Population biology of Octopus digueti and the morphology of American tropical octopods.

Abstract

My dissertation explores octopod ecoloy, morphology and evolution. Using an artificial shelter trap technique (Voight, 1988a), parameters of a wild population of Octopus digueti are monitored for one year. Octopus movement correlates with sea temperature but is reduced under full moonlight. Enlarged suckers reliably indicate male maturity in this species. This first definition of an age class in octopuses allows field growth rates to be compared to those from laboratory studies. Octopus digueti growth in the wild equals that in the lab; average life span may be only 6 months during which time individuals may grow from a hatching weight of 40 mg to over 40g. Individuals show the uniparous life history documented in lab studies. Cohorts appear not due to reproductive synchrony, but to seasonal temperature fluctuations. Genetic differences probably control individual growth rate and life span. To test the reliability of morphology of preserved octopus specimens, ln-transformed measurements are plotted versus ln mantle length. Body measurements are strongly correlated with size; preservation does not therefore eliminate information contained in specimens. Principal component analysis reveals shallow water tropical octopuses vary primarily in arm length, mantle length and sucker diameter. Trans-Atlantic conspecific populations are morphologically indistinguishable. Hypothesized species relationships (Voight, 1988b) are supported, despite considerable overlap among species. No secondary sexual dimorphism except enlarged suckers is present in these species. Octopuses from rocky habitats have longer arms and smaller mantles than do those from sandy habitats. In the Octopodidae, arm length, head width and sucker diameter contribute most size-free morphological variation. Sucker size correlates inversely with depth due to hydrostatic pressure. Arm length and head width variation correlate inversely with latitudinal distribution, and are associated with the number of sucker rows. Subfamilies defined by the number of sucker rows (Voss, 1988a), may represent overtly similar, paraphyletic groups. Cladistic analysis of the suborder Incirrata show that Voss' subfamilies are paraphyletic groups; Robson's (1932) subfamilies are supported. The Argonautida are the most primitive incirrate group, the Ctenoglossa and Octopodidae are sister taxa. The incirrate octopods may have evolved from a deep sea, rather than a shallow-water, ancestor.