Abstract

In mid-May, 2000, one female frogfish (Antennarius striatus) exhibited spawning behavior for the first time since her arrival six months earlier. The female was one of three (2 males, 1 female) frogfish housed in a 180-gallon tank with under-gravel filtration. She had difficulty releasing the egg mass, and one inch of the egg mass was observed protruding from her vent. The following day, the eggs were removed manually, and hemorrhages were evident around the prolapsed vent. Eggs were nonviable. The female was separated from the males by a divider and placed on enrofloxacin (Baytril®, Bayer Corporation, Shawnee Mission, KS). She recovered uneventfully.

A few weeks later, this same female again appeared conditioned and ready to spawn. After two days, a small portion of eggs was seen in the tank. These eggs were poor in consistency and beginning to degrade. At this time, the female exhibited some minor buoyancy control problems. The fish was transported to the Tropical Aquaculture Laboratory for evaluation and treatment.

The female weighed 210 grams and was approximately 16 cm TL. Fins had minor erosions on their tips, and eyes were mildly opaque. The left half of the maxillary portion of the mouth was dark. One lesion near the vent was darkened and slightly ulcerated. Attempts to obtain an ovarian biopsy were unsuccessful. Ovaprim® (Syndel Laboratories, Vancouver, British Columbia, Canada), a gonadotropin releasing hormone analogue (GnRHa) with domperidone, a dopamine inhibitor, was administered at the rate of 0.5 ml/kg. The following day, the remaining egg mass was expressed using gentle pressure. A total of 60 grams of eggs (approximately 90 ml volume) was collected. Prolapsed gonadal tissue was gently reinserted and temporary sutures were placed around the vent. The fish was placed on enrofloxacin (Baytril, 5 mg/kg q 48 hours), and the sutures removed after two days. Although the fish outwardly appeared to have recovered, she was found dead two weeks later.

Problems with a previous spawn and possible concurrent systemic infections appear to have contributed to egg-binding in this fish. Ovaprim facilitated expression of the remaining eggs. Use of Ovaprim and other hormones to induce ovulation may be an effective treatment modality for egg-binding. An accurate diagnosis of the problem is key to success. A good history of the problem, including evidence of spawning activity and conditioning of the female is critical. An ovarian biopsy should be attempted to stage the maturity of the eggs. Immature eggs may indicate another problem, and will not respond to hormonal treatment.

Hormone-Induced Spawning

In nature, sexually mature, conditioned fish undergo final maturation and spawn after exposure to specific stimuli, including photoperiod, water quality (such as temperature, pH, hardness, salinity, total dissolved solids, alkalinity), flooding, tides, barometric pressure, spawning substrates, or the presence of other species.⁴

In the aquaculture of more difficult species, these triggers may not be easily mimicked. Induction of final maturation and spawning can be stimulated by hormonal injection. Although, theoretically, hormones that act anywhere along the brain-hypothalamic-pituitary-gonadal axis can be used, most spawning hormones available act directly on the pituitary or the gonads.

Ovaprim® is a gonadotropin releasing hormone (GnRH) analogue, based upon the salmon GnRH (sGnRH), found within the brains of many teleosts, and active in most^2,5 In addition, Ovaprim contains the dopamine blocker domperidone, In later stages of gonadal maturation, Ovaprim causes therelease of GTH-II from the pituitary. GTH-II acts upon the gonads, stimulating final maturation of oocytes and ovulation in females, and spermiation in males, via gonadal steroid hormones. Domperidone acts as a dopamine receptor antagonist, blocking the inhibitory effects of dopamine on GTH-II release. In some species, such as many of the cyprinids, this inhibition by dopamine is an important spawning hurdle to overcome.⁵ Dosing regimens may vary, but are typically 0.5 ml/kg given in a single dose or divided over two doses, with the initial dose a smaller priming dose.

What is Egg-Binding?

Although egg-binding in birds and reptiles is well-documented^1,3 by comparison, egg binding in fish is poorly understood. Major differences in egg production and reproduction between birds, reptiles, and fish make any analogies difficult if not altogether incorrect. The terms "egg-binding" and "egg-bound" are frequently used in reference to mature, conditioned female fish that do not spawn normally. Although the terms are used by both professionals and lay persons (e.g., aquarium hobbyists), it is likely that inexperienced hobbyists may be using the term much more loosely and incorrectly. Despite this casual use of terminology, no major work has been done in this area. In fact, there is presently no good definition for egg-binding in fish.

Factors that may determine a fish species' or individual fish's predisposition to egg-binding include: status as synchronous vs. group synchronous spawners; complete vs. partial spawning; temperature; age of fish; presence of infectious disease; susceptibility to handling stresses; nutrition; photoperiod, and failure to respond to other spawning triggers.

A better understanding of the germinal epithelium, folliculogenesis, and natural regression, are important prerequisites to tackling the question of egg-binding in fish. PAS-positive granulocytes appear to play an important role in natural regression of gonadal tissue.

The following may be examples of "egg-binding" in different species observed by the authors:

 A group of high-fin black tetras (Gymnocorymbus ternetzi) spawned once and then allowed to condition for over one month did not spawn again.

 A group of long'finned rosey barbs (Barbus conchonius), when spawned continuously,

 Red drum (Sciaenops ocellatus) that were induce-spawned could not be stripped of eggs completely. The following day, following catheterization, remaining eggs were observed to have formed into a gelatinous mass.

 In black sea bass (Centripristis striatus) black masses observed in the posterior ovarian lumen are believed to be degenerated egg masses.

 Injection of snook (Centropomus undecimalis) with human chorionic gonadotropin (HCG) resulted in an apparent collapse of the ovary followed by infiltration of granulocytes into all vitellogenic oocytes.

Unanswered Questions

There are many unanswered questions. Here are just a few. How do we define "egg-binding" in fish? What factors lead to a species' or an individual fish's susceptibility to becoming egg-bound? How common is egg-binding in nature? What percentage of a spawn can be resorbed and then regress naturally without major interruption of the reproductive cycle? How much of an interruption in the cycle will egg-binding cause, and what are dependent factors? What "level(s)" or "category(ies)" of egg-binding lead to sterility? Exactly what role do PAS positive granulocytes play in natural regression and how may they contribute to egg-binding? Hopefully future work will begin to shed light on this topic.

References

1.  DeNardo, D. 1996. Dystocias. In: Mader, D (ed). Reptile Medicine and Surgery. W.B. Saunders Co., Philadelphia, PA. Pp. 370-374.

2.  Powell, J. Syndel Laboratories. Personal communication.

3.  Romagnano, A. 1996. Avian Obstetrics. In." Speer, BL (ed). Seminars in Avian and Exotic Pet Medicine, 5" 4. W.B. Saunders Co., Philadelphia, PA. pp. 180-188.

4.  Rottman, RW, Shireman, JV, Chapman, FA. 1991. Introduction to Hormone -Induced Spawning ofFish (SRAC Publication No. 421). IFAS, University of Florida.

5.  Van Der Kraak, G, Chang, JP, Janz, DM. 1998. Reproduction. In: Evans, DH. The Physiology of Fishes. CRC Press., Boca Raton, FL. Pp. 465-488.