Abstract
Ceratomyxa amazonensis is a cnidarian myxosporean originally described with strongly arcuate crescent-shaped myxospores, absence of vegetative stages and infecting Symphysodon discus, an important Amazonian ornamental fish in the aquarium industry. As part of a long-term investigation concerning myxosporeans that infect discus fish Symphysodon spp. from different rivers of the Amazon Basin, thirty specimens of S. discus collected from Unini River were examined. Plasmodial vegetative stages therefrom were found freely floating in the bile of gall bladders from eighteen fish. Mature myxospores were slightly crescent-shaped, measuring 4.72 ± 0.1 (4.52–4.81) μm in length, 24.2 ± 0.4 (23.9–25.3) μm in thickness with polar capsules 2.31 ± 0.1 (2.29–2.33) μm in length and 2.15 ± 0.1 (2.13–2.17) μm in width. Strong morphological differences were observed between the newly isolated myxospores obtained and the previously described C. amazonensis; however, molecular assessment, based on 18S rDNA, revealed a high similarity (99.91%), with only a single nucleotide base change. This study provides new data, expanding the original description of the species with a discussion on differences in myxospore-morphology in the context of intraspecific morphological plasticity.
Highlights
The global aquarium market moves millions of ornamental fish worldwide and is the primary mode for international transport of cnidarian myxosporean parasites (Hallett et al 2015)
Following the criteria outlined by Lom and Arthur (1989) and Heiniger et al (2008), measurements taken for each myxospore included spore length (SL), spore thickness (ST), polar capsule length (PCL) and polar capsule width (PCW) in micrometres and posterior angle (PA) in degrees (°)
It is widely recognised that Ceratomyxa spp. myxospores can display a high degree of morphological plasticity (Atkinson et al 2015, Bartošová-Sojková et al 2018); classifications, based strictly on morphology, can result in ambiguous descriptions, especially considering that there is a high level of natural morphological and morphometric variation in myxospores both within and between hosts (Atkinson et al 2015)
Summary
The global aquarium market moves millions of ornamental fish worldwide and is the primary mode for international transport of cnidarian myxosporean parasites (Hallett et al 2015). The Amazon Basin is amongst the most important sources of wild-caught ornamental fishes in the international aquarium industry (Moreau and Coomes 2007), there are few surveys concerning cnidarian myxosporean infections in Amazonian ornamental fish (Mathews et al 2015, 2017, 2020a, b). The three recognised species of the discus genus Symphysodon Heckel, 1840, in the family Cichlidae, are popular, expensive and widely exploited ornamental fish (Bleher et al 2007). The red discus Symphysodon discus Heckel, 1840 inhabits lentic aquatic environments, such as floodplains and flooded forests in the lower Rio Negro, upper Uatumã, Unini, Nhamundá, Trombetas and Abacaxis Rivers in Brazil (Amado et al 2011)
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