Ameloctopus litoralis Norman 1992

Introduction

This distinctive long-armed species is restricted to intertidal coastal waters of northern Australia. Sexes are approximately equal in size (mantle length to around 30 mm, total length to 300 mm, weight to 20 g). This octopus was described in 1992, and there is limited information available on its behaviour and life history. It has no commercial value.

Characteristics

1. Greatly elongated arms, up to 10 times mantle length, equal in length when intact.
2. Arms capable of autotomy at set plane in arm base at level of 4th or 5th sucker pair.
3. Small head with tiny eyes.
4. Mantle thin-walled; dark branchial hearts visible through mantle.
5. Ink sac absent.
6. Funnel organ greatly reduced to two to four small oval pads.
7. Anal flaps absent.
8. Gills with 5-6 lamellae per demibranch.
9. Terminal organ of male reproductive tract lacks diverticulum.
10. Spermatophores with bulbous inflated sperm reservoir.
11. Low skin sculpture without papillae.

Behaviour

Ameloctopus forages over intertidal sand and rubble flats at night, using the long arms to probe down crevices for small crustaceans. Captive animals typically remain within shelter and shoot out the long arms to grab passing prey. This octopus discards arms as a decoy to predators, in the same manner as drop-tail lizards. The wriggling severed arm distracts (and feeds!) the predator while the octopus flees. One arm was observed to writhe for over 4 hours, continuing to crawl and adhere with suckers. In other arm-dropping octopuses, the arm regenerates over 6 to 8 weeks. There is a set level in the arm at which the break occurs. The exact mechanism by which the arm separates is unknown.

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Left: Ameloctopus litoralis with left arm III autotomized. Photograph copyright © 1996, Mark D. Norman. Right: Histological section (longitudinal) of an arm showing the plane where autotomy occurs (arrow).  Histology, B. Abaloz, photograph copyright © 1996, David Paul.

Life history

The males of this octopus have a unique system of reproduction. In other bottom-living octopuses, the male develops a modified third arm from birth, known as the hectocotylus. This arm has a spoon-like tip used to pass spermatophores into the oviducts of the female. Ameloctopus is unique in that it does not develop these modifications until fully mature (i.e., its reproductive tract is full of spermatophores). Instead it has a full-length normal arm while the animal is immature, which can be used for feeding and/or discarding at predators. On maturity, the male drops this arm near the base and develops the spoon-like tip directly out of the stump. He is then able to mate using this special short arm to transfer sperm to the female. Mature females of this species produce around 80 eggs, each 10 mm long. These relatively large eggs would result in benthic hatchlings, young that immediately adopt the bottom-living habits of the adults.

Habitat

This octopus appears restricted to intertidal reefs and flats of mud, sand, coral and/or rubble. It emerges to forage over these reefs during night low tides, typically in depths of 10 cm or less. Presumed benthic hatchlings appear to restrict this species to continuous coastal habitat across northern Australia.

Distribution

Ameloctopus litoralis is known only from the coastal waters of tropical Australia from the southern Great Barrier Reef (23 deg. 30 min.S , 152 deg. 00 min. E), west to the Dampier Archipelago, Western Australia (20 deg. 30 min. S, 116 deg. 30 min E).

Other Names for Ameloctopus litoralis Norman 1992

• Banded Drop-arm Octopus
• Ameloctopus Norman 1992

References

Norman, M.D. 1992. Ameloctopus litoralis, gen. et sp. nov. (Cephalopoda: Octopodidae), a new shallow-water octopus from tropical Australian waters. Invertebrate Taxonomy 6: 567-82

Information on the Internet

• Speciesbank: Ameloctopus litoralis. Australian Biological Resources Study (ABRS). Provides a distribution map and a short video.