Friday, February 5, 2016

Wow! Did You See That Yellow Stingray? by Don Orth

This post is not about the Corvette Stingray; rather it’s the Yellow Stingray Urobatis jamaicensis  (Cuvier, 1816), a ray in the order of stingrays (Myliobatiformes) and the family of American round stingrays (Urotrygonidae).  But they both tell stories of contraints in morphological design over  time.  The stingrays (thie fish) are all extremely depressiform with pectoral fins that surround much of the body and are fused to the head making the Myliobatiform pectoral fin one of the most unique appendages of all vertebrates. These genetic and developmental constraints place limits on the design of the new phenotypes, just as market expectations place limits on the design of new models of the Corvette Stingray.   Advertisements for the Corvette boast that the "Stingray is a perfect driving machine, born from brilliant engineering and precision performance. Its aggressively sculpted exterior is a statement of intent; its driver-oriented cockpit is a creation of purpose."    "Stingray lives at the intersection of race-proven technology and provocative, purposeful design. Every element serves a purpose, from functional exterior vents to intelligent driver controls." It's a low-built car designed for performance at high speeds (sports car) and we are unlikely to see a new hybrid or SUV model Corvette Stingray produced by Chevrolet anytime soon.   
Chevrolet Corvette Stingray Targa C3 by stkone on Flickr
Phylogenetic constraint means "that certain evolutionary pathways are not likely to be followed by a species or group of related species, as a result of prior evolutionary history. In short, yesterday's adaptation may be today's constraint." (J.D. Ligon in McKitrick 1993).  The round stingrays are intermediate group located between the basal flatsharks, Sawfish (Pristidae) and guitarfish (Rhinobatidae), and the more derived Eagle rays (Myliobatidae).  All are derived from a common ancestor with the extremely dorsoventrally depressed body form and expanded pectoral fins.   The body form constraint is evident in all these flatsharks.  Let’s explore the constraint with the Yellow Stingray (the fish) as an example. 

The Yellow Stingray is a benthic dweller found on tropical shallow marine reefs of western Atlantic from Florida south through Caribbean and to northern South America.   Coloration is variable.  The oval-shaped, flattened disk is yellowish – hence the name -- with dark vermiculations and spots that form a variety of patterns on upper surface.   The complex and changeable coloration patterns are likely associated with camouflage. The relatively small Yellow Stingray is often seen buried in sand or resting on rocky substrates.  Their movements increase during nocturnal and crepuscular periods.  
Yellow Stingray left photo by DJ Orth,  and cleared and stained Electric ray (Torpedinidae) by Adam Summers
Before mating, the male and larger female swim together for several meters.   Then the male bites at the margin of the female’s pectoral fin until successfully biting and holding the pectoral fin such that he can swing underneath the female.  This abdomen-to-abdomen position allows the male to insert a clasper for copulation.   In fact, the mature males possess upper teeth that are more loosely spaced with high conical cusps to better grasp the female during copulation  (Source: FMNH, Young 1993). Yellow Stingrays are live bearers, and produce 3-4 live young.   Their nursery habitats are more likely to be in shallower water.

Low reproductive output is another phylogenetic constraint; a few females may produce more than 4 offspring, but that still means the many Yellow Stingrays must survive and grow to maturity to maintain populations. Maximum life span is relatively short (7-8 years) when compared with other elasmobranchs.  Females move to nearshore areas for pupping so that they small neonates may avoid predators and maximize individual growth rates..   They possess a venomous spine, which further protects from harassment. Finally, panoramic vision, adjustable coloration pattern, burial in sand, and low movement during daylight further reduces predation.   

The depressiform body swims via undulations of its enlarged pectoral wings.  This permits the Yellow Stingray to gracefully generate forward thrust without disturbing the substrate and attracting attention.   Watch video  video 1   video 2  to observe this swimming motion in their habitat. 

The extreme depressiform body means that the field of vision is different from other fishes.   The Yellow Stingray needs to see and detect motion or predators in a wide field.   The eyes are periscopic, enabling them to protrude above the substrate when the ray is buried.   Each eye has a covering that allows fine control over amount of light entering the pupil to permit the Yellow Stingray to be active during nocturnal and crepuscular periods.  McComb and Kujiura (2008) demonstrated a 360° panoramic visual field in the horizontal plane; binocular vision was possible in 34° anterior view.  This field of vision is larger than the more basal members of the batoid clade.  However, because of the phylogenetic constraint, the vision of Yellow Stingray is limited binocular forward-facing vision and depth perception is inferior to a fish with frontally positioned eyes.  Because the mouth in positioned ventrally, the Yellow Stingray never sees what it eats.  Rather it uses sensitive touch receptors and electroreceptors to locate prey. This fish is very common in public aquariums, as it readily feeds on shrimps, clams, worms, and small fishes.  
Yellow Stingray eye (left, Keri Wilk) and field of vision (right, McComb and Kajiura (2008)
Over 1,000 species of fish are venomous.  The venom of the Yellow Stingray is distributed via a stout spine on its tail.   If you accidently step on this fish it will use its spine in self defense.   The tail spine can inflict a nasty puncture wound that will bleed profusely and cause swelling and pain for several days.  Here is a treatment plan if you are stung (No! do not pee on it!).  Any explorations in shallow coastal waters should be done with protective beach shoes.  In areas where stingrays live, learn to do the stingray shuffle and you will scare them away.   Or better yet, put on a mask and snorkel and observe the Yellow Stingray up close, without disturbing them. Yellow Stingrays are easily observed by divers and snorkelers.  In fact, some evidence suggests that diver surveys, collected for REEF, may contribute to estimating trends in abundance.
 
Tail spine of a Yellow Stingray © Cathleen Bester/FLMNH (left)  and Do the Shuffle Warning sign
The Yellow Stingray, though widely distributed and commonly sighted, is usually seen by divers singly.  It remains one of many unexplored species of Elasmobranchs, and has been the subject of few scientific investigations outside the Florida region (Spieler et al. 2013).   The Yellow Stingray has a brain that is 3-10 times the size of guitarfish, skates, and electric rays, yet the fish has not been investigated by neurophysiologists or behaviorists.   Current issues relate to recent decrease in Yellow Stingray numbers in the Florida Keys, which may be related to increase in Goliath grouper, Epinephelus itajara, habitat destruction, or harvest (Ward-Paige et al. 2010).  More questions than answers when it comes to the Yellow Stingray.  Wow! They are fun to watch.
 
References

McComb, D.M., and S.M. Kajiura. 2008.  Visual fields of four batoid fishes: a comparative study. The Journal of Experimental Biology 211:482-490   
McKitrick, M.C. 1993. Phylogenetic constraint in evolutionary theory: has it any explanatory power? Annual Review of Ecology and Systematics 24:307-330
Spieler, R.E., D.P. Fahy, R.L. Sherman, J. Sulikowski, and T.P. Quinn.  2013. The Yellow Stingray, Urobatis jamaicensis (Chondrichthyes Urotrygonidae): a synoptic review.  Caribbbean Journal of Science 47(1):67-97. 
Ward-Paige, C.A., R.A. Myers, C. Pattengill-Semmens, and H.K. Lotze. 2011.  Spatial and temporal trends in yellow stingray abundance: evidence from diver surveys Environmental Biology of Fishes 90:263-276.

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