An archaic proverb with several contested versions, including the lesser-known version that attests that the “blood of the covenant is thicker than the water of the womb”. Two different meanings that manage to effectively relate itself to the marine realm also. Who would have known!
Marine organisms are surrounded by dangers from all directions and in many different forms. If they were to remain constantly vigilantly and ready to respond to a predator at every given moment it would be a lifetime of all work and no fun. This is because the avoidance response often prevents individuals from efficiently carrying out the various activities they need to survive and to have a higher level of fitness, or successfulness. This includes foraging for food and reproductive behaviour. Yes, it sounds medieval but reproductive success measures the fitness of an individual.
So organisms that have devised a way to assess various stimuli present in the water to gauge their response of fleeing or maybe getting ready to rumble are more successful. This is because it allows them to carry out their daily life activities without having to react to every single stimulus that indicates a predator is in the mix. These defences are known as inducible defences, or defences that can be turned on when an organisms determines that they are truly in danger. Hence, this defensive strategy saves them the energy of constantly remaining in a hyper-vigilant state and also allows them to be active enough to acquire food and potentially a mate. It a better lifestyle that allows recreation with work!
Inducible defences depend on an organism’s ability to detect chemical cues that are present in the water around them. Predators release cues known as kairomones, whereas injured or consumed individuals release alarm or dietary cues. The way cues work is that certain cues are identifiable as danger to the individual, either from evolutionary mechanisms such as coevolution with a predator populations or shared or learned cues from prey populations. Often times the combination of both alarm cues and predator cues is what make the organism truly squirm and ready to respond to the predator. This is because there are several predators present in marine environments, if organisms react to each one they will spend a lifetime hiding under a rock. Hence, if they use the combination of alarm cues, that suggest a predator is actively hunting, and predator cues, that identify a predator that is their constant nemesis, they are able to grade the magnitude of predation risk and act accordingly.
Marine organisms will often react to alarm cues from their kin, or conspecifics in the most heightened manner, but their reaction to other alarm cues, known as heterospecific cues, depends on if they have a greater inkling for their neighbors, co-existing organisms, or their distantly related cousins, phylogenetically related organisms. Hence, literally is blood thicker than water?
And that was exactly what I set to find out with my independent project. To do so, I first had to acquire my predator and three species of prey, one that was my guinea pig for this experiment and the others were the “neighbor” and the “cousin”. The predator I choose was the purple sea star, Pisaster ochraceus, which is prevalent in the Pacific Ocean and able to eat a range of organisms including mussels and snails. Hence, my chosen preys were the black turban snail, Cholostroma funebralis, common periwinkle, Littorina littorea and the bay mussel, Mytilus trossulus. The black turban snail and common periwinkle can be thought of as distantly related cousins as the common periwinkle is common the Atlantic waters and the black turban snail the Pacific. Whereas, the bay mussel and black turban snail are neighbors, belonging to the same Pacific Ocean and found within the same communities. The black turban snail was the organism I used for my experimentation trials.
For my experiment I fed three different diets to my sea starts. I did this by placing each sea star in a different tank filled with water and placing different preys in each tank. I then allowed them to eat their meal overnight. In the morning, I first made sure they had eaten, and sure enough some snails and mussels were unaccounted for. I then acquired the water from either the sea water table, which was my no-cue control, or from the three predator set ups and placed it into smaller containers. A single black turban snail was then placed into the water and the time taken to reach the water line was recorded as the escape time, a common anti-predator response of the snail. Then I recorded the mass of each individual.
The results were interesting. I found that the different diets did not result in a significantly different escape time in comparison to the control or even the other two heterospecific diets. This was very unexpected. However, when I ran the same results with mass as a factor, significance was found! The results showed an interaction of cue and mass such that in the conspecific, or kinship, diet larger individuals left the water much faster than smaller individuals, but in the heterospecific diet and control the larger individuals left the water much slower.
A potential reason for this size-dependent response is that larger individuals often have greater energy reserve to both be able to quickly exist the water when needed and furthermore, leave foraging activities without facing starvation. Whereas, smaller individuals often need to feed more and for longer periods and thus can face a greater cost of leaving the water. But, this may not necessarily leave them more vulnerable. Often larger animals are more detectable to prey and hence this may be why they need to leave the water, whereas smaller individuals can more efficiently take refuge and hence can remain in the water. This also allows smaller individuals to return to feeding much quicker. A consequence of this can be that individuals of the same species can occupy different regions within same community, resulting in differences in spatial distribution. This can affect how different species interact with the black turban snail of different sizes.
But unfortunately there was no definite answer to how the black turban reacts to heterospecific diets. It is possible that both diets cause a response and hence are not different from each. It is possible that the perils of both its neighbour and cousin send the black turban snail into distress. I would love to carry this experiment out again with a greater range of prey and explore this question further.
If I’ve got you interest take a look at this dramatic video of sea stars feeding! Its rather amusing: