Reproductive Behavior, Embryology, and Larval Development of Four Species of Pygmy SunfishBy Maurice F. Mettee and Christopher Scharpf
reprinted from American Currents, Winter (Feb.) 1998
Introduction: Are Pygmy Sunfishes Sunfishes?
The pygmy sunfishes of the genus Elassoma include six described species, all of
which are native to swamps and backwater areas of the southeastern United States and
middle Mississippi Basin:
Elassoma evergladei Jordan
1884, the Everglades pygmy sunfish, extends from South
Elassoma okefenokee Böhlke
1956, the Okefenokee pygmy sunfish, is restricted to
Elassoma boehlkei Rohde and
Arndt 1987, the Carolina pygmy sunfish, is found in
Elassoma okatie Rohde and
Arndt 1987, the bluebarred pygmy sunfish, is endemic
Elassoma alabamae Mayden 1993,
the spring pygmy sunfish, was previously known
Since the description of E. zonatum in 1877, the taxonomic position of the genus Elassoma has been the subject of much controversy. Originally, the fish was thought to be a cichlid (Jordan, 1877), but that opinion soon changed. Hay (1881) and Jordan and Gilbert (1882) placed E. zonatum into its own family, the Elassomatidae, because they believed it was intermediate between the pirate perch (family Aphredoderidae) and the centrarchids. Boulenger (1895) placed Elassoma into the sunfish family Centrarchidae because of the similarity and kinds of vertebrae; he was also probably the first to postulate that Elassoma was a dwarf sunfish.
Over the years, several investigators have presented evidence they felt was sufficient to exclude Elassoma from the Centrarchidae. After examining the olfactory organs of three centrarchid species and E. zonatum, Eaton (1956) stated that Elassoma was a neotenous sunfishthat is, capable of being sexually mature as a juvenile. Branson and Moore (1962) surveyed the acoustico-lateralis systems of 26 centrarchid species and the three described Elassoma species at that time; they concluded that while elassomids were closely related to centrarchids and possibly shared a common ancestry with them, they had specialized and diverged sufficiently to be considered a separate family. Moore and Sisk (1963) stated that the eye structure of Lepomis and Elassoma were markedly different. Roberts (1964) examined the chromosome complements of 20 centrarchid species and found that while E. zonatum possessed the modal centrarchid number of 48 diploid chromosomes, its chromosome morphology differed significantly from that of any other centrarchid species. This led him to the conclusion that while Elassoma is distantly related to the sunfishes, it still differs to the extent that it should be placed in a separate family. Similar findings based on biochemical studies were presented by Avise and Smith (1977).
One aspect of the life history of Elassoma which has received little investigation, but which might be important in providing additional information regarding its relationships to centrarchids, is reproductive behavior. Because elassomid fishes are usually found in slow-moving waters that are less than one foot deep and choked with aquatic vegetation, observations on their reproductive behavior would be difficult; consequently, all reports on their spawning behavior have been based on aquarium experiments.
Several investigators have indicated that the spawning habits of elassomid fishes are similar to those of other centrarchids, since the male constructs a nest into which the eggs are deposited during spawning. Such behavior was observed in E. evergladei by Axelrod and Shaw (1971), Innes (1969), and Axelrod and Schultz (1971). Contrarily, Nachstedt and Tusche (1954), Sterba (1961), Breder and Rosen (1966), and Branson (1974) stated that this species was not a nest builder. Shortt (1956) observed that the eggs of E. okefenokee were deposited in "moss," but did mention a nest. After he had observed spawns of E. zonatum in aquaria, Poyser (1919) speculated that this species preferred to spawn over a nest, but if bottom conditions were unfavorable, it would alternately spawn on algae or aquatic vegetation. In a paper on the life history and ecology of E. zonatum at Mound, Louisiana, Barney and Anson (1920) noted that the eggs of this species were always found scattered about in the aquatic vegetation. It is obvious from this summary that the reproductive behavior of Elassoma is incompletely known and in need of additional research before it can be compared to that of the Centrarchidae.
This article is an adaptation of the senior authors Ph.D. dissertation (Mettee,
1974), which documented the reproductive behavior of the four Elassoma species
known at the timezonatum, okefenokee, evergladei, and the then undescribed alabamaewith
the intent of comparing it with that of the family Centrarchidae. Composite descriptions
of the embryology of elassomid fishes were also presented, as well as information on
growth rates and fin development.
Materials and Methods,
All reproductive studies were conducted in the laboratory. The fishes were contained in four 40-liter and six 20-liter all-glass aquaria. Continuous air was supplied by aquarium pumps and air stones. Water temperature was controlled within 3°C by tube-type aquarium heaters with internal thermostats. A 15.5 hour light period was maintained throughout the study using daylight supplemented with fluorescent light banks on an automatic timer. Because elassomid fishes will not readily eat dry foods, they were fed live brine shrimp (Artemia) nauplii either daily or every second day, depending upon their size and breeding condition.
In order to duplicate their natural environment as closely as possible, aquatic plants, principally of the genus Ceratophyllum, were collected with breeding stocks of elassomid fishes and used in the spawning aquaria. Specimens were transported from the field to the lab in Styrofoam boxes and then placed into a 40-liter aquarium filled with 21°C distilled water. After a period of 7-10 days, five or six mature females were transferred into each of two 20-liter aquaria with physical conditions similar to those of the holding tank. Using aquarium heaters, the water temperature in these two aquaria was gradually raised 2.5-4.5°C over a period of 10-14 days until the female abdomens began to enlarge, indicating egg production. This temperature was maintained for another 7-8 days, at which time one or two males of the same species were introduced into each tank with the females. After a period of 2-3 days, during which the males established territories, spawning usually occurred.
Within 10 minutes after spawning, the eggs were transferred into 50 ml petri dishes and maintained under similar physical conditions. Photographs of live eggs were taken of each embryological stage, upon which the accompanying composite illustrations were drawn.
The prolarvae were maintained in the same petri dishes until they reached
a total length of 8-10 mm. At this time they were transferred into a 20-liter all-glass
aquarium and allowed to grow to adult size. Periodically, specimens were preserved in a 5%
formalin solution for later observation. Because of the small number of eggs produced by a
single spawn, and the high mortality rates of eggs and larvae, several spawns were
necessary in order to complete a series from newly hatched prolarvae to adult.
Based on the following observations, the reproductive behavior of the four Elassoma species studied is very similar.
The color pattern of breeding males of E. zonatum was essentially unchanged, although the colors did intensify considerably. All of the fin membranes, except those in the pectoral fins, became much darker, and the 9-11 vertical bars on the trunk darkened to the extent that the black spot usually found ventral to the dorsal fin origin was indistinguishable. A small crescent similar in size and position to the ones described for E. evergladei and E. okefenokee, but gold in color, was present around the eye, and many small, iridescent gold and blue flecks were scattered about on the cheeks and opercula.
Breeding males of E. alabamae were dark brown to black, and on the trunk were located 6-8 very narrow, irregularly spaced, vertical, iridescent gold bars that extended the entire body depth. An iridescent gold structure similar to that described for E. zonatum was present around each eye, perhaps the most outstanding color characteristic of males of this species was a distinct, clear spot in each of the last four dorsal fin and anal fin membranes which, when viewed collectively, formed a "window" in the posterior end of each fin. This "window" is a valuable key character for this species as it was present in the dorsal and anal fins of all male individuals used in this study.
The Sidling Threat Display
The Wiggle Waggle Display
Figure 1. Stages of the reproductive behavior of elassomid fishes. A = male approaching female. B = wiggle waggle dance of male. C = female approaching the spawning site. D = the spawning act.
Vegetation and Egg Deposition
Depending on the species, the male continued to guard the eggs for the next 72-100 hours. If another individual approached, it was confronted by a Sidling Threat Display and chased from the area. When the eggs were being collected for observation, it was not uncommon for the male to bite on the end of the pipette; if that failed to stall collection efforts, he would eat his own eggs. Once the eggs were removed from the spawning site, the male would renew his efforts to spawn with another female.
After witnessing several spawns of each of the four elassomid species, it became evident that the lack of aquatic vegetation as a suitable spawning medium may have been the reason why Axelrod and Shaw (1967), Innes (1969) and Axelrod and Schultz (1971) have observed these fishes spawning on the bottom rather than in aquatic vegetation. As mentioned in the Materials and Methods section, most of the vegetation collected with elassomid breeding stock was Ceratophyllum, a thick-growing, fine-leafed plant. Elassomid eggs were always found attached to leaves of Ceratophyllum, except in cases where this plant was either not available or in a decomposing state, at which time the eggs were found on the bottom. Because of their semi-adhesive nature, the eggs would become covered with debris soon after they reached the bottom of the aquarium. In his efforts to clear away the debris, the male would clean an area that could be construed as a nest by those familiar with the bedding habits of centrarchid species. Photographs that lend support to this idea were given in Axelrod and Shaw (1967). The first sequence of photographs showed specimens of E. evergladei spawning in what appeared to be dying strands of Ceratophyllum or some closely related plant on the bottom of the aquarium, while the second photograph depicted a male E. evergladei guarding eggs that had been scattered about in healthy strands of aquatic vegetation that were floating away from the bottom.
Comparison and Conclusion
A later study by Walsh and Burr (1984) confirms that E. zonatum, like other pygmy sunfishes, deposits its eggs in aquatic vegetation rather than in cleared nests.
According to Mayr (1969), behavioral taxonomic characters are often superior to
morphological characters in the study of two closely related groups. From this study, it
is evident that the behavior of elassomid and centrarchid fishes is not similar. Based on
the morphological, chromosomal, biochemical and behavioral differences, as given in Eaton
(1956), Branson and Moore (1962), Roberts (1964), Avise and Smith (1977), and this study,
it is our opinion that the elassomid fishes have specialized to an extent to justify their
being placed into a separate family, the Elassomatidae.
The embryological stages illustrated in Figure 2 are based on the observation of eggs
collected from 17 spawns of E. okefenokee, 10 spawns of E. evergladei, six
spawns of E. alabamae, and 24 spawns of E. zonatum. Because there is
variability in developmental rates within groups of eggs from the same spawn, observations
were made on a time schedule, and developmental stages were assigned based on the stage
demonstrated by the majority of eggs at that time.
Figure 2. Composite illustrations of the embryology of elassomid fishes. A = unfertilized egg. B, C = fertilized egg within 10 minutes after fertilization. D = one-celled embryo. E = two-celled embryo. F = four-celled embryo. G = eight-celled embryo. H = 16-celled embryo. I = 32- to 64-celled embryo. J = early high blastula. K = late high blastula. L = early gastrula. M = late gastrula. N = neurula, end view. O = neurula, lateral view. P = early larval stage. Q = 12-14 somite stage. R = 16-18 somite stage. S = 20-24 somite stage; brain and eyes prominent; heart is pumping colorless blood. T = prehatch larva; morphological development of larva appeared complete; all areas of brain visible; blood cells light pink in color. U = newly hatched larva.
Table 1. Developmental rates for four species of Elassoma.
During the hatching process, movements inside the egg became more frequent
and violent until eventually, by using the tail as a lever, the larva ruptured the
chorion, freeing the posterior end of its body. After a short rest the larva would shake
Description of the Prolarvae and Postlarvae
Prior to metamorphosis, elassomid larvae cannot be distinguished from each other; therefore, descriptions included herein pertain to pygmy sunfish larvae in general unless otherwise specified.
Newly hatched larvae (Figure 2-U) were tadpole-like in shape, except for a large ventro-lateral bulge caused by the enlarged yolk sak. No mouth was visible. The eyes were without pigment. A small pectoral fin bud which consisted of a fan-shaped membrane without fin ray primordia was present on either side of the larvae posterior to the eye and dorsal to the yolk sac. No pelvic fin buds were present. The major areas of the brain were distinguishable. The heart beat rate remained at 100-115 beats per minute for E. okefenokee, 145-150 for E. evergladei, 140-144 for E. alabamae and 134-136 for E. zonatum, and the blood pathway around the yolk sac and through the vessels of the body was visible. When viewed from the dorsal side, four pairs of gill arches and the rhythmic movements of the gill covers were observed.
Several morphological and behavioral changes were observed after the transition from
prolarval to postlarval stages. The standard lengths at which these changes occurred are
given for each species in Table 4.
Table 4. Standard lengths (mm) at which Elassoma change from prolarvae to postlarvae and postlarvae to juveniles.
Food and Larval Mortality
Temperature and Larval Growth
In his discussion on larval metabolism and growth, Blaxter (1969) indicated that
ambient temperature was one of the most important influences on the rate of development.
During this study, individuals of E. okefenokee and E. zonatum that were
maintained at lower temperatures (23°C and 21°C, respectively) metamorphised at an older
and longer standard length than did specimens of E. evergladei. The growth pattern
for individuals of E. alabamae differed from the other elassomid species. Even
though they were maintained at a lower temperature (23.5°C), the larva of this species
grew faster than those of E. evergladei (25.5°C). Once they had lived through the
"critical phase" between the time of yolk absorption and active feeding, growth
in E. alabamae larvae was rapid for approximately 40 days, after which it slowed
significantly for 270 days.
Pen drawings of the sequence of fin development in elassomid fishes are given in Figure 3. The standard lengths at which various fin primordia were first observed are listed in Table 5.
Fin development was complete at standard lengths of 8.0-9.0 mm for E. okefenokee, 6.4-7.0 for E. evergladei, 5.3-5.7 for E. alabamae, and 8.0-8.5 for E. zonatum.
The fishes of all four elassomid species studied here are covered with cycloid scales except for E. okefenokee and E. alabamae, which do not have scales on the tops of their heads. Small scales were first observed on late prolarvae of each species; by the time metamorphosis had occurred, scales covered the entire body.
Since the completion of the senior authors initial study on elassomids, several additional studies have come forth indicating an even more distant relationship between pygmy sunfishes and centrarchids, and perhaps no relationship at all. Humphries and Lauder (1985) found no evidence to support the notion that elassomids are a sister group of centrarchids. Johnson (1984, 1993) presented evidence that elassomid affinities lie outside the Percoidei, the large perciform suborder that includes such familiar fishes as groupers, perches and darters, butterflyfishes, marine angelfishes, and sunfishes. Johnson and Patterson (1993) expounded on this belief, finding that elassomids share some derived features with synbranchids (swamp eels), mugilomorphs (mullets), gasterosteiformes (sticklebacks, seahorses, etc.), mastacembelids (spiny eels), and atherinomorphs (rainbowfishes, killifishes, etc.). They even proposed a name for this new groupSmegmamorphaan acronym using the initials (S-M-E-G-M-A) of the six taxa which comprise the group. In addition, the name derives from the Greek and Latin smegma, meaning cleansing or cleansing agent. In this usage, the name refers to the authors "expectation that grouping these taxa will have the effect of cleaning up or tidying the systematics of higher teleosts . . .".
More recently, Johnson and Springer (1997) presented evidence that in every aspect an elassomids skeleton is trying to be like a sticklebacks. A formal rationale for placing elassomids into Gasterosteiformes is being prepared (G. D. Johnson, pers. comm. with CS).
Until their relationships are more clearly and definitively resolved, most
ichthyologists retain pygmy sunfishes in the order Perciformes, within their own suborder
(Elassomoidea) and family (Elassomatidae) (Nelson, 1994; Helfman et al., 1997). Please
note, however, that many publications, including the popular How to Know the Freshwater
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scientific names (Robins et al., 1991), still place pygmy sunfishes among the
centrarchids. This will no doubt change in future editions.
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© 1998 North American Native Fishes Association. May not be republished without written permission from NANFA.