Control of Neobenedenia sp. Infestations in the Ocean Tank at Gran Acuario Mazatlán

Abstract

The Gran Acuario Mazatlán (GAM), Mexico’s largest aquarium, is located at the entrance of the Gulf of California. In April 2023, fish in the Ocean Tank (OT) began exhibiting disease symptoms, followed by significant mortality, primarily affecting Caranx caninus, Chaetodipterus zonatus, and Lutjanus colorado. Parasitological examinations identified heavy infestations of Neobenedenia sp. (Monogenea: Capsalidae), with parasites extensively covering body surfaces and gills of some individuals. To control the outbreak, an environmental management strategy was implemented, involving a gradual reduction in water temperature (from 26 °C to 22 °C) and salinity (from 35 ppt to 26 ppt), increased cleaning frequency, and parasite egg abundance monitoring. Over time, egg abundance declined, and by July, fish behavior and feeding rates had returned to normal, with parasite-associated mortality dropping below 2%. Preventive measures, including strict quarantine protocols for new fish, have been implemented to maintain optimal conditions in the OT. This case highlights the challenges of managing parasite outbreaks in large, multi-species aquaria and underscores the importance of environmental control strategies in marine animal care.

1. Introduction

The Gran Acuario Mazatlán (GAM) is the largest aquarium in Mexico and one of the most important in Latin America. It opened to the public on 6 May 2023, and is located in Mazatlán City on the Mexican Pacific coast, at the entrance of the Gulf of California (also known as the Sea of Cortez), a marine ecosystem renowned for its rich biodiversity. This region is home to approximately 900 species of teleost and cartilaginous fish, over 150 species of birds, 115 species of reptiles, 36 species of mammals, 31 species of cetaceans, 5 of the world’s 7 sea turtle species, and 5000 species of macroinvertebrates [1]. The GAM is dedicated to enhancing public knowledge of Sea of Cortez biodiversity through engaging exhibitions as well as educational, research, and outreach activities.

The primary attraction at GAM is its Ocean Tank (OT), which spans 427 m², reaches a depth of 7.6 m, and holds 2,583,000 L of water. The tank houses a wide diversity of fish, ranging from small pelagic species to sharks, while maintaining high water quality standards. However, in 2023, an outbreak of parasite-associated fish mortality occurred. This article describes the incident and the control measures implemented. While fish parasite infestations are a known challenge in aquariums, published reports on their management remain scarce. By detailing this outbreak and the implemented control measures, this study aims to contribute to existing knowledge and aid aquarium professionals in improving parasite management strategies.

2. Case Report

At GAM, water quality in each tank is maintained within optimal parameters by its own dedicated life support system. However, in April 2023, a few days before GAM’s official opening, fish in the OT began exhibiting disease symptoms, such as reduced appetite, cloudy eyes, erratic swimming, lethargy, loss of vertical orientation, and scale loss, followed by mortality. At that time, the OT housed 754 specimens from 19 fish species (

Table 1. Fish species present in the Ocean Tank at Gran Acuario Mazatlán during the Neobenedenia sp. outbreak.

Fish Species	Fish Family	Common Name	Number of Individuals
Diodon hystrix	Diodontidae	Spot-fin porcupine fish	3
Anisotremus interruptus	Haemulidae	Bacoco	11
Caranx caballus	Carangidae	Green jack	15
Caranx caninus	Carangidae	Pacific crevalle jack	31
Chaetodipterus zonatus	Ephippidae	Pacific spadefish	127
Trachinotus rhodopus	Carangidae	Gafftopsail pompano	3
Lutjanus argentiventris	Lutjanidae	Yellow snapper	427
Hoplopagrus guentherii	Lutjanidae	Barred snapper	10
Lutjanus novemfasciatus	Lutjanidae	Pacific dog snapper	11
Lutjanus colorado	Lutjanidae	Colorado snapper	65
Diodon holocanthus	Diodontidae	Long-spined porcupinefish	27
Pteroplatytrygon violacea	Dasyatidae	Pelagic Stingray	1
Aetobatus narinari	Aetobatidae	Spotted eagle ray	2
Carcharhinus altimus	Carcharhinidae	Bignose shark	5
Gynglimostoma cirratum	Ginglymostomatidae	Nurse shark	9
Negaprion brevirostris	Carcharhinidae	Lemon shark	2
Bodianus diplotaenia	Labridae	Mexican hogfish	3
Hyporthodus acanthistius	Epinephelidae	Rooster hind	1
Trachinotus sp.	Carangidae	Pompano	1

). By 16 May, mortality had risen to 15%, primarily affecting three species: Caranx caninus, Chaetodipterus zonatus, and Lutjanus colorado. Additionally, complete mortality was observed in a smaller tank (24,300 L) containing 204 specimens of Acanthurus xanthopterus. During the outbreak, average water conditions were as follows: temperature 26 °C, salinity 35 ppt, dissolved oxygen 6.5 mg/L, pH 7.9, and nitrite 0.05 mg/L.

Parasitological examinations of deceased and moribund fish revealed heavy infestations of Neobenedenia sp. (Monogenea: Capsalidae) (Figure 1), with parasites completely covering body surfaces and gills of some individuals. Identification was conducted by one of us (F.N.M.S.) based on morphological characteristics, following descriptions of Neobenedenia melleni and N. girellae [2,3]. Species-level identification was not possible, as it requires molecular analysis for accuracy, which was beyond the scope of this study. A total of twenty specimens of L. colorado, five of A. xanthopterus, and five of C. zonatus were visually examined. Initially, fish body surfaces were inspected under good illumination. Although the monogeneans are translucent, their movements produced a flashing effect that facilitated detection. Subsequently, smears of skin, fins, and gills were obtained using a scalpel and examined under a light microscope at 100× and 400× magnifications. Based on this diagnostic evaluation, immediate action was taken to mitigate the issue. Fish exhibiting disease symptoms were removed from the exhibition tank and placed in a 1000 L tank, where they received a 3 min freshwater bath, followed by a 50 min formalin bath (0.17 mL/L) the next day. However, this treatment did not resolve the problem, as the parasite remained in the exhibition tanks. Since chemotherapeutic methods were not feasible in that environment, the staff opted for environmental management strategies to eradicate the monogeneans.

The control method involved gradually reducing both water temperature and salinity. Temperature was lowered from 26 °C to 22 °C over five days (18–22 May), while salinity was decreased from 35 ppt to 26 ppt over one month (18 May–17 June). Additionally, tank cleaning frequency was increased, with backwashing performed for 25 min once daily and siphoning twice daily (morning and evening). Monogenean infestation levels were monitored indirectly by estimating the abundance of monogenean eggs in the water. To collect the eggs, new 60 cm long cotton threads were placed in the tanks every two days, as Neobenedenia produces eggs with filaments that become entangled in the threads. These threads were examined under a stereomicroscope 48 h after placement. This cleaning regimen and egg monitoring protocol began on 18 May and continued for two months. In the second month, however, the threads were replaced and examined weekly.

Egg abundance decreased over time. Initially, each cotton thread was nearly covered with eggs, but after one month, only about one-third of each thread was covered. By July, conditions returned to normal, as indicated by typical fish behavior, increased feeding rates, and rare egg detection. During June and July, parasite-associated mortality was less than 2%. To prevent another outbreak, the cleaning regimen has been maintained, with backwashing reduced to 15 min. Water temperature and salinity have been kept at an average of 23 °C and 31 ppt, respectively, while egg monitoring continues on a monthly basis. Prior to introducing new fish into the exhibition tanks, they are quarantined for three to four weeks and treated with a 3 min freshwater bath followed by three 50 min formalin baths (0.17 mL/L), with two days of rest between treatments. These control measures have ensured that the OT and other tanks continue to operate under optimal conditions.

3. Discussion

Addressing a mortality event like the one that occurred at GAM requires immediate action to minimize both animal and economic losses. However, applying antiparasitic treatments in oceanic tanks is challenging due to the large water volume and high species diversity. Moreover, Neobenedenia has biological traits that complicate control efforts. This monogenean follows a direct life cycle, is hermaphroditic, and capable of self-fertilization; a single adult can produce over 3000 eggs within days and develop from egg to adult in about two weeks, allowing a rapid outbreak [4]. Additionally, controlling capsalid monogenean infections is further complicated by the resilience of its eggs, which float in the water column and are more tolerant to treatments than juvenile or adult stages [5,6].

The monogenean control method implemented at GAM was based on previous studies demonstrating that lowering both temperature and salinity can significantly slow parasite development [7,8,9]. Additionally, its application was inspired by a prior report suggesting that hyposalinity is an effective strategy for controlling Neobenedenia in large, multi-species display aquaria [10]. This report highlights the case of the South Carolina Aquarium, where staff successfully eradicated Neobenedenia infestations by implementing prolonged hyposalinity, replacing the repeated use of copper and organophosphate treatments.

The case reported here is not uncommon, as monogeneans have long been recognized as a significant threat to fish in captivity [11]. The detrimental effects of monogeneans, including Neobenedenia spp., on fish include excessive mucus secretion, hemorrhage, tissue damage from feeding activities, and inflammatory reactions caused by the parasite’s attachment hooks, all of which can impair physiological processes and lead to disease outbreaks and mass mortalities in cases of heavy infestations [12,13,14,15].

The species of Neobenedenia responsible for the outbreak at GAM has yet to be identified and requires molecular analysis. Among the species known to occur in the region, N. melleni and N. girellae have been reported as serious pathogens affecting farmed marine fish in the southern Gulf of California [9,16]. Notably, N. girellae is a cosmopolitan parasite capable of infecting a wide range of fish species, including wild, farmed, and aquarium fish [15,17]. For instance, N. girellae has been detected in 40 fish species from 12 families of imported marine ornamental fish at a public aquarium in the Mexican state of Yucatán [18]. This suggests that Neobenedenia remains a persistent threat to animal welfare at GAM, emphasizing the need for continued preventive strategies.