Abstract
The entire skin surface of octopus embryos, hatchlings and juveniles bears scattered tufts of tiny chitinous setae within small pockets, from which they can be everted and retracted. Known as Kölliker’s organs (KO), they disappear before the subadult stage. The function of these structures during the early life of the octopus is unknown, despite having been first described nearly two centuries ago. To investigate these organs further, general trends in size of KO distribution and density were analyzed in hatchlings and juveniles of 17 octopod species from all oceans, representing holobenthic, holopelagic and meropelagic modes of life. The size of the KO is fairly constant across species, unrelated to mode of life or hatchling size. The density of KO is similar on ventral and dorsal body surfaces, but hatchlings of smaller size tend to have a higher density of KO on the aboral surface of the arms. Analysis of a series of post-hatchingOctopus vulgarisshows KO size to be constant throughout ontogeny; it is therefore a consistent structure during the octopus early life from planktonic hatchling to benthic juvenile. New KO are generated on the skin of the arm tips during the planktonic period and initial benthic lives of juveniles. Their density, on both the mantle and arms, gradually decreases as the octopus grows. In older benthic juveniles, the KO degrades, losing its setae and the base of its follicle becomes exposed as a nearly circular stump of muscle. It is estimated that fully everted KO increase the body surface area by around two-thirds compared to when the KO are retracted. This modular mechanism of body surface extension and roughness probably influences flow-related forces such as drag and propulsion of the moving surface of the young octopus while it is of small size with a relatively large surface area. In addition, the distribution of these organs on the aboral surface of the arms of the octopus and their birefringent properties suggest a role in camouflage. Further research is needed to test these hypotheses of KO function in live animals.
Highlights
Cephalopods have a uniform mode of early development: they all hatch with a complete set of internal organs and systems as in adults, including the gonads, that will develop when sexual maturation begins (Budelmann et al, 1997)
To determine how the Kölliker’s organs (KO) density pattern changes with animal age and growth, particular emphasis was given to the study of KO in O. vulgaris, using reared and wild specimens across a size range of 1.1 to 8.3 mm mantle length (ML) (Table 2)
As far as we know, the setae of other animal groups do not have the capacity to splay and radiate into the everted form observed in the octopod KO, with the possible exception of the long chaetae of trochophore larvae from sabellarid polychaetes (Pennington and Chia, 1984)
Summary
Cephalopods have a uniform mode of early development: they all hatch with a complete set of internal organs and systems as in adults, including the gonads, that will develop when sexual maturation begins (Budelmann et al, 1997). The external morphology of some species changes notably during development: octopod embryos, hatchlings and juveniles have clumps of chitinous, bristle-like transitory structures, called Kölliker’s organs (KO), present on the surface of the skin, which has a punctate appearance (Figures 1, 2). These organs are present in species belonging to all three basic modes of octopod life, that is holobenthic, holopelagic and meropelagic (Lincoln et al, 1998). We follow earlier studies on octopod KO (Boletzky, 1973; Brocco et al, 1974) by using the term setae to describe the simple, chitinous unit rodlets which make up the characteristic KO tuft
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
