Pushing the boundaries: the land-walking, sex-changing shrimp Merguia oligodon

 Let me tell you about a shrimp that walks on land. Late one night on a low tide, my student Dina and I were stalking the mangrove fringe. It was very dark, and with our flashlights we went probing about the mangrove prop roots and under driftwood. From under a piece of wood, pairs of red eyes were reflecting back our lights. What could they be? We flipped the wood, and out jumped a handful of shrimp! They were leaping about the mud and sand, looking for all the world like a bunch of grasshoppers.  We caught a few to study. A bit of probing into the literature, and Dina and I figured out what they were: they are called Merguia oligodon, and don’t they look awesome?

The dude on the right decided to perch on a lumpish crab for safety. Notice how it keeps one pair of antennae pointing forward, and another pair pointing backwards? It's very characteristic for Merguia.

(plate from our paper, showing color variation)

We learned that the genus Merguia only has two species, both are semi-terrestrial. They are the ONLY shrimp lineage to jump around on dry land*  (1). You may sniff and say that it’s not much of ‘land’, since we’re only talking about the muddy mangrove fringe. Still, to paraphrase Armstrong, one small jump for Merguia, one giant leap for shrimpkind. What sorts of physiological adaptations were needed for caridean shrimp to breathe air? We still don’t know.

Merguia oligodon is ‘edgy’ in still another way: they are sex-changers! By looking at the sizes and figuring out their sexes, we deduced that they start their life out as males and later transition to females. While this may sound really wild, it’s actually not uncommon among caridean shrimp (the larger group to which Merguia belongs). Caridean shrimp have a whole galaxy of possible sex roles(2), from the standard two sexes (the most common mode), to male-to-female hermaphrodites (like in Merguia), to other rarer/stranger sexual strategies.

We published our discoveries in the Journal of Crustacean Biology (3). This is only the 2nd report of Merguia from Philippine waters - the first report (4) was from Bongao, Tawi-tawi in 1930, as part of the Dutch Snellius expedition (5) to what was then the “Dutch East Indies” [now Indonesia - but somehow they surveyed Bongao too?].

In our case we found Merguia on Taklong Island, Guimaras. Taklong happens to be the ground zero of the M/T Solar bunker oil spill of 2006 (6), to date the biggest oil spill to ever hit the Philippines. Oil spills do their worst damage at the air-water interface, so the fact that we discovered a thriving population of shrimp specialized for that zone is, I think, a hopeful sign of recovery of the mangrove forests of Taklong. 

These shrimp have only been reported from a handful of locations in the Indo-West Pacific: Kenya, Myanmar, Thailand, the Ryukyus of Japan, a couple of places in Indonesia, and now two spots in the Philippines. Does this mean they are very rare? Or perhaps they're actually common, just that few people go mucking about the mangrove fringe at night? If you’ve seen this amazing shrimp in your locality, I’d be delighted if you would let me know =)

*note - after I posted this, colleagues pointed out that there are other mangrove-associated shrimp that crawl out onto land (some Macrobrachium and Alpheus species). But only Merguia jumps about actively. 

References

1. Abele, L. G. (1970). Semi-terrestrial shrimp (Merguia rhizophorae). Nature, 226(5246), 661-662. https://decapoda.nhm.org/pdfs/31852/31852.pdf

2. Chandler, J.C., Elizur, A. & Ventura, T. (2017) The decapod researcher’s guide to the galaxy of sex determination. Hydrobiologia 825, 61–80. https://doi.org/10.1007/s10750-017-3452-4. https://link.springer.com/article/10.1007/s10750-017-3452-4

3. Malay, M. C. M. D., & Rañises, D. M. L. (2019). Observations on the habitat, color polymorphism, and sexual system of the semi-terrestrial shrimp Merguia oligodon (De Man, 1888)(Decapoda: Caridea: Merguiidae). The Journal of Crustacean Biology, 39(6), 764-769. https://academic.oup.com/jcb/article-abstract/39/6/764/5555763

4. Holthuis, L. B. (1947). The Decapoda of the Siboga Expedition part IX. The Hippolytidae and Rhynchocinetidae collected by the Siboga and Snellius Expeditions with remarks on other species. Siboga-Expeditie 39a^ 8, 1-100.

5. van Riel, P. M. (1930). The Snellius Expedition. Nature, 125, 762-762. https://www.nature.com/articles/125761a0.pdf

6. Murga, A. (2019). Saving an island from the worst oil spill in the Philippines: The case of Guimaras. https://news.mongabay.com/2019/10/saving-an-island-from-the-worst-oil-spill-in-the-philippines-the-case-of-guimaras/