No, don’t be ridiculous
It is a fine evening and you are working on your allotment. Suddenly a lettuce savages you. Considerably shaken, you sit on the grassy path, where a dandelion launches an unprovoked and vicious assault. You yell “I thought I was safe!”, just as an apple tree tries to strangle you. Fantasy, of course, because in biology there are reliable rules of thumb. Yes, plants can be toxic and even sting you. If you are an insect then the Venus fly-trap is best given a wide berth, while woe-betide the small visitor that steps incautiously onto the waxy rim of a pitcher plant. Moments later it will be glissading to its doom in the stagnant pool at the base of the pitcher that serves as the plant’s stomach. Carnivorous plants may verge on the Gothic, but we scarcely need the sabretooth to remind us that the world is full of hazards. But surely some animals you can trust. Sponges, for example.
Yes, of course
Lacking a nervous system, collectively sponges pump billions of gallons of sea-water through an intricate system of internal canals, harvesting suspended bacteria for food before the filtered water is expelled through large openings (the ostia). Now sponges certainly possess spicules which would provide an awkward mouthful for any would-be predator, but sitting peacefully on the sea-floor can you imagine a more inoffensive group? As they say, “Wouldn’t harm a fly”. Life as a suspension feeder works fine if the sea-water remains a very dilute soup, but in this respect the ocean depths represent the very margins of existence. Here, apart from when a dead whale crashes to the sea-floor, food is in critically short supply. What then is a sponge to do, except to turn to carnivory? Devoid of a mouth (let alone teeth), a gut and an ever-handy anus, how this is possible?
The solution is to transform the surface of the sponge into a sort of Velcro: cunningly designed hook-like spicules readily snag passing animals. This shift to carnivory means that the acquiferous system used for suspension feeding often becomes redundant, but not invariably. In one deep-sea sponge (Chondrocladia) the hydraulic system is retained and serves to inflate translucent balloon-like structures covered with those lethal spicules. Once ensnared, the balloon speedily collapses around the victim, followed by digestion. Now here is another biological rule of thumb: clever ideas are seldom only invented once. This one involves a foraminiferan (usually just called forams and relatives of the amoeba. This single-celled organism (Spiculosiphon) ingeniously mimics sponges by adopting their cast-off spicules to build a body that looks remarkably similar to some carnivorous sponges, complete with elongate tentacles. The one difference is forams trap their prey using their sticky surfaces rather than using those snagging spicules.
It all depends on the question
Perhaps it is merely academic whether one ends one’s days glued to a sponge or vanishing down the gullet of a shark, but sponges are not the only deep-sea animals to turn their hands to murder. In the bivalve molluscs we see a skilful redeployment of an anatomy originally designed for suspension feeding and otherwise exemplified by the unremarkable lives of the oyster and cockle. The bivalve anatomy is far more complex than that of a sponge, but the fundamental principles are much the same: sea-water is drawn in (through the inhalant siphon), filtered (through a gill) for food, and finally expelled via the exhalant siphon. Feeding is ceaseless but placid; one almost feels like humming Beethoven’s Pastoral Symphony. Unless, that is, you encounter the septibranchs. These bivalves are equipped with tentacles hyper-sensitive to water movements. Any passing animal is engulfed by the inhalant siphon that engorged with blood shoots out with lightning speed. This hydrostatic surge results from the sudden movement of an interior partition (or septum; hence the name septibranch), but the spasm also generates an in-rush of water. This sweeps the luckless prey to the mouth and then in short order to a grinding stomach.
So evolution can transform the benign into ravening monsters, but is there any reverse traffic? What about the spiders? Fangs dripping venom and with hapless prey cocooned in silk, these animals exemplify carnivory. Spiders can even tangle with birds. And when it comes to hunting, the jumping spiders (saltatory, hence salticids) are well to the fore. With acute vision and an alarming intelligence, these spiders have been labelled “Eight legged cats”. There is even one jumping spider (Evarcha) that not only hunts mosquitoes but has a decided preference for those females engorged with blood obtained shortly beforehand from a dozing human. This spider (and many others), however, shows a softer side because the young also drink nectar (although let’s hope it has not too fermented).
Sipping nectar for valuable sugars may be one thing, but could we ever envisage a herbivorous spider? Well, we can. The story revolves around a spider (Bagheera) that has learnt how to insinuate itself onto acacias that are zealously defended by ants to the mutual benefit of plants and insects. Some salticids are adept at disguising themselves as ants and then delivering a horrible surprise. So once established do these visitors run amok, hoovering up the ants in all directions? No, because Bagheera has converted itself into a vegetarian, feasting on tips of the plants (botanists know them as Beltian bodies) rich in nutrients. So plant and meat eaters can certainly swap roles. But food is food, and some examples read more like a visit to an extraterrestrial biosphere. Consider the bone-worms (Osedax). Infesting the carcasses of whales, these extraordinary creatures obtain nourishment by sinking roots into the skeleton, etching the surrounding bone to access the organic matrix. The alien theme continues with a body that dangles in the water. Lacking any gut, its principal function is the production of eggs. If that accounts for the female, what about the boys? Not so easy to find because they are absolutely tiny and are a classic example of dwarfed males. Nestling close to the female oviducts, they have only one thing on their minds. And this sort of thing has probably been going on from the time of the dinosaurs to judge from signs of Osedax infestation in the bones of giant marine reptiles.
Text copyright © 2015 Simon Conway Morris. All rights reserved.
Danise, S. and Higgs, N.D. (2015) Bone-eating Osedax worms lived on Mesozoic marine reptile deadfalls. Biology Letters 11, 20150072.
Goffredi, S.K. et al. (2005) Evolutionary innovation: a bone-eating marine symbiosis. Environmental Microbiology 7, 1369-1378.
Harland, D.P. and Jackson, R.R. (2000) ‘Eight-legged cats’ and how they see – a review of recent research on jumping spiders (Araneae: Salticidae). Cimbebasia 16, 231-240.
Jackson, R.R. et al. (2001) Jumping spiders (Araneae: Salticidae) that feed on nectar. Journal of Zoology, London 255, 25-29.
Jackson, R.R. et al. (2005) A spider that feeds indirectly on vertebrate blood by choosing female mosquitoes as prey. Proceedings of the National Academy of Sciences, USA 102, 15155-15160.
Kübler, B. and Barthel, D. (1999) A carnivorous sponge, Chondrocladia gigantea (Porifera: Demospongiae: Cladorhizidae), the giant deep-sea clubsponge from the Norwegian trench. Memoirs of the Queensland Museum 44, 289-297.
Kuja, J.O. et al. (2012) Nectar meals of a mosquito-specialist spider. Psyche 2012, e898721.
Maldonado, M. et al. (2013) A giant foraminifer that converges on the feeding strategy of carnivorous sponges, Spiculosiphon oceana sp. nov. (Foraminifera, Astrorhizida). Zootaxa 3669, 571-584.
Meehan, C.J. et al. (2009) Herbivory in a spider through exploitation of an ant-plant mutualism. Current Biology 19, R892-R893.
Morton, B. (1987) Siphon structure and prey capture as a guide to affinities in the abyssal septibranch Anomalodesmata (Bivalvia). Sarsia 72, 49-69.
Reid, R.G.B. and Reid, A.M. (1974) The carnivorous habit of members of the septibranch genus Cuspidaria (Mollusca: Bivalvia). Sarsia 56, 47-56.
Rouse, G.W. et al. (2004) Osedax: bone-eating marine worms with dwarf males. Science 305, 668-671.
Tresguerres, M. et al. (2013) How to get into bones: Proton pump and carbonic anhydrase in Osedax boneworms. Proceedings of the Royal Society of London, B 280, 20130625.
Vacelet, J. and Boury-Esnault, N. (1995) Carnivorous sponges. Nature 373, 333-335.
Vacelet, J. and Duport, E. (2004) Prey capture and digestion in the carnivorous sponge Asbestopluma hypogea (Porifera: Demospongiae). Zoomorphology 123, 179-190.