AUTUMNAL ADVENTURES with AMPHIPODS

In which a perplexed investigation into tiny uninvited houseguests is undertaken

By Holly McKelvey

Autumn is a season known only to the temperate latitudes; in my days in the tropics, I forgot how the dry sharp crisp of autumnal air begins to overtake the lazy mugginess of summer, and how the morning fogs take longer and longer to burn off, until one day they don't. Instead, cold in the tropics is linked with elevation, and as you climb upwards from jungle to cloud forest, and beyond to the paramo perched above, you experience chilliness again – by surprise – in much the same way that autumn creeps in.

As fall arrives in my temperate climate home, therefore, I begin to wax romantic about time spent working in the high-elevation tropics, lost in the mists of the cloud forest. There are the early mornings, waking with the song of a hundred new bird species; the hikes through the mist where each trail's next curve is obscured until you turn around it; then there's the sheer beauty of standing amid saplings in a reforestation plot high on a ridge and watching the clouds roll and recede down the valley, as the tropical sun pushes them back. There's sitting on a bench with binoculars just after the rain, to watch the myriad of birds come out to catch worms, and marveling at the sheer variety of color; and there's the insects and the birds and the shifting of boulders in the river, that all blend together into a profound silence of nature.

But also, there's the pure, joyful mystery of solving the weird little quirks of a brand new environment, fueled by the curiosity of a scientist's mind. It was a tropical non-autumnal autumn two years ago exactly that I sent out a bemused but intrigued call for help, in response to a peculiar invasion in my remote house in the mountains in Costa Rica:

Anyone know what these are?? They show up in the water, which I would expect; but I have been finding them all over the floor of [the] house! Mysterious. Unless we have walking shrimp? That just…. all died at the same time? In all the corners throughout the entire house?

It was pretty strange. These little creatures, which up until that point I had only seen in the bathwater, were dispersed across literally every floor in the house (I slept with the door open so the dog could go out at night, meaning outdoor beasties had free access to inside... just not usually in these numbers). The question was thus multifold: what were they? where did they come from? why had they come into the house and died there? and how on earth had the arrived in such multitudes?

Without any answers to my appeal, and following two repeat occurrences of mass-amphipod death on every floor surface in the house, I decided to investigate the matter with the limited (only) material available to me in my remote, off-the-grid corner of the cloud forest: an on-again off-again internet connection. A couple hours' studious learning later, this blog entry emerged in triumph from the cloud forest:

The mystery has been solved!

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... my mystery species is, of course, an amphipod: that's a 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 the Intro to Aquatic Macro-Invertebrates, are generally between 5-20 mm – this fits, as mine are about 5-8 mm.

The question of "what" had been solved. Now, the question of "why":

To figure out their strange behavior, I dug around for info specifically on terrestrial amphipods and found a report by Thomas R Fasulo, which describes the behavior of 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, “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 the house is like. Check. 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 dead, dry 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. The amphipods are able to hop inside 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 - all exactly as I have been observing!

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 in even distribution across the floor 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!

Oddly, after I had done the homework of identifying the creatures and determining why they were coming into the house, I didn't see them again. The torrential downpours had only lasted for a few days, before giving way to gentler rains that did not drive the amphipods to the drier land inside. A brief few days of surrealism followed by normality; insofar as a city kid might experience normality in such a setting, that is.

These mysteries of the cloud forest are the stories that come back to me as the autumn mists of temperate northern Germany roll in. The crispness of the seasonal change calls back the crispness of the cloud forest's elevation in Costa Rica's otherwise tropical warmth. And the rains here, which bring no amphipods with them into my third floor city apartment, make me reminiscent of the torrential downpours that drove the forest's little beasties out of their rain-soaked habitat and into my high-up house in the clouds.

Holly is a graduate student in Environmental Management at the University of Kiel, Germany, where she cycles a lot, drinks tea, and enjoys the brief lapses of sunshine. She can be reached at holly.mc.kelvey@etu.univ-poitiers.fr