Invertebrate Adaptations

This is a design I made for my Spring 2009 Invertebrate Zoology class's T-shirt design contest. My design won the contest and was subsequently printed on shirts. This picture differs from what appears on the shirt in that I cropped off the bottom text ("Earlham College Invertebrate Zoology 2009") as I thought tying this design to a particular course and school would limit the scope of its appeal. The design as it appears on the shirts can be viewed here.

What follows is a brief description of each of the evolutionary adaptations depicted on the shirt, as well as commentary on the accompanying pictures. All of these adaptations are mostly or completely restricted to invertebrates. Architecture is a bit of an exception, as humans build elaborate structures, but the scale and complexity of termite mounds are not rivaled by any non-human vertebrate.

Silk: This multi-purpose substance is produced by caterpillars and at least one species of beetle, but most famously by spiders. Silk is proportionately stronger and more durable than concrete. Spiders can produce different types of silk to serve numerous functions, depending on the lifestyle of the species: webs or other trap lines to catch prey, a case for eggs, immobilization of prey, a safety line to suspend the spider if it falls, a long thread that enables the wind dispersal of juveniles, and more. Humans have thus far been unable to synthetically replicate the properties of silk. The spider I drew has the body plan of a typical orb weaver but is not meant to be any particular species.

Chromatophores: These pigment cells lie just under the skin of cephalopods such as cuttlefish (pictured here), octopuses, and squid. Some vertebrates such as fish possess chromatophores, but the color transformation they can undergo is nowhere near as rapid and dramatic. By rapidly changing the position and orientation of pigments in chromatophores, a cephalopod can instantly alter its coloration. Cephalopods use this ability both to blend into their surroundings and to communicate with each other through complex patterns and rapid color flashes. The cuttlefish I drew on the left is displaying the zebra pattern, which is used to indicate aggression. The polka-dot pattern of the cuttlefish on the right? Well, I just made that up.

Cryptobiosis: Tardigrades, also known as waterbears, are microscopic meiofauna--animals that live in the interstitial spaces of substrates like soil or sand. They are active only when they are submerged in water. Some species line in the ocean or bodies of fresh water; others, however, will live in habitats that are just damp enough to cover the tardigrade in a thin film of water. Should a tardigrade's water source evaporate, the tardigrade does not die. Instead it enters a state of suspended animation called cryptobiosis. The tardigrade ceases all motion, slows its metabolism to an almost-complete stop, and covers its body in a sugary coating that prevents further water loss. A tardigrade can survive in this state for years, waiting for water to wake it from its dormancy. Tardigrades can use cryptobiosis to cope with other environmental stresses that would be fatal to most animals, including oxygen deprivation, extreme temperatures, and changes in salinity. The line art of the arrow in this illustration was thin and faint so it is hard to see; I have never designed a T-shirt before and this one bit of too-thin line art was a novice mistake.

Architecture: I was able to find only one reference for what a termite mound looks like in cross section. You can see an image of the original diagram here (scroll down to “termite mound algorithms” ). Mound-building termites boast the second-most sophisticated architecture in the animal kingdom, second only to humans. A single termite mound can be several meters tall, and depending on the termite species the external structure can vary in appearance from a fusion of numerous asymmetric columns to a flattened, wafer-thin tombstone. The structure of a termite mound is efficiently designed to regulate air flow and temperature. Hot, oxygen-depleted air from the subterranean tunnels where the termites live rises up through channels in the mound and exits through its porous surface. As this air leaves, cooler outside air flows in and eventually reaches the inhabited tunnels.

Kleptocnidae: This may or may not be a real word, but it is the only succinct term for this adaptation that I have found. Kleptocnidae is the ability of nudibranchs (sea slugs if you wish to be common; Opisthobranchia if you—like me—wish to be insufferably obscure) to preserve the stinging cells (nematocysts) of the stinging cnidarians (primarily jellyfish) that they eat. The nudibranchs keep these cells alive and functioning and incorporate them into their own bodies so that they become capable of stinging in self-defense. Some nudibranchs, such as the blue sea slug Glaucus atlanticus illustrated here, feed almost exclusively on stinging cnidarians, including the Porteugese Man-of-War. I fudged the scale in this illustration, however. G. atlanticus is only about two centimeters long, meaning the Man-of-War would have to be much larger or the nudibranch would have to be much smaller than they appear here, but I decided in favor of drawing both species at a similar size to maximize detail.

Neoblasts: These are the stem cells which planaria retain throughout their bodies for the entirety of their lives. The presence and abundance of neoblasts grants planaria a phenomenal level of morphological plasticity even as adults. Thanks to neoblasts, planaria can survive for months without food by slowly reabsorbing their own tissues and shrinking in size. Food-deprived planaria look like dwarf versions of well-fed adults. Once the planarian has the chance to feed again, it will gradually rebuild the lost tissues and regain its former size. However, what a planarian’s neoblasts are most famous for are an almost-limitless capacity for regeneration from traumatic injury. A planarian can be cut into as many as eight pieces and still survive. The neoblasts in each separate piece will differentiate into the cells of whatever organs or body regions the fragment lacks, even reconstructing an entire new head. Some species use this ability to reproduce asexually by splitting in half below the pharynx. In my captive planarian populations, temporarily-tailless individuals and tail-only individuals are an everyday sight.