Here we report for the first time that adult Hapalochlaena lunulata (Quoy & Gaimard, 1832), which has a vestigial ink sac, is capable of inking. Ink was released under three different agonistic conditions: female-female aggression, rejection of mating attempt, and when attacked by a predator. We observed no apparent reaction to the ink by the other animals involved in these interactions.
Ink sacs are found in most shallow-water coleoid cephalopods (octopuses, squids, and cuttlefish), but the function of inking as a defensive behavior of cephalopods rarely has been investigated. Released ink is believed to (1) provide a smoke screen for escape; (2) serve as a pseudomorph; or possibly (3) block olfactory or taste receptors (Schäfer 1956, Moynihan and Rodaniche 1977, Hanlon and Messenger 1996, Boletzky 1997). Cephalopod ink elicits jetting in squids (Gilly and Lucero 1992) and increases ventilation rate in cuttlefish (Boal and Golden 1999). This may indicate its function as an alarm pheromone to warn conspecifics, but such behaviors also could be elicited if the ink was a general irritant. In fact, some predators may be attracted to ink (Grüninger 1997).
In octopuses, the reduction or loss of an ink sac may have evolved independently in at least three shallow-water taxa that have developed other effective defenses. Ameloctopus littoralis Norman, 1992, a small, intertidal octopus, lacks an ink sac but readily autotomizes its long arms (Norman 1992). The undescribed "Wonderpus," another long-armed, shallow-water octopus capable of arm autotomy, has a severely reduced ink sac (M. D. Norman and F. G. Hochberg, pers. comm.) and does not ink despite considerable disturbance (C.L.H., pers. obs.). Blue-ringed octopuses, Hapalochlaena spp., are known best for their potent, tetrodotoxin-laden venom (Norman 2000). The genus is diagnosed by the presence of multiple iridescent blue rings or lines on the body and arms and marked reduction of the ink sac in adults (Robson 1929). Although Robson did not speculate on the vestigial ink sac in adults, he did note its drastic reduction in size in the type species, H. lunulata (Quoy & Gaimard, 1832), compared with Hapalochlaena maculosa (Hoyle, 1883).
Hatchling Hapalochlaena are able to expel ink: H. fasciata (Hoyle, 1886) (Tranter and Augustine 1973 [taxonomy according to Stranks and Lu 1991]); H. maculosa (Stranks and Lu 1991); H. lunulata (R.L.C., pers. obs.). However, H. fasciata does not ink after they are 4 weeks old (Tranter and Augustine 1973). Juvenile (5- to 6-mm mantle length) H. maculosa expel ink, but the reservoir degenerates with age (Stranks and Lu 1991) until neither production nor release of ink occurs in adult H. maculosa (Roper and Hochberg 1988, Stranks and Lu 1991). Tranter and Augustine (1973) also reported that H. fasciata does not eject ink as adults. Recently, two undescribed species of Hapalochlaena were noted to ink as adults (sp. 3 and sp. 5 of Norman 2000). We therefore can no longer assume that degeneracy of the reservoir in Hapalochlaena spp. obligates the loss of the ability to ink.
During laboratory studies on H. lunulata obtained from Indonesia, we recorded seven [End Page 255]
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instances when adults released a single, poorly defined cloud of brown ink. In each case, the small puff of ink diffused in seconds. The octopuses ejecting the ink did not jet from the inked area. We observed no apparent direct reaction to the ink by the other animals involved in these situations.
Two adult octopuses (one male and one female) inked as they attempted to escape from an attack by a stomatopod, Odontodactylus scyllarus (Linnaeus, 1758). Another male inked as it jetted away from a nearby stomatopod. A female inked as she rejected the approach of a male with which she had previously mated, and a brooding female inked while a male mounted her, attempting to mate. An...