Introduction

The Glaucus atlanticus, or blue glaucus (Forster, 1777) is the only species of the genus Glaucus and only one of two species in the family Glaucidae.The animals float upside down at the surface of the sea, keeping afloat by swallowing air which is stored in their stomachs.Their diet is mainly made up of hydrozoans including the Portuguese Man o’ War - they even eat the stings. As well as getting nourishment from the stings, they use them for their own defence.
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Distribution

Distribution and ecology



Feeding
  • the blue glaucus feeds mainly on hydrozoans although they are also known to be cannibalistic
  • one of the specialities of their diet is the Portugese Man-O-War, Physalia physalis (Linnaeus, 1758), best known for the very painful stings they can give if you are unlucky enough to get caught in their tentacles
  • the glaucus will eat the tentacles, both the fired and unfired stinging cells (nematocysts) and will pass these stings into special pouches in their cerata which they then use for defence


Distribution
  • They are found throughout the tropical Atlantic, Pacific and Indians oceans
  • Glaucus live a pelagic life - this means they go where the winds and currents take them
  • keeping air in their stomachs helps them to stay afloat on the surface of the oceans
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Physical Description

Diagnostic Description

Description

A distinctive, truly pelagic silvery blue mollusc, up to 6 cm long, with three pairs of arms. Floats upside-down just under the sea surface, feeding on the colonial hydroids Physalia, Porpita and Velella. Habitat: offshore. Distribution: pantropical. N.B. must be handled with care as it can give an extremely painful sting, due to the nematocysts taken from the prey species Physalia, which itself gives a nasty sting.
  • Richmond, M. (Ed.) (1997). A guide to the seashores of Eastern Africa and the Western Indian Ocean islands. Sida/Department for Research Cooperation, SAREC: Stockholm, Sweden. ISBN 91-630-4594-X. 448 pp.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage: Glaucus atlanticus

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 0
Specimens with Barcodes: 11
Species With Barcodes: 1
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Wikipedia

Glaucus atlanticus

Glaucus atlanticus (commonly known as the sea swallow, blue angel, blue glaucus, blue dragon, blue sea slug and blue ocean slug) is a species of small-sized blue sea slug, a pelagic aeolid nudibranch, gastropod mollusk in the family Glaucidae.[1] It is closely related to Glaucus marginatus, which is sometimes included in Glaucus.[2]

These sea slugs feed on other pelagic creatures including the venomous cnidarian, the Portuguese Man o' War. Because the sea slug stores stinging nematocysts from the cnidarian within its own tissues, a human picking up the sea slug may receive a very painful sting. For more information on the effects of the nematocysts, see Portuguese man o' war.

Characteristics[edit]

The blue sea slug (here shown out of water, and thus collapsed) is one of the smallest members of its biological family, Glaucidae

At maturity Glaucus atlanticus can be up to 3 centimetres (1.2 in) in length.[3] It is silvery grey on its dorsal side and dark and pale blue ventrally. It has dark blue stripes on its head. It has a tapering body which is flattened, and has six appendages which branch out into rayed, finger-like cerata.[4]

The radula of this species bears serrated teeth.[5]

Distribution and habitat[edit]

This nudibranch is pelagic, and occurs throughout the world's oceans, in temperate and tropical waters. Regions where this slug is found include the East and South Coast of South Africa, European waters, the east coast of Australia and Mozambique.[6] This species floats upside down on the surface tension of the ocean.

Life history and behavior[edit]

G. atlanticus preys on other, larger pelagic organisms: the dangerously venomous Portuguese Man o' War Physalia physalis; the by-the-wind-sailor Velella velella; the blue button Porpita porpita; and the violet snail, Janthina janthina. Occasionally, individual Glaucus become cannibals given the opportunity.

G. atlanticus is able to feed on Physalia physalis due to its immunity to the venomous nematocysts. The slug consumes the entire organism and appears to select and store the most venomous nematocysts for its own use. The venom is collected in specialized sacs (cnidosacs), on the tip of the animal's cerata, the thin feather-like "fingers" on its body.[7] Because Glaucus concentrates the venom, it can produce a more powerful and deadly sting than the Man o' War upon which it feeds.[7]

With the aid of a gas-filled sac in its stomach, G. atlanticus floats at the surface. Due to the location of the gas sac, the sea swallow floats upside down. The upper surface, actually the foot (the underside in other snails), has either a blue or blue-white coloration. The true dorsal surface (downwards in G. atlanticus) is completely silver-grey. This coloration is an example of counter shading, which helps protect it from predators from below, sides, and above.

Like almost all heterobranchs, Glaucus is a hermaphrodite, having both male and female reproductive organs. Unlike most nudibranchs, which mate with their right sides facing, sea swallows mate with ventral sides facing.[8] After mating, both animals produce egg strings.

References[edit]

  1. ^ Lalli, C. M.; Gilmer, R. W. (1989). Pelagic snails: the biology of holoplanktonic gastropod mollusks. Stanford University Press. p. 224. ISBN 978-0-8047-1490-7. Retrieved 13 Jan 2010. 
  2. ^ WoRMS. "Glaucus". World Register of Marine Species. Retrieved 5 August 2012. 
  3. ^ "Glaucus atlanticus (blue sea slug)". The Natural History Museum. Retrieved 2013-04-13. 
  4. ^ Piper, R. (2007). Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals. Greenwood Publishing Group. pp. 42–43. ISBN 978-0-313-33922-6. 
  5. ^ Thompson, T. E.; McFarlane, I. D. (2008). "Observations on a collection of Glaucus from the Gulf of Aden with a critical review of published records of Glaucidae (Gastropoda, Opisthobranchia)". Proceedings of the Linnean Society of London 178 (2): 107–123. doi:10.1111/j.1095-8312.1967.tb00967.x.  edit
  6. ^ Glaucus atlanticus Forster, 1777. World Register of Marine Species, Retrieved 3 April 2010.
  7. ^ a b Rudman, W. B. (6 November 1998). "Glaucus atlanticus Forster, 1777". Sea Slug Forum. Retrieved 26 February 2011. 
  8. ^ Debelius, H.; Kuiter, R. H. (2007). Nudibranchs of the world. IKAN-Unterwasserarchiv. ISBN 978-3-939767-06-0. 

Further reading[edit]

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