A remarkable method of defense has been documented for Tremoctopus violaceus. Jones (1963) noted that earlier reports had documented the occurrence of unidentified cnidarian tentacles on the arms of immature T. violaceus, and the suggestion had been made that this might be a defensive strategy, with the octopus actively acquiring these tentacles as weapons rather than just the result of a chance encounter. Subsequent observations by Jones revealed that these tentacles were from the Portugese Man-of-War (Physalia) and were attached in an orderly fashion to each row of suckers on each of its four dorsal arms, with none found on the four ventral arms. Jones noted anatomical features of the suckers that he proposed are adaptaions for holding cnidarian tentacles (see Jones 1963). Jones speculated that these weapons might also be used to aid in capturing prey.
Only the tiny males and females 7 cm or less in length have been observed carrying tentacles. Norman et al. (2002) speculate that males of this species may be so small in part because the evolutionary benefits of larger size cannot compensate for the loss of this effective weapon system. The fecundity benefit for females that comes with larger size, in contrast, might be sufficient to compensate for outgrowing the "tentacle weapon system". Reproductive competition among males may also drive the reduced size of males. Females have been found with multiple male arms in their mantle cavities (Thomas 1977), suggesting the possible importance of competition among males for fertilization opportunities. Maturing at a small size might allow earlier and greater investment in sperm than would otherwise be possible.