Tuesday, July 19, 2011

Stingrays and Skates

Atlantic Stingray
Some people are scared of stingrays, some people like them, and then there are those very few people who live for stingrays. We had the opportunity to meet one such researcher from the University of Rhode Island and his Coastal Fellow who is following in his footsteps. John (Jack) Szczepanski was preparing for a joint meeting of ichthyologists and herbatologists in Minneapolis, when he and his Coastal Fellow, Peter Schooling (Marine Affairs - URI ’13) took a break from their preparations and research to share a brief lunch with us and discuss their interests and research.
Szczepanski gave us some basic details about sting rays to get us started. Stingrays are elasmobranches; like skate rays and sharks they have a skeleton made entirely of cartilage. Certain rays have reinforced jaws which allow them to consume hard species such as crabs, whelks, and snails. Stingrays have strong chemoreception and use their sense of smell to find their food. They store urea in their tissues to control salt intake because unlike most fish they don’t filter out the salt water, rather they store all the nutrients, which makes them taste bad and smell like ammonia. Stingrays are commonly used as lobster bate. They are also sometimes used as faux sea scallops in the Midwest and many species of rays are eaten in certain cultures. Electric rays, which are often found in Rhode Island, appear large and blobby and have an electric organ-muscle. The poisonous barbs on a stingray’s tail (that famously and tragically killed Steve Irwin) can break off and likely do not grow back.
Electric Ray (the red area is the
electric field emitted)
Generally, stingrays move inland during their remarkably long (11 month!) gestation period. An egg case is absorbed in the side of the mother stingray, who eventually gives a live aplacental birth. This type of reproduction is known as ovoviparity; in other words, the ray embryos develop in eggs that are held within the mother until they are ready to hatch. Some rays are rather large when they are born. For example, bull nose rays are generally between 18 and 30 cm at birth. Stingrays do not have a set breeding season. In contrast to stingrays, skates lay eggs, which is one of the major differences between the two otherwise similar species.
Szczepanski with the stingray
that stung him in the hand
on his honeymoon!
Large groups of stingrays (particularly the eagle ray and bull nose ray) migrate north from tropical waters in the summer. They are new but no longer uncommon in RI and have been seen in Narragansett Bay as early as May. Their migration this far north that early in the season may be indicative of climate change. Szczepanski believes their migration patterns may also be indicative of ecological changes because they are generally not commercially fished. Much of Szczepanski’s research takes places in Delaware Bay, which serves as a breeding ground for sharks that are sand-born and then spend the rest of their adult lives offshore. Delaware is very species rich, but comparisons can still be made to RI despite our fewer and smaller populations. Perhaps the most important correlation that can be drawn between the two water bodies is that they are both estuaries – breeding grounds/nurseries for numerous species.
Szczepanski with a stingray
on his honeymoon.
Szczpanski has gone out on many surveys to assess populations of stingrays (including one on July 20, 2010 while he was on his honeymoon and ended up getting stung in the hand!) While out on the boat, he measures the disk width from wing tip to wing tip and length from nose to pelvic fins, determines the sex of the stingray, and then checks for its stomach contents. Szczepanski then weighs and identifies the food from the belly of the stingray. Skates can have their stomach pumped to remove the contents for measurement. Little skates, which are common in Rhode Island eat a variety of food. Among other things, Szczepanski is trying to determine if their diets are more specific in Delaware, where they have a greater variety of food to choose from, than in Rhode Island. Clear nose skates, which are also common in Rhode Island tend to feed on squid, wheat fish, worms, crabs, shrimp, and more. Bull nose rays eat anything from whelks to mud snails and hermit crabs and sometimes even razor clams. Cow nose rays have strong jaws and plate-like teeth which are used for crushing. They, too, eat razor clams. Szczepanski believes that the mechanisms stingrays use to find food are – in order of importance – smell, sight, and electroreception.
Over the course of two years, Szczepanski will perform more than 20 surveys per bay (about one every month) in the Delaware and Narragansett Bays. He hopes to monitor how many stingrays are caught during each trip and their weights and species type. His work takes a serious dedication because he has almost no funding aside from his graduate studies research allowance. Szczepanski says he is grateful to have the cooperation of fishermen who allow him to examine the stingrays caught in their nets.

Elizabeth Gooding

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