Stomatopod Rumblings

I thought I had covered the basics of every aspect of stomatopod biology in my stomatopod post, as did all the readers (thanks again for the wonderful feedback!). But I missed one part: acoustics and its role in communication and ecology. It’s obvious that the raptorial appendage will make a loud pop when it strikes, due to the cavitation bubble produced; stomatopods can also stridulate by using the muscles in their uropods; this is another pathway to communication, since crustaceans can hear. I was alerted to my omission by this recent article, so might as well explain it fully.

In fact, the reason why I neglected acoustics in the comprehensive post is because I knew nothing about its use in stomatopods. It turns out I wasn’t the only one. It’s only been studied once in the lab (Patek & Caldwell, 2006); this paper is the first to look at it in the wild, under natural conditions, using Hemisquilla californiensis as the study organism.

ResearchBlogging.org

Staaterman, E., Clark, C., Gallagher, A., deVries, M., Claverie, T., & Patek, S. (2011). Rumbling in the benthos: acoustic ecology of the California mantis shrimp Hemisquilla californiensis Aquatic Biology, 13 (2), 97-105 DOI: 10.3354/ab00361

What they did was set up three units to record sounds made by the stomatopods in 2 places off the coast of Santa Catalina Island, California. These allowed the authors to characterise the nature of the sound produced (they’re referred to as rumbles), localise the sound (to make sure it comes from the stomatopod burrow and isn’t environmental) and see when during the day they are produced.

Using this data, the authors wanted to know how lab results differed from those in the wild, if there is any communal aspect to sound-production, and how the sounds play into their behaviour.

Two things should be noted about the study design: the first is that there is always difficulty with doing such data collecting because of environmental noise pollution, in this case from ships. The second is that the recordings took place at the start of the mating season, meaning the data collected here may be part of mating rituals and not representative of “normal” behaviour.

It turns out that the rumbles are produced in batches (termed “rumble groups“), within 0.25s of each other, and repeated for anything between several minutes to several hours (the total is referred to as a “rumble bout“). The diagram above shows an excerpt from one such rumble bout: each column labelled a and b is a rumble group (look closely and you’ll see that a is 3 lines, b is 2 lines), and this repetitive sequence lasting for over a minute is the rumble bout.

Occasionally, this rhythmic pattern was disrupted by a “chorus”, where many stomatopods produced sound at will.

However, all sounds are produced at predictable times: in the mornings and evenings. Nothing occurs at during at mid-day and in the afternoon – although this may be an artefact of the study site, given that there are ships disrupting proper recording – and there are occasional spurts during the night, but nothing spectacular. Clearly then, these have some sort of behavioural aspect, or else they wouldn’t be so predictable.

What behaviours might these be? Hints come from the figure above: there are two distinct rumble patterns (a and b), and each rumble group is produced simultaneously by several individuals from different burrows. That they occur so regularly hints that it might be coordinated, but how and why isn’t known.

The daily pattern are consistent with what we know of their ethology: during the day, they shut their burrows, but in the mornings and evenings, they either head out for food, or they personally guard the entrance, and it’s at these times that communication, either antagonistic or cooperative, will need to happen. That this is observed in the field confirms that the rumbles aren’t just chance, but are actually produced purposefully.

Given how aggressive stomatopods are towards each other, the best explanation, and the first one offered by the authors, is that this is a way to mark territories. In my post, I hinted that they might use chemicals for this, but using sound and vibration is more likely – and the data is right here, unlike for the chemicals.

The authors note another potential reason: female attraction. The earlier lab study had found that only males produce sound. Given that the study took place during mating season, it may be that the observed choruses are actually males competing for females, with the sound (loudness, frequency, or a combination of factors) being a signal for the male’s attractiveness, kind of like in frogs.

Is there anything else basic I missed of stomatopod biology? (I know, I didn’t do their nervous system and physiology much, because that would require a ton of basic zoological background info first, too much for that post.)

Lab Study ref: Patek SN & Caldwell RL. 2006. The stomatopod rumble: sound production in Hemisquilla californiensis. Marine and Freshwater Behaviour and Physiology 39, 99–111.

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