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Physiology of Stingray Venom

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Stingray Barb

As mentioned earlier, the spine containing the sting is only used as a form of defense for stingrays.  If a predator comes too close to a stingray, either touching its wings or making the ray feel threatened, the tail will fling up in a whip-like response causing the spine to thrust into the “attacker” releasing the venom along with fragments of the stingray’s spine.  In some instances, the entire spine may even break off and get stuck in the wound (Magalhaes et al., 2006). Fortunately for the stingray, broken spines are able to regenerate quickly in order to again ensure the organism’s protection.  Furthermore, stingrays are quick and agile, easily arcing in circles and flicking their spine sideways or backwards over their bodies in order to hit their enemy (Diaz, 2008).

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The stingray barb can be as long as 37 centimeters, however, the average length is about 3.81 centimeters (Dehghani et al., 2009; Tennesen, 2005). According to a survey conducted on the spine characteristics of 51 species of stingrays living in African, Arabian to Chagos-Madlive archipelago waters, the spine length and disk width do not correspond with the size or type of stingray (Schwartz, 2007). The spine itself is made up of vasodentin, a bone-like cartilaginous substance which contains serrated edges that are able to penetrate the victim’s skin (Diaz, 2008). These jagged edges along the spine often cut up the victim’s flesh on the stinger’s way out of the wound (Tennesen, 2005). This saw-edged spine is covered by a thin layer of skin referred to as the integumentary sheath.  This sheath controls the ventrolateral grooves that contain the venom glands.  The venom secretory cells are the specific cells within the epithelium layer that house the venom and secrete it during the sting (Dehghani et al., 2009). These cells are cylindrical or elliptical in shape and contain fusiform vesicles rich in protein (Dehghani et al., 2010). When the spine gets thrust into the victim, the epithelial sheath comes apart, releasing the venom into the wound (Evans and Davies, 1996). It is the mucus covering the spine which contains this venom and which causes pain in the victim following the sting (Tennesen, 2005; Dehghani et al., 2009). Many aquatic organisms have similar mucus layers which can sometimes fall under attack by microbes in the water.  In catfish, a gel-like layer of material made of proteases and antibodies is secreted to the skin surface in order to protect the mucus layer from such attacks.  Potamotrygon stingrays, found in South American river systems, contain peptides in their mucus layers which are thought to act as host-defense effector molecules protecting the epithelium layer from these attacking microorganisms (Magalhaes et al., 2006).  

The location, size, and number of stings vary in different stingray species, habitats, and ages.  The number of spine serrations are dependent on the gender of the stingrays with males having up to twice as many as females. The number of serrations also corresponds with stingray habitat. The rays that live primarily in open water have an increased number of serrations while freshwater stingrays and those that reside along the bottom of bodies of water have lower numbers (Schwartz, 2007). The distribution of the venom secretory cells along the stinger also varies among species.  These particular cells may occur either around or inside the ventrolateral grooves resulting in more or less severe envenomations.  The secretory cells may also appear in different positions relating to the stingray’s epithelial layer.  They can be in multiple rows among the epidermis layers or in separate layers below the epidermal cells (Dehghani et al., 2010). Furthermore, stingray species that reside in freshwater habitats have secretory cells located along the entire epidermis of the stinger as opposed to those in marine species being located only within or around the ventrolateral grooves.  The differing locations of these secretory cells along the stingers explain why the freshwater species are capable of causing more severe envenomations than the marine stingrays (Pedroso et al., 2007). Freshwater species also contain more specialized cells containing high protein levels suggesting that their tissue extract is more toxic than that of the marine stingrays (Barbaro et al., 2007).



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