Read the science and health sections of any major news website these days and it seems like all we’re hearing about is infectious diseases: ebola, measles, poliovirus, E. coli. As humans, today we have access to vaccines, water purifiers and sanitizers, antibiotics, and many more marvels of modern healthcare to aid us against disease. Undoubtedly, we have reduced human morbidity and mortality related to disease and lengthened our lifespans, but do you ever wonder how wild animals—especially those species often viewed as “primitive” and “less complex” like sponges, jellyfish, anemone, worms, or mollusks—cope with disease?
Many invertebrate groups may seem primitive, but these lineages evolved hundreds of millions of years ago, and the fact that they’re still around in multitudes suggests that whatever their immune systems are, they definitely work!
An organism’s outermost layer serves as a primary line of defense, because it is a physical barrier against invaders. For vertebrates, it’s the skin. Many invertebrates, like starfish and cephalopods, also have a thick layer of skin, sometimes covered in spines or color-changing skin cells, but always serving to protect the animal. Bivalves have shells made of calcium carbonate, and of course lobsters and crabs have tough exoskeletons made of chitin. Still others secrete mucous, either to protect their delicate skin or to directly ward off predators. Mucus, which is continuously renewed, can trap tiny invaders and then slough them off the body. Moreover, mucus layers are rich in microbial species, which interact in complex processes that appear to play fundamental roles in disease and immunity.
We are still learning about the details and complexities in the evolution of animal immune systems, a tall task considering how enormously varied in their evolutionary histories. Here are a few specific examples:
- The comparatively simple (but effective!) immune system of the multi-celled, tiny hydra includes a thin epithelium full of antimicrobial peptides, which help prevent infection.
- The jellyfish, composed of about 95% water, is a wonder of nature. In some cases, it has been found that jellyfish have an immune system that presents bacterial-specific responses.
- By comparison, the defensive mechanisms employed by coral are rather sophisticated (or at least studied in better detail). A coral’s immune system can identify and respond to specific pathogens. Because the coral’s immune system can also identify allografts, coral polyps can fuse into colonies, while other opportunistic infections can be fought off.
- Some worms are employed as model organisms because of their simplicity, which is conducive to laboratory studies aiming to understand basic systems in animals. Such studies have revealed that worms possess innate immunity. The signal cascades that initiate the worm’s response to an infectious agent provide the immune system with the ability to recognize different pathogens.
- Bivalves, like worms, rely solely on their innate immune response. Phagocytic cells in the blood eliminate infections by attacking invasive organisms with harsh enzymes or reactive oxygen species. And like the hydra, bivalves also possess antibacterial peptides.
- Like the other invertebrates mentioned above, the immune systems of crabs cannot develop antibodies to fight infection. Instead, the crab produces compounds that bind to invading bacteria, viruses, and fungi, inactivating these disease-causing agents and even serving as a clotting agent that can help with wound control.
The variety of immune responses in invertebrates may be surprising—it may also have you wondering why starfish on the West Coast have fallen victim to a recent mass die-off. Officially known as “sea star wasting syndrome,” more than 20 species have fallen victim, in many areas eliminating every individual. Recently, scientists identified the likely infectious agent, a densovirus, although questions remain about what environmental factors may have contributed to the severity of the outbreak. Hopefully, you’ve also heard of coral bleaching in response to warming waters. Coral bleaching occurs when the photosynthetic zooxanthellae living within the coral’s tissue die or leave the host, as warming waters create an inhospitable environment. Because of their ecological importance, coral disease is an active area of research.
While coral and starfish are among the more charismatic marine invertebrates, all of the above organisms, it should be noted, depend on specific temperatures and other environmental factors for their immune responses to function normally. As oceans continue to change in response to human activities, what will the future bring?
By: Liz Boatman
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