Carnivorous trees of the sea: Notodendrodes not as harmless as it looks

Remember Notodendrodes and the spicule tree? Don't they look so much like harmless trees sitting around sunbathing like their plant counterparts? Not all tree forams are harmless. The microscopic marine world is full of surprises, like trees waving around their long sticky network 'feet' to trap and devour any traveler that happens by. Here's some wonderful shots of Notodendrodes caught in the act:

The top left image shows a clump of Artemia caught by Notodendrodes, a big carnivorous tree foram. Note how the reticulopodia (pseudopodial networks) stretch between the branches like spiderwebs. Top right: SEM of the reticulopodial mesh of another species of Notodendrodes. Bottom: The tree foram in its natural setting, with a copepod attached (arrow). (Suhr et al. 2008 Mar Ecol Prog Ser)

There some nice foram videos on this YouTube page, including shots of reticulopodia and a fairly large foram moving about in situ. This movie by a Japanese researcher includes clips of Artemia being captured starting at 0:50.

Many forams are voracious predators, devouring anything from fellow protists to crustaceans and echinoderm and mollusc larvae. The following is Astrammina rara's rather impressive menu; all but two species were happily consumed:

However, not all forams are carnivorous. Some are mediocre at best at capturing prey, and some, like Crithionina, are quite bad. This suggests a range of feeding habits from detritovory to carnovory to omnivory. Note how Gromia (not a foram, despite looking vaguely similar; placement somewhat uncertain, though most likely either close to forams or a cercozoan) fails to capture any prey. Also, dead specimens failed to catch prey, indicating the capture is intentional and requires a fully functioning cell, and not an accidental adhesion to something sticky. In fact, there is evidence for specific targetting of certain prey, which wouldn't be much of a stretch as many forams are quite picky in choosing their test material.

I think this has some interesting – perhaps borderline philosophical – implications. Towards the end of the ciliate kleptoplasty post I mentioned how the traditional ecological terms often fail to describe the majority of life, which happens to be microscopic and play by some different rules. There's a greater problem in the approach of traditional ecology towards microbial life, however, and it even surfaced in a random chat with some ecology grad students. Namely, the treatment of all things microbial as the "bottom of the food web", ie. prey species created by evolution to feed cute fluffy animals. They have a similar attitude to plants as well: 'producers'. Fungi are 'decomposers'.

Probably to people tracking bird migration out in the field, such crude terms do just fine, and we all must make crude approximations somewhere (or drown in details). However, as in any simplification, there's always a danger of skimming over interesting outliers. I disagree with the blanket treatment of protists (and bacteria, and anything else) as the "bottom of the food web" for two reasons:

1. There are plenty of intricate interactions resulting in elaborate food webs (and, more generally, 'interaction webs'); a plethora of fascinating relationships is lost when one blurs them all into the 'prey for animals' category.

2. Feeding by animals forms but a very tiny part of the overall diversity of microbe-animal interactions. An ecological framework must account for symbionts (mutualists, parasites and commensals) along with predation. Toxoplasma, arguably the most successful parasite of vertebrates ever, is a wonderful example of 'lower trophic levels' leeching 'up' the food web and running the show. You can't really draw an arrow from a cat or human to the modest apicomplexan, as it doesn't really consume its slaves. But you can't really not draw that arrow. It's complicated.

(In fact, if organisms besides humans had Facebook, most of their relationship statuses would be set to "It's complicated". Groan all you want... =P)

Lastly, our forams mentioned above also have ecological consequences on the megafauna in their environments. Astrammina rara is benthic, meaning it lives on the ocean floor (or, technically, any substrate). Suhr et al (2008) mention past studies indicating lower-than-usual densities of marine fauna in particular areas; these areas seem to match up with Astrammina's distribution. Presumably, the effects of predation on small fauna and larvae can be seen on the larger scale.

Furthermore, the carnivorous forams seem to affect the survival strategies of the fauna around them (in hindsight, unsurprisingly): some echinoids have brood protection and settling strategies that may well have evolved in response to the lowly single celled protists they rightly fear. The authors suggest that the failure of Astrammina to capture larvae of the echinoid Acodontaster may be a result of the latter evolving a specific chemical defense against it.

The 'scum' from the bottom of the foodweb can come up to bite some 'higher' organisms in the ass – whodathunk?

Reference
Suhr, S., Alexander, S., Gooday, A., Pond, D., & Bowser, S. (2008). Trophic modes of large Antarctic Foraminifera: roles of carnivory, omnivory, and detritivory Marine Ecology Progress Series, 371, 155-164 DOI: 10.3354/meps07693