Image cred: Hanson, Paul, Monika Springer, and Alonso Ramirez. "Capítulo 1: Introducción a Los Grupos De Macroinvertebrados Acuáticos." Revista De Biología Tropical 58.4 (2010). SciELO. Web.
Image cred: Hanson, Paul, Monika Springer, and Alonso Ramirez. "Capítulo 1: Introducción a Los Grupos De Macroinvertebrados Acuáticos." Revista De Biología Tropical 58.4 (2010). SciELO. Web.
The mystery has been solved!
So I’ve been doing some reading up on macro-invertebrates of Costa Rica. I started out here with an Introduction to Groups of Aquatic Macro-Invertebrates, just by googling macroinvertebrados Costa Rica. I was immediately able to identify that my mystery species is an amphipod, which according to Wikipedia is defined as a sort of crustacean that hasn’t got a carapace (hard outer shell), has varied thoracic legs (as opposed to isopods, whose thoracic legs are all the same), has a laterally compressed body, and can be both aquatic and terrestrial. Bullseye! Costa Rican amphipods, according to Hanson et al in the Intro to Aquatic Macro-Invertebrates, are generally between 5-20 mm (mine are about 5-8 mm). However the amphipods described on this site are predominantly aquatic, almost half living in groundwater and the rest largely inhabiting deep water - though some live in aquatic vegetation (Hanson et al).
There is a stream nearby Tom’s house that could provide aquatic vegetation as a habitat for amphipods, but it’s too far away to explain this quantity of shrimp coming into the house! So the research continued…
I googled “land amphipod” and found this report by Thomas R Fasulo, which describes terrestrial amphipods in Florida (which, like here, is humid, warm and experiences heavy rainfall - a similar environment, and therefore potentially host to similar species). Though amphipods can live on land, it says, “they still require moist habitats” (the cloud forest is definitely a moist habitat). They can live in the top 13 mm of soil, particularly when it is damp and composed of leaf litter or organic matter, which is exactly what the area around Tom’s house is like. It goes on:
"After rains, large numbers of amphipods can migrate into garages or under the doors of houses. There they soon die. Amphipods do not have a waxy layer on their exoskeleton as do insects. They lose or gain moisture from their environment. Too much of a water loss results in desiccation while too rapid a gain is also lethal. This is why they migrate out of rain-soaked soil to drier areas where they usually end up dying anyway. Most species are active at night…. Once dead, they turn a reddish color" (Fasulo).
This makes perfect sense for what I have been seeing. It has been raining heavily for the last two nights - and both yesterday and today I have found hundreds of amphipods across the floor. The fact that they can die from too much water explains why they have been coming inside to where it is drier. Tom’s door does not lie flush with the ground, so the amphipods are able to hop in easily. Once inside, they dry out (I have mostly just seen them in the front room - presumably they begin to dry out or die before they can make it to further rooms), and turn a reddish color - this is all exactly as I have been observing!
It seems that the type of “lawn shrimp” I am seeing here is similar both in biology and in behavior to the ones discussed in this article, observed in Florida. No, it is not raining shrimp through a mystery leak in the roof; no, there are no floods that are flowing in through the house while I am asleep and leaving behind shrimp as they recede; no, the dog is not tracking in literally hundreds and then managing to shake them all off before she reaches her bed (which is amphipod-free). They are getting in here on their own initiative to escape these heavy rains, and desiccating once inside.
Case closed!
References
Hanson, Paul, Monika Springer, and Alonso Ramirez. “Capítulo 1: Introducción a Los Grupos De Macroinvertebrados Acuáticos.” Revista De Biología Tropical 58.4 (2010). SciELO. Web.
United States. Department of Agriculture. Institute of Food and Agricultural Sciences, University of Florida. Terrestrial Amphipods or “Lawn Shrimp” (Crusacea: Amphipoda: Talitridae). By Thomas R. Fasulo. Doc #EENY-220 (IN377). 2001. Web.
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