Floating in the ocean, dependent on the currents to carry them, millions of zooplankton begin their life. Although invisible to the human eye, under a microscope a magical world is revealed. Over 7000 species of zooplankton fill our oceans and serve an essential role in marine food webs (Perrier et al. 2015). Some zooplankton remain small throughout development but others grow and transform into fishes, crabs, lobsters, and other animals we are familiar with. The larval forms can be unrecognizable from the mature form.
Several species undergo multiple stages of development as zooplankton before reaching adulthood. Crabs are a wonderful example of this strategy. The number of planktonic larval states depends on the species of crab. These planktonic stages, known as zoea, have an appearance that resembles a shrimp unicorn hybrid. The settling state, known as megalopa, looks more like a distorted microscopic crab.
Many species of plankton need to find a habitat in which to settle so they may grow or they will not survive. Collectively, the sessile animals which settle onto a substrate are known as epifauna. This group includes sponges, tunicates, bivalves, echinoderms, bryozoans, etc. Alexandra Hiller of Harilaos Lessios’ lab and Carmen Santiago of Mark Torchin’s lab at the Naos Marine Lab are studying recruitment of these communities. To understand how long it takes for settling to occur and the succession of this system, artificial media are introduced and monitored regularly. The two primary media are baskets filled with rocks and construction fencing, covered in mesh, and settlement plates. Both are suspended from docks and trees into the ocean. In just a couple weeks some juvenile species can be found amongst the bright orange plastic but waiting a month provides a treasure trove of magnificent organisms.
At first glance porcelain crabs pour from the baskets, scurrying every which direction. After carefully corralling of the crabs, to prevent autotomy (dropping of limbs), brittle stars emerge with delicate, spined arms.
Hard, yet squishy, clear plastic like blobs with visible digestive tracts are identified as juvenile tunicates. Tube worms create an extensive network of intertwined calcified tubules which cover the rocks. When those rocks are removed from the water, the tube worms crawl out of their homes in search of refuge. Closed anemones appear as fleshy cushions out of the water but after returning to the sea water, they slowly open to reveal their tentacles. Small, flat, undulating organisms with eye spots are identified as flat worms, but not the parasitic kind. Special care is given to centipede like fireworms whose bristlelike hairs (setae) cause intense pain, rash, redness, and swelling on contact lasting several days. The compound that causes this inflammatory response is called complanine (Nakamura et al. 2008).
The most flamboyant clade found in the baskets are nudibranchs. Not only do these gastropods come in a rainbow of colors with clashing patterns and textures, some can actually produce their own light through bioluminescence making them glow in the dark (Vallès and Gosliner 2006). Although they have a shell as larva (Page 1995) they lose it as an adult because they are so awesome they don’t need one anymore. What looks like boas one either side of their foot or tiny horns are called cerata. The cerata not only aid in respiration and gas exchange, they also protect the nudibranchs. Some nudibranchs will eat cnidarians (e.g. anemones), to steal their nematocysts (stinging cells). The nudibranch then stores the nematocysts in the tips of the cerata so to use for their own defense (Greenwood and Mariscal 1984). If that wasn’t badass enough, those cute “ears”, called rhinophores, are used for distant chemoreception and rheoreception (detection of water currents) and for contact chemoreception and mechanoreception they use their oral tentacles (Cummins et al. 2009).
As awesome as nudibranchs are, there was another animal in the baskets which tends to get more attention, the snapping shrimp. These shrimp belong to the family Alpheidae. There are about 600 species of snapping shrimp in 46 genera. These shrimp, also known as “pistol shrimp”, form symbiotic relationships with several different groups of animals (e.g. sponges, cnidarians, mollusks, echinoderms, echiurans, other crustaceans, and gobiid fishes) (Silliman et al. 2003). A rather friendly clade indeed. However, these little guys pack quite a punch. With one snap of their large claw, these shrimp have asymmetric claws, a sound is produced loud enough to be heard a kilometer away. If several alpheidae are snapping at once, it can interfere with submarine sonar (Anker et al. 2006). The sound is produced when the shrimp closes its claw so fast it alters the pressure of the surrounding water, imploding a pocket of air known as a cavitation bubble. The water jet from a snap can be received by other snapping shrimp and used to communicate or to stun prey (Versluis et al. 2000).
Despite decades of marine research, we are barely grasping a fraction of what goes on in the waters that cover 3/4ths of the Earth’s surface. In some aspects, we know more about space than we do about what happens in the depths of our oceans. Our oceans are teeming with life, waiting to be discovered and filled with questions as well as opportunities. So next time you are going for a swim in the ocean or maybe a snorkel, remember that you are in a soup of baby animals so tiny they are invisible to the human eye. Or maybe listen to the crackling of the water, that’s the snapping shrimp communicating, hunting, or both. Whatever you do, remember you are never alone.
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