The Grijalva and Usumacinta rivers, the most important rivers in southern Mexico, begin their course in Guatemala and flow through the states of Chiapas and Tabasco (^{Pease et al., 2012}). In the Mexican side of this basin, ^{Miller (2005)} documented the presence of 115 fish species, 41 out of which are endemic. There are also two species of aquatic mammals, the Neotropical river otter (Lontra longicaudis, Olfers 1818) and the Antillean manatee (Trichechus manatus manatus, Linnaeus 1758), considered near-threatened and endangered, respectively, by the IUCN (^{Self-Sullivan & Mignucci-Giannoni, 2008}; ^{Rheingantz & Trinca, 2015}).

Over the last 15 years, Mexican freshwater ecosystems have been affected by the presence of the invasive armoured suckermouth catfish (Pterygoplichthys pardalis, Castelnau 1855 and another species of the genus) (^{Mendoza et al., 2007}). This species is tolerant to extreme changes of temperature, flow, salinity and quality of water (^{Capps et al., 2011}; ^{Bijukumar et al., 2015}). Additionally, its early reproduction, nest construction, parental care, high fertility rate (800 to 1500 eggs per female) and larval survival have facilitated its invasion in water bodies from all over the world (^{Tello et al., 1992}; ^{Hoover et al., 2004}).

Armoured catfish interactions with other species include its predators (^{Power, 1984}; ^{Willard, 1985}; ^{Bistoni et al., 1995}; ^{Aguilar & Di-Bernardo, 2004}; ^{Kasper et al., 2008}; ^{Bonino et al., 2009}; ^{Borteiro et al., 2009}; ^{Cabral et al., 2010}; ^{Ríos-Muñoz, 2015}) and the Florida manatee (T. m. latirostris) (^{Nico et al., 2009}). This last interaction is controversial, because it is not clear whether it is beneficial or harmful to manatee’s health. ^{Nico et al. (2009)} mentioned that catfish attached to manatees to feed on the epibiota that live on their skin and searching for protection from its predators. In Florida, no injured manatees had been observed due to catfish attachment and in surveys analysing catfish gut contents, no epibiont (e.g., copepods, nematodes, ostracods) was found only algae (^{Gibbs et al., 2010}). However, when manatees have a catfish attached, they have been observed agitated, irritated, discomforted and attempting to dislodge catfish; these reactions have been also observed when larger jacks and Archosargus probatocephalus, Walbaum 1792, peck the manatees (^{Nico et al., 2009}; ^{Gibbs et al., 2010}). This suggests that some structures of fish oral apparatus (teeth) are harming manatees.

Manatees in southern Mexico are considered abundant (^{Colmenero & Hoz, 1986}; ^{Álvarez et al.,1988}), but in the last two decades, orphaned and injured animals have been rescued and transferred to different institutions (e.g., zoos, aquariums, universities, wildlife rescue centres) (^{Ortega-Argueta & Castelblanco-Martínez, 2018}) of the states of Chiapas and Tabasco to provide them with veterinary clinical assistance. In most of the facilities where these animals are housed, the water used to fill the pools comes from the hydrological network of the lower basin of the Grijalva-Usumacinta, which increases the frequency and intensity of the contact among catfish and manatees.

Three adult female manatees are kept in an artificial lagoon (AL) (6824 m² total area) in Palenque, Chiapas, Mexico (17º 29’10’’ N, 92º 01’10’’ W). This lagoon is connected to a management pool (MP) (8.40 m wide, 7.20 m long and 2.10 m deep), where veterinarians inspect animals and keepers conduct training sessions. The water filling these artificial water bodies is turbid and comes from a nearby lagoon (1 km away). Two of the manatees arrived in August 2011 from an aquarium from Veracruz, Mexico. One of them has a total length (TL) of 2.75 m (13 years old), weighing 650 kg and the other has a TL of 2.87 m, and weighs 700 kg (14 years old). The third one (TL=3.28 m, unknown age and weighs 850 kg) was rescued from the wild in May 2013 from nearby Catazajá wetlands (Figure 1A). Upon its arrival, none of the manatees presented wounds, just the wild one presented a buoyancy problem derived from ingestion of plastic garbage. Since their arrival, the females that came from the aquarium lived in the MP, due to the difficulty to handle them in the AL. On the other hand, the wild female spent a few weeks in the MP while recovering from the buoyancy problem; after the veterinarians discharged her, she was released to the AL.

Figure 1 Same female manatee on three different dates: A) May 2013 the day she arrived to the facilities without wounds, B) December 2013 she presented wounds through all the dorsal region, C) January 2017 the wounds totally disappeared, D-E) Rounded and irregular cutaneous lesions presented through all the dorsal area. Photo A courtesy of María Silva. 

In December 2013, as part of a routinely veterinary examination, the water of the MP was removed and while examining the manatees’ skin we observed rounded and irregular lesions through the dorsal region (head, thorax, pectoral flippers, abdomen, fluke) (Figure 1B, D-E); no other clinical sign was observed. Cutaneous lesions of the three manatees were similar in appearance (width from 0.4 to 2.1 cm; length from 0.5 to 2.3 cm), irregular, sometimes well defined, with an abnormal thickening of the skin showing a rough surface and a light grey to off-white coloration. Occasionally the lesions showed very prominent borders and a whitish depressed centre (crater-like). These lesions were interpreted as ulcers in healing process. No systemic or topical medication was administered to the individuals. During each examination, we observed that wounds were healing slowly from the periphery to the centre. This type of healing process has been observed previously by one of the authors in other injured manatees from the Mexican Caribbean (^{Pérez-Flores et al., 2010}). Finally, in January 2017, we performed the last clinical evaluation and observed a total recovery of cutaneous wounds (Figure 1C).

After inspecting the MP carefully, we didn’t find any object that could cause these kinds of injuries, except for the presence of 120 suckermouth catfish. Therefore, in May 2014 we decided to establish a protocol to reduce the population of catfish in AL and MP, and to monitor the manatees’ skin healing process. First, we removed the water from the MP every 20 days to take out catfish and track the healing process of the wounds. Second, periodic extraction of catfish from the AL was performed by trawling three consecutive days every six months for two and a half years. Since we performed the first trawling, we decided to release the two manatees held in the MP into the AL to avoid contact between the manatees and the catfish.

The eradication of catfish from the MP was achieved in five months by removing catfish every time we emptied the MP (every 20 days) and closing the gates that connect with the AL. Meanwhile, hundreds of catfish were removed from the AL since the first trawling; however, complete eradication was not possible due to the characteristics of the lagoon.

It was previously assumed that the interaction between P. pardalis and manatees was not harmful for the latter (^{Nico et al., 2009}; ^{Gibbs et al., 2010}). However, ^{Williams (2005)} and ^{Nico et al. (2009)} mentioned that catfish sometimes bite the skin of manatees, causing them to squirm and roll, flip the fluke, increase vocalizations, and rub their body against some superficies (e.g., branches, logs, stream bottom). Despite the fact that manatee skin is thick, we believe that this kind of behaviour is displayed as a consequence of pain or discomfort caused by the scrapping-feeding movements of P. pardalis (^{Adriaens et al., 2009}). Catfish present a row of rake-like teeth, which they use to scrape up to one and a half meters deep in different kind of substrates they have invaded (^{Mendoza et al., 2007}; ^{Adriaens et al., 2009}).

The parts of the body where we observed the wounds (head, snout, dorsal and ventral regions, and caudal fluke) are the same parts where catfish are usually found attached (^{Nico et al., 2009}). Furthermore, the length of Pterygoplichthys sp. found in the MP (28.0 to 44.6 cm) coincides with the length (30-40 cm) of catfish attached to manatees in Florida (^{Nico et al., 2009}). ^{Williams (2005)} and ^{Nico et al. (2009)} observed up to 40 catfish attached to a single manatee for a period of four minutes; however, it only takes one armoured catfish attached to a manatee to provoke a change in latter’s behaviour (^{Gibbs et al., 2010}). Due to low visibility, we only observed catfish attached (a maximum of 2 minutes) to the manatees during the training sessions, but when manatees swam on the surface most of the fish detached.

We hypothesize that two factors induced catfish to injure manatees. First, the long period of time spent by manatees in the MP. The MP is a small concrete reduced space where manatees cannot escape from catfish; this could increase the frequency and intensity of the attachment to the skin causing the observed wounds. Second, walls and floor of the MP were constantly cleaned, preventing the accumulation of organic particles; thus, the only source of food for Pterygoplichthys were the algae present in manatees’ body, and once the algae were removed, they probably began to scrape manatees’ skin.

We assumed that the main factor that contributes to heal the wounds of the manatees was the declining and eradication of catfish population from AL and MP respectively. This process must be carried out through the periodic extraction followed by killing, due to the rapid rate of colonisation of this species (^{Schmitter-Soto et al., 2015}).

This interaction has probably been carried out since catfish invaded the fluvial-lagoon systems of Chiapas and Tabasco; however, the turbidity of the water makes it difficult to document it. The reduction of size of water bodies during the dry season could favour the contact between manatees and catfish; this might be harmful for manatees’ health, since it is speculated that dermal scavenging is associated with the papilloma virus (^{Nico et al., 2009}). For these reasons, it is necessary to create and implement strategies to control or eradicate armoured catfish populations, since there is evidence that this species is increasingly abundant and could be a potential threat to the long-term population viability of the endangered Antillean manatee in the South of Mexico.