Monday, September 26, 2016

Green Sunfish Is One Tough Sparring Partner, by Don Orth

Green Sunfish (Lepomis cyanellus Rafinesque) is a very common sunfish that may occur in streams, rivers, ponds, and shallow weedy margins of lakes.  No matter where you find them, they will usually be in close proximity to weedy vegetation, large boulders, or woody cover.   Although their maximum size may not be very impressive, they make up for the small size with spunk.   The world record Green Sunfish  was 2 pounds, 2 ounces – that must have been one big spunky sunfish.   The generic name is derived from Greek words that refer to the scales on the operculum, while the species name is derived from the Greek word for blue, in reference to the blue wavy lines on the cheeks. 
Green Sunfish adult. Photo by Nate Tessler.  Source. 
To positively identify a Green Sunfish you will have to check a number of characteristics.   Coloration is bluish-green on back and sides with reflections of yellow and emerald.  Blue wavy streaks are present on the cheeks.   The belly is pale yellow or white. The ear flap is black with a white or yellow-orange margin; it is not elongated and the bony margin of the gill cover is hard and inflexible.   Black blotches are usually present near the base of soft dorsal and anal fins.  The body is not as deep bodied as other Lepomis.   Body depth is approximately equal to the distance from the tip of the snout to the origin of the dorsal fin; measure to be sure!  It has a large mouth for a sunfish with the upper jaw extending to the middle of the pupil.   The pectoral fin is rounded and, when bent forward, will not extend beyond the front of the eye. Juveniles and hybrids can be more challenging to identify.  Green Sunfish have a complete lateral line with 41 to 52 scales, which is intermediate between Micropterus and other Lepomis.   When breeding, the male Green Sunfish develops fringes of white or salmon or yellow on the dorsal, caudal, anal, and pelvic fins.   Females and subordinate individuals will not develop the fringes and have a dorsal ventral banding pattern. 
Juvenile Green Sunfish.  Photo by Dan Johnson.  Source
Why the opercular ear flap?  All Lepomis sunfishes have a dark ear flap.   Why the spots on the soft dorsal fin?   Only Bluegill, Bantam Sunfish, and Green Sunfish have the blotches.  Think about it! 

Green Sunfish is tolerant of a wide range of environmental conditions. Therefore, it is often found in sloughs and ditches where no other sunfish is present.  It is usually one of the few sunfish that will persist in small streams of the Great Plains along with Creek Chubs Semotilus atromaculatus, Fathead Minnows Pimephales promelas, and Black Bullheads Ameirus melas.  Because of their broad geographic distribution and wide environmental tolerance, the index of biotic integrity in the Midwest USA has a Green Sunfish metric (Metric 6, Proportion of individuals as green sunfish).    That always seemed  speciesist, and the meaning of metric 6 should reflect proportion of highly tolerant fish.    Some streams are naturally harsh and the Green Sunfish is able to persist in harsh environments.    That should not influence the assessment of biological impairment.  Harsh places are not necessarily impaired.   However, Green Sunfish have become widespread outside its native range where they may suppress native fishes (Fuller et al. 2016). 
Distribution of Green Sunfish (Fuller et al. 2016)  USGS
The Green Sunfish seldom get large enough to attract the following of recreational anglers.  However, it is usually so common, especially in intermittent creeks, that it is a good fish for young anglers to target.  Green Sunfish provided me many an evening meal while camping during my dissertation research studies on Glover River, Oklahoma.  Here the Green Sunfish was the dominant fish species, comprising 46% of total biomass (Orth and Maughan 1984).    Green Sunfish are also popular as an aquarium fish for the native fish enthusiast because they feed readily and may persist under less than ideal water quality conditions.   

Sunfishes are notorious for hybridizing in nature.  Many species overlap in range, spawning time, and share limited nesting habitats.  More than 20 Lepomis hybrids have been reported in nature (Childers 1967). Natural hybrids are produced when one species at very low abundance and females of rarer Lepomis breed with an aggressive male of the dominant species (Avise and Saunders 1984).   These F1 hybrid exhibit adult sex ratios strongly biased toward males and, consequently, hybrid sunfish have been bred specifically for pond stocking.  The Green Sunfish x Bluegill cross is popular. Click here for photo of the F1 hybrid and parentals.  

 Green Sunfish show homing tendencies and defend territories as do other sunfish and Micropterus basses.   A territory is “any defended area,” and the Green Sunfish is an aggressive defender of their territory.   Bernard Greenberg first wrote about territoriality and social hierarchies in the Green Sunfish.  Hierarchies developed in groups of immature Green Sunfish only 0.75-1.5 inches standard length and less than one year old.   Establishing a dominance hierarchy early in life avoids wasting energy on agonistic display patterns.  Rather the dominant individuals “drive their subordinates, that is, they make quick dashes at them, and the latter swim away, usually without being nipped”  (Greenberg 1947, p. 269).    Green Sunfish show three aggressive behaviors: (1) gill-flare threat display; (2) weaving; and (3) nipping.  Green Sunfish pairs will occasionally test one another by a gill flare display.   The gill flare threat involves “spreading their gill-covers and moving head-on toward each other; the threatened individual turns broadside and, with body held rigid, vibrates in a weaving fashion” (Greenberg 1947, p 270).  The gill flare shows the observer the margined patch of black of the opercle flap, which roughly resembles an eye.    View the video of a Green Sunfish showing the gill flare display and you will see how effective this display. 
Video of Green Sunfish gill flare display.   by CarpGuy  Source

The spot on the soft dorsal fin also darkens when the Green Sunfish is interacting with another.   Greenberg observed that the fighting individual “almost always has these spots intensified, but, if it loses the fight, the spots disappear.”     Although many others have described aggressive behavior and territorial defense and displays, the experiments of Bernard Greenberg established that the Green Sunfish engaged in aggressive behavior and defend territories long before they became reproductively mature.

Greenberg also observed changes in the coloration of the eye during agonistic encounters.  The iris is normally red.  Hence the Green Sunfish is sometimes referred to as a goggle eye.   However, when Green Sunfish are fighting the red becomes more prominent.  In a Green Sunfish that loses in an encounter the black pigment spreads to the rest of the iris and the red is obscured.  Consequently the black eye signals a sign of defeat in the Green Sunfish.  
Close up of head of Green Sunfish.  photo by  Source.
The fighting pattern of two Green Sunfish involves one individual on the attack with gills flared and the other weaving to avoid being nipped.  The attacker may make a series of vigorous nips at the head or body and may even lock jaws.    In aquaria,    groups of Green Sunfish establish a pattern of dominance within 24 hours. Individuals learn to display subordinate patterns.   Further maze experiments by Greenberg supported the notion that dominant individuals in groups facilitated learning among members.   In an experiment comparing Green Sunfish in isolation and in groups, the individuals in groups grew faster over the period of study (Allee et al. 1948).  Reasons for enhanced growth are difficult to establish, although the authors did observe fish in social groups displaying more ready feeding.   Also, the high ranking fish within a social hierarchy ate more and grew faster than low ranking ones.

The Green Sunfish is an excellent example of a common opportunistic sunfish species that has a rich set of social interactions that have not yet been fully explored.  What you may see as aggressive interactions in aquaria are adaptations that will facilitate group success and resource distribution in wild populations.  Also, Green Sunfish have double cones in their eyes that make them sensitive to polarized light (Cameron and Easter 1993).  The adaptive significance of polarized light sensitivity has yet to be explored.  We can only speculate and devise new experiments. 

Allee, W.C., B. Greenberg, G.M. Rosenthal, and P. Frank.  1948.  Some effects of social organization on growth in the Green Sunfish Lepomis cyanellus.  Journal of Experimental Biology 108:1-19.
Avise, J. C., and N.C. Saunders. 1984. Hybridization and introgression among species of sunfish (Lepomis): Analysis by mitochondrial DNA and allozyme markers. Genetics 108:237-255.
Cameron, D.A., and S.S.Easter.1993. The cone photoreceptor mosaic of the green sunfish, Lepomis cyanellus. Visual Neuroscience 10:375-384.
Childers, W.F. 1967. Hybridization of four species of sunfishes (Centrarchidae).  Bulletin of the Illinois Natural History Survey 29:159-214.
Fuller, P., M. Cannister, and M. Neilson. 2016. Lepomis cyanellus. USGS Nonindigenous Aquatic Species Database, Gainesville, FL. Revision Date: 1/20/2012
Greenberg, B.  1947. Some relations between territory, social hierarchy, and leadership in the Green Sunfish (Lepomis cyanellus).  Physiological Zoology 20:267-299.
Orth, D.J., and O.E. Maughan. 1984.  Community structure and seasonal changes in standing stocks of fish in a warm-water stream. American Midland Naturalist 112:369-378.

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