Text and Photos by Emilie Novaczek Spoon Cove, Newfoundland There’s lots to see while SCUBA diving the North Atlantic coast; long-toothed wolffish, scowling eelpouts, graceful winter skates, fields of magenta coralline algae, and if you’re lucky, a glimpse of a humpback whale in the distance. In all that diversity, the sea urchins are some of my favourites – and it’s all about the hats. The most common urchin in Newfoundland, and throughout the Maritimes, is Strongylocentrotus droebachiensis, a bit of a mouthful for the unassuming green urchin. We see hundreds of green urchins on every dive, and in the shallows, we find them covered with anything they can get their tube feet on: rocks, shells, even the odd golf ball. St. Thomas Cove, Newfoundland. Photo by Christopher Power Echinodermata refers to the phylum of animals that includes seastars, sea cucumbers, sand dollars, crinoids, and our hat-wearing urchins. Echinoderms share a few key features, including a couple that sound like superheroes: Wolverine’s regeneration and Spiderman’s ability to scale smooth surfaces at any angle. We’ll get back to regeneration later on. Echinoderms move around the seafloor (or simply move food to their mouth) using hundreds of sticky tube feet. These feet are part of the water-vascular system, a network of seawater-filled tubes. Urchins move using the tube feet on their ventral side by contracting small muscles that force water into the tubefoot to take each step. The end of each tube foot is very, very sticky. They are often described as suction cups, however recent research[1] indicates that echinoderms likely use a bio-adhesive, rather than suction, to achieve their Spiderman-like climbs. Urchins also use these sticky tube feet to pick up and hold onto rock, shells, golf balls, and other treasures. But why? Spoon Cove, Newfoundland Behavioural ecologists call urchin hats “covering behaviour”. That name is related to the first and most prevalent hypotheses about the phenomena: the urchins are covering themselves to provide shelter from sunlight, predators, or both. Experiments conducted on Paracentrotus lividus, the purple sea urchin common to the Eastern Atlantic Ocean, confirmed the light hypothesis. Researchers in Ireland found that when the urchins were exposed to full spectrum UV light, more individuals would pick up their hats and/or move to the shady corners of their tanks to avoid harmful UV radiation[2]. Around the same time, another scientist in California was studying covering behaviour of Pacific rose flower urchins, Toxopneustes roseus[3]. The rose urchin study wasn’t conducted in a lab; instead urchin behaviour was observed in their natural habitats. What they found was that at the sample site with the greatest wave energy, there was also the most covering behaviour among the urchins. Harbour Grace, Newfoundland So which is it? Sun safety? Or are these hats more like seat belts and kneepads, weighing urchins down and protecting them from wave damage? On this side of the Atlantic, researchers tested several factors simultaneously to trace the covering behaviour to it’s source. In a laboratory, green urchins were exposed to common predators, wave surge, waving algae blades, and sunlight[4]. As it turns out, the predators were a bust: their presence had no significant impact on the rate of covering behaviour. The hats are not camouflage. Urchins may have some scary looking spines, but they still have predators! Like the Irish purple urchins, green urchins exposed to UV light were found to cover up more. However, UV exposure wasn’t the most important factor. This study found that green urchins on the east coast of Canada, like rose urchins in California, wore more hats when they were exposed to wave surge, and/or in contact with moving algae blades. Not all urchins wear hats though; the Canadian study found that smaller urchins were more like to cover up. Seastars can regenerate lost arms, sometime many at a time (see? I told you we’d get back to this). Though it’s less dramatic, urchins are constantly regenerating lost and broken spines. But regeneration takes energy. It may be a safer bet, particularly for a small urchin who is vulnerable to dislodgement and damage, to pick up some extra weight and a little sun protection at the same time. Bacon Cove, Newfoundland [1] More information on tube feet: http://echinoblog.blogspot.ca/2013/01/echinoderms-dont-suck-they-stick.html [2] Verling et al. 2002: http://link.springer.com/article/10.1007%2Fs002270100689?LI=true [3] James 2000: http://link.springer.com/article/10.1007%2Fs002270000423?LI=true [4] Dumont et al. 2007: http://www.sciencedirect.com/science/article/pii/S0003347207000796 Emilie is a marine conservation biologist and PhD candidate at the Memorial University of Newfoundland. She is also the Chair of the Biology Graduate Student Association at Memorial. Emilie knows all about catch-and-release aquariums as she has been a volunteer scientific diver for the Petty Harbour Mini Aquarium for the past four years. t: @maptheblue Thanks so much for writing this post Emilie. We can only hope you'll be able to join us on a Back to the Sea dive one day! P.S. Another thing we love about Emilie are her awesome pictures, like this one of an urchin's mouth, known as Aristotle's Lantern.
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By Dr. Chris Harvey-Clark This guest post is brought to you by Dr. Chris Harvey-Clark who is the University Veterinarian as well as the Director of Animal Care in the Department of Psychology at Dalhousie University. In an interview on CBC's Mainstreet program, Dr. Harvey-Clark spoke to the success of small-scale aquarium such as the ones found in St. Andrew's, New Brunswick and Petty Harbour, Newfoundland. We reached out to him following this interview at which point Dr. Harvey-Clark agreed to serve as one of our valuable advisors. He has taken the time to tell us more about one of his areas of interest in the post below. People might be surprised to know that a big marine predator that kills fish using electrical shock is coming in increasing numbers to shallow waters around Nova Scotia! This late summer visitor is the Atlantic torpedo ray, Tetronarce (formerly Torpedo) nobiliana, a large, enigmatic member of the skate and ray family (the Batoids) found from tropical to temperate waters on both sides of the North Atlantic inshore and in deep waters. By far the largest of 17 species of electric rays worldwide, the torpedo ray can weigh 90kg and have a body disc diameter approaching 2m in mature females. This species uses electrogenic organs comprised of modified muscle cells in the lateral margins of the body disc to generate controlled DC current bursts in excess of 200 volts. This shocking power can snap the back of a mackerel in tetanic convulsions and is also used for discouraging predators. A friend of mine who was shocked by this species while diving lived to tell the tale and likened the sensation to putting your finger into a dryer socket. Dr. Fred Whoriskey views the first ever satellite tagged Atlantic torpedo ray Photo credit: Dr. Chris Harvey-Clark The electrogenic tissues of torpedo rays have been extensively studied at the cellular and molecular level, with thousands of citations in the scientific literature. Some of the earliest work on the neurotransmitter acetylcholine and its effects on muscle tissue were first characterized in torpedo ray tissues. It is a paradox that despite extensive study at the cellular level, little is known of the ecology, movement and behaviour of T. nobiliana. In fact, decades of fishing for use in neuroscience research depleted local populations of this species in the vicinity of the Marine Biological Laboratory at Woods Hole Mass. The fact remains that virtually nothing is known about the basic biology of T. nobiliana. The size and age structure of the Atlantic population, depth, substrate and temperature preferences, onshore/offshore movements of this species, prey preferences, longevity, reproductive parameters and life cycle are all poorly known. The IUCN (International Union for Conservation of Nature) Red List indicates the species is data deficient, in common with the majority of sharks, skates and rays. In this respect, our knowledge of T. nobilana resembles the former state of knowledge of many large charismatic species such as sharks, tunas, sea turtles and many marine mammal species prior to the development of modern electronic tracking technology beginning two decades ago. Like the curious case of the dog that failed to bark in the night, the fact that this species is rarely reported as bycatch despite intense commercial fishing within its known range begs the question: where do these animals go? What is their role in the seasonal summer assemblage of large pelagic and forage fish species that occurs in boreal seas around Europe and North America annually? The habit and habitat of this species remains a mystery. Observations exist of occasional individuals in shallow water sand and mud bottom habitats from Nova Scotia to the Florida keys and from northern Scotland to West Africa, into the Mediterranean, usually from fisheries bycatch inside continental shelf depths. Fishbase and similar database sources cite depth data for this species from shallow water to 800 meters and report their presence as rare fisheries bycatch in the Mediterranean. Several references claim the rays are benthic bottom dwellers when younger and become more pelagic dwellers as they get older, but there is little evidence in the primary scientific literature to support this claim.
In the fall of 2015, Dr. Fred Whoriskey and myself tagged a female Atlantic torpedo ray with a satellite tag near Halifax, NS. The tag was programmed to pop up to the surface 95 days later, and report its position and other data to a geosynchronised satellite. I had theorized that the rays were following the shallow continental shelf migrations of forage fish like herring and mackerel - north in the summer and south in the winter. Imagine my surprise when the tag reported 95 days later from an offshore location over 900 km out in the North Atlantic, from an area where the bottom is in excess of 4000 m. This single record indicated that in at least one case, this species does in fact act as a pelagic animal, quite amazing for a ray we had found dug in to the bottom while scuba diving in 20 m of water. This discovery has led to plans for a more extensive study of the movement and behaviour of this species. Volunteers interested in helping the torpedo ray tagging team can contact me at Dalhousie University: chclark@dal.ca. |
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