Behavioural Ecology – Habituation and morphology in the green shore crab

Behaviour Ecology, University

In a component of my Behavioural Ecology module (BSX-2018) we began learning about habituation, one of my favourite subject areas, which we were then able to study through a practical experiment using green shore crabs obtained from The Menai Straits, North Wales.

Habituation: Diminishing of an innate response to a frequently repeated stimulus.

The experiment:

In order to successfully carry out the habituation and morphology experiment, we paired up and collected a crab from the bucket sitting at the front of the lab after carefully reading the practical handout and stating our practical hypothesis.

We placed the crab in a large tray of salt water, setting the timer for thirty minutes for each individual crab, and the experiment was carried out on three crabs in total, per group. Though, Rosie and I opted to use a forth crab following our enjoyment of the experiment, and because we were keen to experiment using a Female after already using three Males. And that way, it would be interesting to distinguish differences in results between the two sexes.

Asked to label each of our chosen crabs, my friend and I excitedly named each of ours (appropriately, of course…)

Fred, Leo, Barney and Penny. 

Fred, our first crab and the largest we experimented with, was noticeably aggressive nearing the beginning of the experiment and would raise his claws as an indication of a warning signal to fend off predators. And our third crab (Barney) was unfortunately missing his right claws, which could’ve given him a disadvantage throughout the experiment in comparison to the fully-clawed crabs, since he had less limbs to assist him in turning himself back around.

The task was to gently flip the crabs over on their backs, distinguishing whether the individual was a Male or a Female. So we could time how long each of the crabs took to get themselves back over to an upright position. We carried this out for thirty minutes for each of the crabs, noting down in a table the duration of time the crab took to turn back over (without assistance!)

Hint: The Male’s abdomens are more triangular in shape whereas the abdomens of Females are more rounded, as pictured below.

Crab

We then measured the crabs carapace height and width alongside their left and right minor and major claws – also referred to as cutters and crushers – described which crabs were small (< 45mm) and which were larger (> 45mm) and recorded the information into a table.

The hypothesis being: When an animal is exposed to a stimulus, they’ll react differently to it over time and will no longer become affected, or stressed, by it. Similar to if a Human was constantly exposed to the ringing of a car alarm. At first, the alarm would be annoying and would cause distress, but over time and following more exposure to the stimulus, it would become more tolerable and significantly easier to deal with. The more times we flipped the crab over, the longer they’d take to flip themselves back over, since they learned that the situation wasn’t threatening and therefore didn’t invest as much energy into manoeuvring themselves.

We noted that different crabs had different reactions. Therefore, some were quicker at flipping themselves in comparison to others. For example, one of our crabs (who we named Leo) took approximately one second to turn back around to begin with, and a crab monitored by our friend took several minutes to obtain an upright position. Though, a series of factors could contribute to this, including: size, sex, exposure to the stimulus and possibly whether or not the animal has experienced a similar situation in their natural environment.

Additionally, one of the crabs we experimented with (Penny) appeared to be pregnant! As were many of the crabs used throughout the practical since it was mating season; we had permission from our lecturer to flip them over. So this may have also had implications on the results gathered. Ie: Her reaction rate could’ve been quicker due to predation risk and her maternal instinct to protect herself and her offspring.