RARELY SEEN ALIVE, th.e octopus Argonauta argo is an open-ocean, surface dweller. Like its relatives, the mgonaut possesses eight arms and 0 flair of highly developed eyes. Thefemale argonaut, however, has the unusual ability to secrete {J fragile externa I shell. All photographs by G. K. Jensen Argonaut Octopus in a Parchment Shell by THOMAS M. ILIFF!:: HOUSED IN A DELlCATt:: shell, the argonaut, or paper nautilus, lives in warm, open-ocean waters around the world. It is seldom studied, however, since it rarely washes ashore alive. In March 1980, the author was fortunate to find a live female argonaut (Argonaut{J. argo) stranded on a beach in Bermuda. The animal appeared to be in good condition and fed actively in the aquarium in which it was kept for six days. During this period, the argonaut was photographed and its behavior was observed. When discovered, the Bermuda specimen had been stranded on a sandy beach by receding tides and onshore winds. It had drawn back into its shell, but when placed in an aquarium, it readily expanded the front portion of its body and covered both sides of the shell with its pair of webbed arms. The tips of the other arms remained thrust back into the shell, holding it in place. Like other octopuses, the argonaut has eight arms surrounding its head and a pair of highly developed eyes. The female argonaut is the only modern octopus to possess an external shell-like covering. This structure, which is actually a modified egg case, serves as a sea-going brood chamber in which the female hangs thousands of tiny eggs. It differs from the true shell of the chambered nautilus (another cephalopod mollusk, to which it is not directly related) in that it lacks internal partitions and is thin and fragile. Pearl-white and parchmentlike, the shell is secreted by highly modified and extended webs of the first pair of arms. The male argonaut, which has no shell, is considerably smaller than the female (it is about Yz to I inch in size. while the female measures from 4 to 8 inches). One of the male's arms which may be ten times as long as its body, 1/2 to 1 inch ~!. 2 7 t02.54 centimeters 4 to 8 inches ~ 10 to 20 centimeters July-August 1982 225
July - Augu sl 1982 227 ii{io t::scf.nt WElHH:O ARMS veil the shell of Ihe Jf/na/r (lrgona ul. The li/),1 ojlhe remainink arm.1 a rr 111 rned back into llil'.1 11.('1/ 10 hold i/ in I}/ace as llip oclo/jus move.1 aboul. Pigmenlsacs, or Clir{J1naloplwres, give thl' u/'/jpi slujl/cp of Ihe web a rusly-red CO/O I: Possibly to distract potential predators, the argonaut, like its relatives the squids and other octo puses, can eject an ink when disturbed. In addition, its color pattern, called countershading (dark shades on the upper surface and reflective silve r on the underside), serves as excellent camou Rage for an anima l swimming near the surface of the sea. Quick-Change Artist has numerous small suction disks a nd a long threadlike organ specialized to carry a nd store sperm. During mating, this arm brea ks o ff and lodges in the ma ntle cavity of the fe male, where fertilizcltion of the eggs ta kes place. Originally, this detached arm fo und within female specimens was incorrectly identified as a species o f parasitic worm and was given the scie nti fic name of H('cIOC(ltyll1s. Though its true ide ntity was eventually discoven:d. the modified arm of male octopllses and squids is still ca ll ed the hectocotylus. Beca use the detached ann is ve ry active, it was also once thoug ht that the "I'm could swim f're e ly in the ocean searching for the female, though this is highly unlike ly since the arm lacks a ny sense of direction. Although the argonaut most frequently sw ims near the surface of the sea, it has been reported from depths as great as 11,000 leet. It normally feeds o n sea butterflies (planktonic pteropod mollusks) and small fishes. In turn, it is preyed upon by sailfish and o ther lal-ge pe lagic fishes. 11. 000 feel = 3.400 meters When alarmed, the argonaut is capable of putting on an impressive color display. Within two-thirds of a second, it can turn dark red. The upper surface of the web covering tlte shell exhibits a profusion of rusty-red chromatophores. These a re small and elastic sacs, each co ntaining only one color of pigment. Whe n muscles radially surrounding the chromatophore contract, the pigmentcontaining sac is stretched out into a large, fl at plate. Whe n the muscles relax, the e lastic ch romatophore contracts, and the color becomes confined to a small area and is much less apparent. E.xpansion a nd contraction or the chromatophores is con trolled by the animal's nervous system, with visua l info rmatio n providing the primary stimulus. The lower part of the web retains its highly reflective, iridescent silver color. Du ri ng its active periods in the aquarium, the Bermuda specimen ra nged from midwater to the surface, moving upward a nd bac kwa rd by forcefully expelling a jet of water from the protruding funnel. When undisturbed and kept in darkness, it remained on the bottom of the ta nk
DU.ICATELY SCULPTURtD h(llve.~ oj tltr nbijnl, /10 rcltmentlike shell are secreted by the webbed (Inns oj the orgona!l t andjwed along one margin to form (J knl or ridge. The shell serves both os an egg case or brood chamber and os a retreatjor the je1l/.0le. with most of its body withdrawn into the shell. This body position, in which the arms are turned back into the shell and the suction disks are displayed forward, is similar to the posture adopted by bottom-dwelling octopuses when hiding in a hole. To tempt the argonaut to feed, several small shrimps and bait fish were placed in the tank. At first the animal moved toward the fish and appeared ready to grab one, but then stopped and took no further action. After about 24 hollrs, several of the fish were killed and dropped into the tank 2 to 4 inches in front of the argonaut. It immediately reached out and grabbed a dead fish in its tentacles before quickly devouring it. In this manner, the argonaut ate six bait fish and two shrimps. On the evening of the sixth day of captivity, the argonaut fed as usual on a shrimp and appeared in good condition. By the following morning, however, the animal had crawled out of its shell and was lying on the aquarium floor, obviously near death. Previous investigators had only been ahle to keep such specimens alive from one to two weeks-quite similar to the author's experience. But that short time at least provided a rare glimpse into the behavior of a truly fascinatmg open-ocean species. o 2 to 4 inches- 5 to 10 centimeters 228 Sea Frontiers