Abstract

The seahorses and their relatives represent a circumglobal family of marine fishes. However, some seahorse populations (Hippocampus sp.) are currently threatened with overharvesting for use in traditional medical practices, the coastal tourism curio trade, and the pet trade. In an effort to increase public awareness of their fascinating life histories as well as the conservation issues surrounding these fishes, the John G. Shedd Aquarium is currently exhibiting twenty-two species of syngnathids in a temporary, special exhibit. The information presented here was gleaned from summarized data from medical records, necropsy records, and histopathology reports from 11/93 through 10/98. These records were largely generated from wild-caught specimens acquired from the pet trade for this special exhibit.

The two primary clinical conditions encountered were vibriosis and gas/air entrapment with its attendant postural and buoyancy control problems. Vibriosis has been the most commonly diagnosed clinical condition among all of the syngnathid species maintained at the Shedd Aquarium. Vibriosis in captive syngnathids was characterized by high morbidity and high mortality. There were three predominant clinical presentations: ulcerative/erosive dermatitis, sudden death with no premonitory signs, and a syndrome characterized by bilateral edema of periorbital tissue as well as edema of the soft tissue around the tubesnout. Pathologic findings included ulcerative dermatitis, bacterial cellulitis/myositis, and bacterial septicemia. Osteomyelitis and peritonitis were less frequently encountered. Septic fish often had one or more of the following histologic changes: reactive endocardium, pericarditis, necrotizing hepatitis, and renal necrosis. Vibrio alginolyticus was the species most frequently isolated from kidney cultures. Vibrio alginolyticus was also the bacterial species most frequently found in live foods fed to the fishes, i.e., adult artemia, artemia nauplii, and mysid shrimp. Treatment with antibiotics has been only moderately successful. In vitro antibiotic susceptibility patterns from twenty-six V. alginolyticus isolates suggested that ceftazidime and chloramphenicol would be the most efficacious therapeutic choices. Prevention currently involves administration of a bivalent vibrio bacterin for V. anguillarum and V. ordalii (Alpha-Dip 2100, Alpharma, Bellevue, WA) at the time of acquisition with a booster after two weeks. Additional measures under current investigation for vibriosis prevention include use of an immunostimulant in conjunction with administration of an autogenous bacterin as well as use of a live food supplement that has antibacterial properties. Installing in-line UV sterilizers in the live food holding tanks may help to reduce the incidence of food-borne vibriosis.

Gas entrapment disorders have been restricted to seahorses (Hippocampus sp.). Male seahorses seem to be predominantly (if not exclusively) affected. Gas entrapment disorders are characterized by low morbidity and low mortality. These disorders typically present in one of four ways: gas entrapment in the brood pouch, subcutaneous emphysema of the tail, gas entrapment in cranial spaces, and hyperinflation of the swim bladder. By far, gas entrapment in the brood pouch and subcutaneous emphysema of the tail are most commonly encountered. Clinically, these fish have problems maintaining normal posture and controlling their buoyancy in the water column. Often, but not always, brood pouch gas entrapment presents as a grossly and symmetrically distended pouch. In caudal emphysema, the gas bubbles in the soft tissue of the subcutis of the tail can be easily observed with the naked eye. Pathologic findings often include gas-filled pseudocysts lined with endothelial tissue suggesting gas-dilatation of lymphatic vessels. Chronic cases often exhibit fibrosis. Granulomatous inflammation may also be present, especially in the subacute to chronic cases. No infectious agents have been identified. The putative etiology is chronic, low level supersaturation of the water column. It has been postulated that the extensive vascular bed that supports the physiologically dynamic brood pouch of male seahorses may be uniquely predisposed to gas embolization (S. Raverty, personal communication). Treatment attempts are most successful in acute cases where there is minimal fibrosis. So far, fine needle aspiration of the gas-filled pseudocysts appears to be the only reliable form of treatment. Reoccurrence of the lesions is common. Identifying and eliminating the source of the supersaturation is advisable but can be rather challenging in practice.

Most of the parasites encountered were metazoan. Extra-intestinal metazoa were most frequently found and usually involved encysted, quiescent digeneans and cestodes. Intestinal metazoa were the second most frequently observed parasites with cestodes being more commonly encountered than nematodes or digenetic trematodes. Both praziquantel and fenbendazole are prophylactically administered during the quarantine period. It is notable that monogenetic trematodes have never been encountered in the syngnathid species exhibited at the Shedd Aquarium. Lernaeocerid copepods were observed in the branchial cavities of two Lined Seahorses (Hippocampus erectus).

Protozoal and dinoflagellate parasites have been seen but with less frequency than the metazoa. Notable cases include enteric coccidiosis in four Weedy Sea Dragons (Phyllopteryx taeniolatus). One of these cases was unresponsive to treatment with orally administered sulfa-trimethoprim. An outbreak of amyloodiniosis (Amyloodinium ocellatum) in a group of Pygmy Seahorses (H. zosterae) responded well to continuous bath administration of chloroquine diphosphate. Three cases of biliary microsporidiosis with a history of sudden death were seen in Spiny Seahorses (H. barbouri). One case of microsporidial branchitis was reported in a Hippocampus whitei. Twelve cases of uronemiasis were documented. It is noteworthy that no cases of cryptocaryonosis (Cryptocaryon irritans) were reported in this collection of syngnathids.

Acknowledgements

The authors wish to thank Lisa M. Doering, William M. Hana, Annelisa M. Kilbourn, and Robert J. Van Valkenburg for their valuable contributions to this work.