A project undertaken at the College of Marine and Environmental Sciences, James Cook University, and supervised by Associate Professor Kyall Zenger
The Southern Blue-Ringed Octopus (Hapalochlaena maculosa) is an Australian endemic species occurring along the southern coastline of the Australian continent. Despite their relatively large species range, H. maculosa are unique among many marine invertebrates in that they lack a planktonic larval phase. Their limited dispersal ability suggests that the connectivity of H. maculosa populations in the wild might be particularly vulnerable to habitat fragmentation and/or any geographic barriers to recruitment and gene flow. So far it was known that the morphology of H. maculosa can differ subtly between geographic regions, and preliminary molecular analyses have revealed that the genetic differentiation between individuals of east and west coast populations is substantial. In our research we used genome wide sequencing to investigate the relatedness of H. maculosa individuals sampled from populations across its geographic range. By doing so, we aimed to unravel both fine-scale and broad-scale patterns of connectivity and genetic structuring within the species. This information enabled us to identify whether there were any signs of local adaptation, any barriers restricting species connectivity and also to assess the possible existence of any subspecies and/or hybrid zones.
Additionally, we investigated several aspects of mate choice behaviours in H. maculosa. Several features of H. maculosa’s life history make it an interesting and unique animal model for studying mechanisms and impacts of sexual selection. Like most cephalopods, H. maculosa has only a very brief life span with a relatively high investment towards reproduction. Hapalochlaena maculosa lives for seven months, and both males and females of this species designate their final one to two months seeking out copulations with multiple partners during a single terminal-breeding season. During this time, males allocate up to approximately fifty finite spermatophores to copulations with different females, and females store the sperm they receive from different males until they lay a single egg-clutch of approximately fifty eggs that the females will guard until hatching. As both males and females of this species have a limited set of gametes and are highly promiscuous, this mating system is ideal for investigating dynamics of both male and female mate choice mechanisms and how they can change with subsequently encountered mates during a breeding season. Our laboratory studies focused on whether males or females were selective of the mates they chose and how they chose them. Similarly, we were interested in whether males strategically allocated more or less spermatophores with females based on their mating histories in a manner that would be consistent with predictions of sperm competition. Finally, we also aimed to address which males’ stored sperm ended up fertilising the egg clutches of females, and whether there were consistent patterns in male behaviours or phenotypes that led to higher paternities.
Broad-scale Genetic Structuring:
- Hapalochlaena maculosa forms a clinal species complex across its geographic distribution from the southwest Australian coastline to Victoria and Tasmania.
- The genetic divergence between distal ends of this range is consistent with the genetic differentiation observed between H. maculosa and its sister-taxon H. fasciata. However, limited gene flow is maintained throughout the H. maculosa distribution via small migrations between adjacent populations.
- The genetic structuring of sampled populations in this study was highly affected by both limited gene flow and strong patterns of local adaptation. This suggests that H. maculosa populations diverge rapidly and would be particularly susceptible to speciation if any barriers to dispersal/gene flow were to occur across its current species range.
- Diversity indices within populations indicate that individuals occupying the same habitat are highly related. Despite this, indices also suggested that inbreeding might be rare in this species. These field data strengthen findings in the above paternity studies that post-copulatory fertilisation patterns in H. maculosa might favour offspring to unrelated parents.
- No indication of female preference of specific male traits, however females mated for longer with larger males.
- No indication of male preference for female traits or size, however males spent more time copulating with novel females and females that had previously mated with higher numbers of competing males.
- Males mounted other males as frequently as they mounted females. However, male-male mounts were shorter than male-female mounts, suggesting that the males might not be able to discriminate the sex of conspecifics until after they have attempted to copulate.
- High-levels of multiple paternity were found in all genotyped egg clutches.
- There was no relationship between copulation time and relative paternity patterns, suggesting that extended copulations might be a form of mate guarding rather than sperm loading.
- There was a non-significant trend for paternity to be biased to males that had mated with females earlier in her breeding season (first-male paternity bias).
- In general the paternity patterns matched the relative sperm signatures observed in female oviducal glands after egg-laying, which suggests that females are not selectively choosing to fertilise their eggs with stored sperm from particular males. However, this was not always the case. In the unintentional pairing of related individuals, paternity was significantly biased to different males despite the majority of sperm remaining in the female’s oviducal gland belonging to the related male. This anecdotally suggests that post-copulatory mechanisms might bias paternity to genetically compatible males, possibly to avoid inbreeding.
Cognition and Sensory:
- No indication of visual signalling using chromatic or polarised light properties. This was consistent with the observation that this species is only active nocturnally.
- Both males and females were found to detect conspecifics via chemical cues in the water.
- Females responded to chemical signals differently based on the sex of the detected conspecific, however males did not.
- Females that reacted strongly to a male’s odour were more likely to be unreceptive to copulation attempts from that same male, and spent less time in copulation with these males compared to males whose odour elicited a weaker response. This suggests that response to conspecific odours might be related to agonistic behaviour.