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Behavior

Cool Fact! Octopuses spawn with ease when temperatures are lower

Juárez, Oscar E., et al. “Effect of maternal temperature stress before spawning over the energetic balance of Octopus maya juveniles exposed to a gradual temperature change.” Journal of Experimental Marine Biology and Ecology 474 (2016): 39-45.

Introduction:

The YP (Yucatan Peninsula, between the Caribbean Sea and the Gulf of Mexico, figure 1) is the home to a large octopus fishery. Temperature on the shelf varies between 22 and 26C, in part because of seasonal upwelling. At the very western edge of the YP the bottom water can get as warm as 31C in the summer and down to 24C in the winter. Scientists are worried about how the octopus population will respond to the prediction of a .6 to 2 degree Celsius increase in sea surface temperature by the end of the 21st century. Potential environmental change scenarios include: longer summers and shorter winters, faster transitions between seasons, and changes in upwelling.   All of these could impact the water column temperature, which may have adverse effects on marine organisms, like Octopus maya.

Figure 1: Yucatan Peninsula Image source: https://en.wikipedia.org/wiki/Yucatán_Peninsula

Figure 1: Yucatan Peninsula Image source: https://en.wikipedia.org/wiki/Yucatán_Peninsula

Marine biologists know that for octopuses, temperature influences reproduction and embryonic development. Octopuses require a lot of energy for growth so efficient conversion of energy via respiratory metabolism in the body is crucial to ensure proper bodily functionality. Respiration is in part a function of temperature, so changes in temperature could affect the octopuses’ ability to work at optimal levels.   In that respect, temperature is a control on the geographical distribution of octopuses; if they were to live somewhere out of optimal temperature range, their chances of survival decrease.

Figure 2: Octopus maya. (Photo: Universidad Nacional Autonoma de México) http://fis.com/fis/worldnews/worldnews.asp?l=e&id=63647&ndb=1

Figure 2: Octopus maya. (Photo: Universidad Nacional Autonoma de México) http://fis.com/fis/worldnews/worldnews.asp?l=e&id=63647&ndb=1

Octopus maya (figure 2) spawning is prompted by temperature changes. They spawn during the transition from winter to summer. Previous laboratory experiments have demonstrated that in 31C (summer temperature) far fewer octopuses spawned and the survival rate of the juveniles was zero. They also found that water kept at 24C (winter temperature) did not inhibit reproduction at all. In a tank where temperatures were decreased to simulate the transition from summer to winter, the octopuses only spawned when temperatures were at or below 27C. It was concluded that above 27C the mothers experienced stress, and thus have adapted not to spawn when temperatures are too warm. Pervious studies of octopuses have concluded that the energetic cost to transition from embryo to larval state is increased at higher temperatures.  Other work has found that Octopus maya juveniles prefer temperatures around ~24C.

Based on available information, and in light of predicted increases in ocean temperatures, scientists aimed their study at determining if thermal stress on the mother octopuses affected the ability of their hatchlings and juveniles to cope with changes in temperature. In a bigger picture, researchers hope that the known impact of temperature on observable characteristics of a species enables them to study how the rate of environment changes will impact the performance and genetic variation in a population.

Methods:

Scientists designed their study with two groups: one where the mother octopuses were not thermally stressed, and one where they were. Then they looked at how the juveniles responded to increases in temperature with respect to their bodily functions and their ability to thermally regulate.

Sample collection: Scientists collected 50 specimens from Sisal Harbour, Yucatan, Mexico. The octopuses were acclimated to the tank environment at a ratio of 1:1 male/female and were able to pair freely.

Stressing out the mothers: Three controlled environments were set up: a winter, a summer, and a seasonal transition.   The winter tank was maintained at 24C, the summer tank was maintained at 31C, and the seasonal transition tank was kept at 31 for ten days, then the temperature was decreased by 1C per day until 24C was reached, and then kept at 24C for the duration of the experiment.   The experiments were run for 40 days. Fifteen octopuses for each treatment type were individually tanked and observed. Conditions, diets, and cleaning methods were identical for all three treatments.

Observing the Hatchlings: The offspring of three mother octopuses from each treatment were collected then divided into four groups and fed at will. Forty-eight of the hatchlings were from stressed mothers and forty-eight of the hatchlings from mothers who were exposed to a change in temperature (T) were used for the experiment. The hatchlings were split into groups of twenty-four so that there were two groups each with the same thermal history (stressed out mother, or mother exposed to the T change).  Each hatchling was placed in its own tank and fed for 46 days. Then the sets of 24 hatchlings were exposed to a constant T of 24C or an increase in T. The goal was to see if hatchlings from different thermal backgrounds during development responded differently to changes in temperature.

Factors measured in juveniles: In the juveniles and hatchlings researchers observed growth rate (weight at end of experiment compared to weight at the beginning of experiment), survival, and phenotypic plasticity (difference between the growth rate of the group kept at constant T and the group exposed to an increase in T).

Environment and Oxygen Consumption: The scientists used flow cells, oxygen sensors, food, and math to determine the oxygen consumption of the octopuses; check out the article section 2.3: Energetic balance to read about their setup in detail!

Preferred Temperature: To determine the preferred temperature of the juveniles the scientists used a thermal gradient set up in a PVC tube, then observed where young juveniles with mixed backgrounds decided to settle in the tube.

Results:

Scientists found that Octopus maya juveniles that hatched from eggs from a mother that was under temperature related stress where not as healthy as juveniles from unstressed mothers.

Growth: Eggs from stressed mothers weighed less, by almost half, compared to eggs form unstressed mothers. The eggs from stressed mothers had a lower survival rate and a slower growth rate than eggs from unstressed mothers, as well. Although, growth rate of the eggs from stressed out mothers was not hindered by increases in water temperature.

Oxygen Consumption: Oxygen consumption of hatchlings from stressed out mothers was higher than hatchlings from unstressed mothers. Juveniles from unstressed females that were exposed to the increase in T used less oxygen then juveniles from stressed mothers.  Oxygen consumption after a meal was highest in juveniles from stressed mothers overall. The highest consumption was by juveniles from stressed mothers who where exposed to an increase in T, followed by offspring of stressed and unstressed mothers maintained at 24C, and the least consumption was by the offspring of unstressed mothers that were exposed to an increase in T.

Conclusion

Temperature stress impacts the health (size and survival) of octopus offspring.

Juveniles who spawn from stressed moms have a higher metabolic rate: they need more calories while at rest; they consume more oxygen.

Regardless of thermal history of the mother, though, the juveniles are able to adapt to changes in temperature increases.

The observation that growth rate was not adjusted for increases in temperature regardless of thermal history suggests that the adaptation of thermal responses is imprinted on the Octopus maya genome. In other words, adjusting to their environment does not require a lot of additional energy.  Although, the size of the octopuses is expected to get smaller over time.

Application

By monitoring the size of octopus caught in fisheries in the YP scientists could determine how temperature is changing on the shelf in the Caribbean Sea and the Gulf of Mexico.

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