In a previous post about corals, we discussed the symbiotic relationship between stony corals and their partner algae, Zooxanthellae. Here we’ll talk a little more about individual coral polyps and their anatomy.
As we saw in pictures in the last post, coral polyps have tentacles that they can use in hunting, feeding, fighting and even to clean themselves off. These tentacles aren't just arms, they're also weapons: most contain special stinging cells called nematocysts towards the tips. If you've ever been stung by a jellyfish, you've encountered nematocysts before.
The entire exterior layer of the coral is covered by the outer epidermis, the coral equivalent of our skin. At the center of the tentacles is a hole, which is the only opening into the gut--so corals use the same hole to take in food and to eliminate wastes (okay, yes, technically they "poop out of their mouths"). When a coral polyp retracts into the skeleton, it does so by expelling all the sea water from inside the gut using muscle contraction. The layer of skin and flesh which extends between each polyp, connecting them, is called the coenosarc. Although polyps are connected by a simple nerve net, we know relatively little about how polyps relate to each other. That said, they certainly can cooperate: one study found that if you selectively fed one polyp in a bleached colony, it would share nutrients to help keep the polyps immediately around it alive.
Inside the gut, the gastrodermis (interior layer of skin cells) is a digestive surface which helps break down the things a coral polyp eats. It also has fleshy, digestive "curtains" called mesenterial filaments which drape over calcium carbonate ridges called septa (septum is the plural) to give them more digestive area within the gut. If you make a coral polyp angry, it can extrude (stick out) the mesenterial filaments through the mouth to try to digest or at least hurt whatever is attacking it. Corals can actually go to war with each other using these filaments and other weapons (which we'll talk about in a future post!)
The theca is the skeletal "cup" which the coral lives inside, and the basal plate is the bottom of that cup, which the coral is always adding to as part of growth and maintenance (coral calcification and skeletons are also a subject for a future post since they are too awesome and complicated to go into here). Now we have a little more vocabulary for talking about corals, but also a better understanding of how complicated they are--even though many people look at them and see nothing more than a colorful rock.
If you can't wait for our next post to get more information on the amazing world of corals, NOAA has a wonderful educational website on corals available at: http://oceanservice.noaa.gov/education/tutorial_corals/welcome.html.
You have probably heard that a shark must swim in order to breathe or it will die. This does hold true for some sharks, but with over 500 described species, there are bound to be some exceptions (and in this case, there are lots!)
As you may know, sharks are a group of cartilaginous fish. They live across all the oceans of the world from pole to pole. Because they are fish, sharks get their intake of oxygen from the water they inhabit. This is accomplished via a specialized respiratory apparatus of gills. Most sharks have five pairs of gills made up of cartilaginous gill arches that support a vast network of gill filaments. These filaments are highly vascularised, meaning they contain numerous thin blood vessels and provides a large surface area for gas exchange. This also explains the bright red coloration found in shark (and fish) gills.
In order to exchange carbon dioxide for oxygen, sharks must continually pass water over these gill filaments. The oxygen in the water is taken up by the red blood cells, while carbon dioxide is released into the water, similar to the process of respiration in our lungs. The amount of oxygen a shark requires is directly dependent on the way it lives. Deep water and sedentary sharks like the Greenland shark have very low oxygen requirements; fast moving, highly active sharks like the mako have requirements that are on par with warm-blooded mammals. These highly active sharks take advantage of their forward momentum to passively force water through their mouths and over their gills in a process called ‘ram-ventilation’. Unlike most fish, these very active sharks lack the physical apparatus to manually pump water over their gills and are obliged to swim continuously in order to pass enough water over their gills to meet oxygen demand. Unfortunately, this condemns them to certain death if they are prevented from swimming by being hooked or entangled in nets.
Other species of sharks, like Wobbegongs, Cat-sharks, and Nurse sharks, spend a great deal of time resting motionless on the bottom. The Nurse shark is perhaps the most encountered shark species in Florida waters by snorkelers and divers. They are commonly found lying under rock and coral ledges during the day, often piled atop each other. These sedentary sharks are adapted to pump water without the need for swimming. As the shark opens its mouth, its pharynx (or throat) expands, flattening the gill slits and creating a vacuum so water is pulled in. Then the shark closes its mouth and constricts its pharynx, thus forcing the entrapped water through the gill slits and across the filaments. If you ever are lucky enough to observe a resting shark, you can see the entire rhythmic process in action. Many sharks can switch between these methods of breathing to suit their activity level and oxygen requirements. Sharks that breathe both ways include Caribbean Reef, Lemon, Tiger and Sandtiger.
The Sandtiger has one more trick using air. Because sharks lack a swim bladder they are "negatively buoyant"--which means they generally sink when they aren’t swimming (remember that you'll see Nurse sharks lying on the bottom). But clever Sandtigers can actually gulp air at the surface and store it in their stomachs, creating neutral buoyancy which allows them to hang motionless in the water while gently pumping away. How cool is that?
We here at field school love honeybees, in part for their importance to ecosystems (thanks, pollinators!) and in part because they are really neat and give us delicious honey. Recent research published in Frontiers in Behavioral Neuroscience gives us just one more proof of how darn cool bees are: they taste with their claws to decide whether something is worth eating.
Insects taste using their sensilla, which are hair-like structures containing nerve cells sensitive to particular substances. Honeybees have sensilla located on their "mouths", antenna and the end of their legs (which scientists call "tarsi"; you can see the end of one magnified by an electron microscope at the right). This research showed that bees weigh the information they get from their front tarsi to decide whether or not something is worth feeding on.
For this experiment, hundreds of bees had their tarsi covered in sugary, salty or bitter liquids, which prompted them to reflexively extend (or retract) their tongues depending on how they liked the taste. Researchers found that the bees used different parts of the tarsi to detect sugary tastes versus salty ones. The ability to taste with their "feet" lets them check a flower for nectar as soon as they land--which means less wasted effort if the flower is dry.
The researchers also wondered what would happen if they coated one tarsi with tasty sugar water, and the other with something boring or bitter. They found that honeybees may consider input from both legs when evaluating whether to extend the tongue: if they taste sugar first, they will usually ignore the other leg and extend their tongues. But if they taste something undesirable on one leg first, and then sugar on the other, they're about 50% less likely to extend their tongues.
You can check out the original research publication:
Maria Gabriela De Brito Sanchez, Esther Lorenzo, Su Songkung, Fanglin Liu, Yi Zhan and Martin Giurfa. The tarsal taste of honey bees: behavioral and electrophysiological analyses. Front. Behav. Neurosci., 2014 DOI: 10.3389/fnbeh.2014.00025
Or read Science Daily's article on the findings here:
Frontiers. "Do you have a sweet tooth? Honeybees have a sweet claw." ScienceDaily. ScienceDaily, 4 February 2014. <www.sciencedaily.com/releases/2014/02/140204162256.htm>.
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