earth science

Seven-arm octopus

Seven-arm octopus is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Seven-arm octopus rather than just read about it. In short: The seven-arm octopus (Haliphron atlanticus), also known as the blob octopus or sometimes called septopus, is one of the two largest known species of octopus; the largest specimen ever discovered had an estimated total length of 3.5 m (11 ft) and mass of 75 kg (165 lb). The only other similarly large extant species is the giant Pacific octopus, Enteroctopus dofleini.

Key takeaways

Seven-arm octopus belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Seven-arm octopus to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Seven-arm octopus from memory before moving on to harder problems.

Reference excerpt

The seven-arm octopus (Haliphron atlanticus), also known as the blob octopus or sometimes called septopus, is one of the two largest known species of octopus; the largest specimen ever discovered had an estimated total length of 3.5 m (11 ft) and mass of 75 kg (165 lb). The only other similarly large extant species is the giant Pacific octopus, Enteroctopus dofleini. The genera Alloposina (Grimpe, 1922), Alloposus (Verrill, 1880) and Heptopus (Joubin, 1929) are junior synonyms of Haliphron, a monotypic genus in the monotypic family Alloposidae, part of the superfamily Argonautoidea in the suborder Incirrata of the order Octopoda.

Traits

The seven-arm octopus is so named because in males, the hectocotylus (a specially modified arm used in egg fertilization) is coiled in a sac beneath the right eye. Due to this species' thick, gelatinous tissue, the arm is easily overlooked, giving the appearance of just seven arms. However, like other octopuses, it actually has eight. Unlike most octopods, H. atlanticus has a swim bladder, an organ evolved from its digestive system. The swim bladder allows it to maintain neutral buoyancy, neither sinking nor rising in the water column. Older specimens have been found to have shorter bodies and longer arms than juveniles. While juveniles tend to be translucent, one study in 1985 in Norway studied the morphology between two female H. Atlanticus specimens, with one being a dark purple and the other a pale blue/white. Video evidence from the Monterey Bay Aquarium Research Institute (MBARI) was released showing a red specimen as well, giving the species a color range of blue to red when adults.

Distribution H. atlanticus is found worldwide in tropical and temperate waters. The type specimen was collected in the Atlantic Ocean at 38°N 34°W (west of the Azores). It is deposited at the University of Copenhagen Zoological Museum. Since then, several specimens have been caught throughout the Atlantic, as far as the Azores archipelago and near South Georgia Island. In 2002, a single specimen of giant proportions was caught by fishermen trawling at a depth of 920 m (3,020 ft) off the eastern Chatham Rise, New Zealand. This specimen, the largest of this species and of all octopuses, was the first validated record of Haliphron from the South Pacific. It had a mantle length of 0.69 m (2.3 ft), a total length of 2.90 m (9.5 ft), and a weight of 61.0 kg (134.5 lb), although it was incomplete. Another specimen was reported from Skjørafjorden, Norway, where a female was captured in a trawl net at approximately 100 m (328 ft) depth in November 2000. The individual measured about 96 cm (3.1 ft) in total length, though the arm tips were damaged, and weighed 8.5 kg (18.7 lb) when received by researchers. The specimen had a soft, gelatinous body with a reddish coloration and rows of pale suckers, and was later preserved and deposited at the NTNU Museum of Natural History and Archaeology in Trondheim. Evidence of the species in northern waters is also supported by dietary studies of sperm whales, which have contained remains of H. atlanticus in their stomach contents. Because these whales are believed to feed in Arctic and northern boreal waters before migrating south, these findings suggest that the octopus occurs more widely in the Norwegian Sea and nearby Arctic regions than the relatively small number of direct captures of indicate. There have also been multiple sightings of H. atlanticus in Monterey Bay in the Northeast Pacific Ocean.

Mating and reproduction The mating of H. atlanticus is poorly researched, but it is predicted to occur sporadically on the continental coast. Dwarf males of H. atlanticus have been discovered. These males have invested energy for growth into reproductive traits such as a spermatophore and a muscular penis. They also possess a detachable hectocotylus which is used to transfer the spermatophore into their mates. These males qualify as dwarf males as they are only 5-15% of the size of a maximum female specimen of H. atlanticus. The organism's swim bladder helps them maintain buoyancy without expending large amounts of energy searching for mates.

Ecology Isotopic, photographic and video evidence have shown complex interactions between H. atlanticus and jellyfish and other gelatinous zooplankton, from feeding to protection, respectively. H. atlanticus specimen have been observed feeding in mesopelagic areas, particularly at the ridges of seamounts. The ROV Dolphin-3K observed H.atlanticus catching a mid-water jellyfish Periphyllopsis braueri, which was one of the first confirmed captures of a medusae. Stable isotope analyses indicate that H. atlanticus occupies an intermediate trophic level among deep-sea cephalopods, consistent with feeding on gelatinous zooplankton as well as other prey types. Lower levels of iron and zinc have been found in H. atlanticus muscle tissue compared to other rare cephalopods, indicating a "cleaner" prey such as jellyfish. Additionally, H. atlanticus exhibit selective feeding by specifically target high-nutrient areas such as the stomach and reproductive organs of jellyfish. Predators of H. atlanticus include the blue shark, sperm whale, Hawaiian monk seal, and swordfish. This may be in part due to the diel vertical migration and diving that some of these species perform. The octopus has also been found in the stomach contents of shorebirds like Cory's shearwater, which predate on the octopus after it is driven to the surface by other organisms. On rare occasion, lancetfish and some species of tuna will predate on H. atlanticus.

Potential divergence of species Though widely considered monospecific, there is genetic evidence that there are multiple Haliphron species, with at least one in the South Pacific and one in the North Pacific. DNA Analysis of the COI gene from a specimen fragment discovered near the Fernando de Noronha archipelago in Brazil provided further evidence of this divergence. This genetic testing revealed that the specimen from the Brazilian coast shared a closer relationship with North Pacific populations than with those previously documented in the North Atlantic. These findings suggest that the H. atlanticus may actually consist of at least two species, one residing in the North Atlantic and another from the South Atlantic and Pacific Oceans.

Illustrations

Worked examples

Example 1 — a first encounter with Seven-arm octopus

Start with the simplest possible case. Write down what Seven-arm octopus claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Seven-arm octopus before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Seven-arm octopus ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Seven-arm octopus

In research: Seven-arm octopus appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Seven-arm octopus in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Seven-arm octopus is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cephalopods described in 1861, Cephalopods of Oceania, IUCN Red List least concern species, so understanding it makes those chapters shorter.
In everyday life: Look for Seven-arm octopus outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Seven-arm octopus in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Seven-arm octopus means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Seven-arm octopus out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Seven-arm octopus in simple terms?

Why does Seven-arm octopus matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Seven-arm octopus?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Seven-arm octopus.