Phylogenetics, biology, and ecology of two unclassified species of near-shore octopuses in Hawai‘i

Abstract

The current understanding of the biology and ecology of octopuses has focused on a limited number of species compared to the high diversity of species currently described. Within the tropical – subtropical Pacific there have been multiple studies describing the phylogenetic relationships among the near-shore octopus species, in particular within the southern and eastern regions. As we start to better understand species diversity through phylogenetic analysis, there is still limited information describing the biology and ecology of these species. In this study the phylogenetic relationships of two unclassified species of octopuses (Abdopus sp. 1 “Crescent octopus” and Octopus cf. vitiensis) inhabiting the near-shore coral reef ecosystem of Hawai‘i were compared to the octopus diversity of the tropical – subtropical Pacific. In order to better understand the physiology and biology of these two species, the growth rates of wild caught specimens and the effect of temperature on the onset of hatching were observed. Finally δ15N and δ13C stable isotope analyses were used to compare the trophic niche of four octopus species (Octopus cyanea, Callistoctopus ornatus, Abdopus sp. 1, and Octopus cf. vitiensis) that utilize the near-shore coral reef habitat in order to determine if there is a difference between species utilizing the same habitat. The results of the phylogenetic analysis indicate that Abdopus sp. 1 is a distinct species and is closely related to Octopus laqueus. Octopus cf. vitiensis is closely related to but distinct from Octopus oliveri, there were no genetic sequences of the type specimen or other non-Hawaiian Octopus cf. vitiensis available to conduct comparisons to determine if the Hawaiian population is the same or a distinct species. Growth observations of the two species indicate that Octopus. cf. vitiensis is sexual dimorphic with males obtaining a smaller maximum mass and mantle length (observed max 74.2g, 47.9mm) compared to females (observed max 133.0g, 60.0mm). There was no observed difference in size and mass between males and females of Abdopus sp. 1 in this study, possibly due to small sample size. The timing of the onset of hatching of the two species was negatively correlated with temperature, ranging from 27 days at 28°C to 57 days at 20°C for Octopus cf. vitiensis. There was no difference in egg developmental time between species at corresponding temperatures. Results from stable isotope analysis indicate that habitat/location has a greater effect on trophic niche than octopus size for the species and sizes analyzed. Differences in δ15N between species were variable between habitats/locations. This indicates that these species are opportunistic feeders, with prey selection based on what prey are more prevalent or easy to capture in a specific location.