Can You Smell What The Rock(y Intertidal) is Cooking!?: How Mussel Larvae Respond to Settlement Cues

I missed an opportunity to use the above title in a conference talk, so I'm pretty excited to finally have it out in the world. Moving on from the nerd-cissism* though, let me begin with a spoiler: Mussel larvae can smell danger, and swim to safety! Ok, not entirely, and not much more clear than my title, so I'll start from the beginning and you'll see what I mean.

Mussels are "broadcast spawners", meaning mom and dad (yes, there are females and males) release their gametes (sperm and eggs) into the water, resulting in many microscopic fertilized mussel eggs floating around. These "zygotes" (fertilized eggs, figure 1A) grow to look like microscopic versions of mussels that also float around in the ocean as plankton (referred to as "larvae"; Figure 1B), and eventually settle down on algae or a rock somewhere (figure 1C) to continue growth to adulthood (the life-stage people are used to seeing and eating; figure 1D).

Figure 1: Fertilized mussel eggs (A) grow into platonic larvae (B) that eventually settle on algae (C) and rocks (D) and grow into adults (D).

Mussel larvae are capable of swimming, but not very fast since they're microscopic. Thus, since ocean currents are typically pretty strong, larvae mostly just get swept away wherever the current takes them. Ocean currents predominantly move horizontally though, and their vertical movements are much weaker (i.e., water currents mostly move side to side and forwards and backwards, not up and down). So, even though mussel larvae are weak swimmers and can't swim against the strong horizontal current, they can usually swim up and down in the water against the much weaker vertical current. This is particularly useful when a mussel larvae gets ready to settle down on a rock or algae, and needs to swim towards the ocean floor. But, what prompts a mussel larvae to swim towards the the bottom and settle at in a particular place? Understanding this would not only be useful information when researching wild mussel population dynamics, but could also help manage and augment mussel settlement on aquaculture ropes and "leases" (rented underwater areas where fishermen can farm marine species).

Recent marine biology research has shown how some larvae of some species can sense chemical "cues" in the water (dissolved organic compounds usually originating from some other species in the water). We also think these chemical cues are important for larvae to make decisions on when and where to settle. The majority of scientific studies have, so far, only looked at the effects of positive cues though — cues that larvae move toward and that lead larvae to a beneficial habitat (e.g., adults of the same species, which implies healthy habitat). On the other hand, negative chemical cues (cues that larvae avoid) haven't been research very much, nor have situations where larvae might encounter two or more different cues at the same time, which would be much more realistic for nature.

In collaboration with Phil Yund, a senior scientist at the Downeast Institute, we setup experiments to test how blue mussel (Mytilus edulis) larvae respond to multiple possible settlement cues. We started by making "cue" for five different intertidal species - each of which could suggest good mussel habitat (positive cues), bad mussel habitat (negative cues), or not imply anything about mussel habitat (i.e., neutral cues).

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We made chemical cues by soaking the following intertidal species in jars of filtered seawater for a few days...

Possible Positive Cues

- Adult Blue Mussels: the adult stage of the larvae

- Filamentous Algae: an algae species larvae like to settle on (figure 1C)

Possible Negative Cues

- Green Crab: A crab predator of adult blue mussels

- Dog Whelk: A carnivorous snail predator of adult blue mussels

Possible Neutral Cues

- Periwinkle: An herbivorous snails that lives with mussels, but do not eat them

We also had a "Cue-Free Control", which was just clean filtered seawater without any species soaking in it ("unconditioned").

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Next, we took a plastic "choice channel" (figure 2), filled the entire channel with clean filtered seawater, and placed a few drops of an individual chemical cue on one side of it. We then placed an individual mussel larvae that was old enough to settle (we spawned and grew them in an aquaculture lab) in the center of the choice channel, and monitored the larvae for 20 minutes to see which direction it swim - towards the cue (positive response), away from the cue (negative response), or to stayed in the central "Neutral Zone" and did not swim either direction (no choice). As a control to compare larval responses against, we ran the same experiment, but with drops of clean filtered seawater (our "cue-free control") instead of chemical cue.

Figure 2: An overview of the "choice channel" design, along with 3-dimensional representations of the end wells, channel, and center well shown in the lower portion of the figure.

In total, we tested 150 larvae (25 larvae for each cue). Our results showed that mussel larvae responded in two different ways:

1) If larvae responded to a cue as positive, negative, or neutral

2) how frequently larvae responded to a cue (i.e., how many of the 25 larvae swam to a side)

Below, I display these results in a canonical analysis (figures 3 and 4). This takes a point associated with each cue, and moves the point around the chart space depending on whether larvae swam towards that cue more (moves a point further right), away from the cue more (moves a point further left), or stayed in the neutral zone and made no choice more (keeps the point in the center) - see figure 3. The canonical analysis plot also displays how frequently larvae even responded to a cue - regardless of whether the response was positive, negative, or neutral (figure 4).

Figure 3: Plot from the canonical analysis highlighting if a cue had amore positive, negative, or neutral effect on larvae. Note that this is the same plot as figure 4, but with different highlighting.

Figure 4: Plot from the canonical analysis highlighting if many or only a few larvae responded to a cue. Note that this is the same plot as figure 3, but with different highlighting.

We can see that our initial hypothesis as to whether a specific cue would be positive, negative, or neutral (which were purely based on how the mussel interact with other species after they grow up - not as larvae) were all correct! Also, we found that larvae responded to some cues (e.g., positive cue from adult blue mussels) much more readily than they did other cues (e.g., positive cues from filamentous algae). Surprisingly our neutral Periwinkle cue actually caused larvae to swim around a lot, but in no particular direction related to the cue (hence being neutral). It could be that mussel larvae use the Periwinkle cue as an indicator that an intertidal habitat is close, and then swim around looking for more information (positive and negative cues).

So you see!? Mussel larvae can smell danger and swim to safety! And also, not exactly that. We can use the words "choose" and "decide" all we want, but I wouldn't go so far as to suggest larvae possess a level of cognition. It's more likely that these behaviors are the outcome of evolutionary selection exerted by predators, or competitors, on adult mussels. In such a scenario, any adaptation that causes a mussel to settle far from danger/close to good habitat as a larvae will increase that mussel's chances of surviving and reproducing as an adult. As long as the mechanism (e.g., sensing chemical cues) can be passed on from generation to generation (i.e., is "heritable"), that mechanism can become more and more frequent in a population of mussels, until it's everywhere, and all mussels we run experiments on display the behavior.

All in all, I enjoyed this bit of research. If you want more information, feel free to read the peer-reviewed article in Journal of Experimental Marine Biology and Ecology! Alternatively, if you missed most of what I said and skipped to the end, see this research mentioned in US News and World Report.

Lastly, I'll leave you with this video of a mussel larvae swimming towards positive cue from an adult mussel. Hope you enjoyed my post!

*based on a quick google search, I may have just invented the word "nerdcissism"; Yes, I recognize the circular logic in doing so.