We documented the first evidence on how suckermouth armored catfishes have evolved in phenotypic divergences when inhabit lotic and limnic habitats in Vietnam. Indeed, all growth and reproductive results indicate that Pterygoplichthys spp. in lotic had faster growth, higher fecundity, and longevity; these results were consistent for all sampling months throughout a year with a large number of specimens. Furthermore, these populations are within the same climate region, suggesting that they may experience the same climatic conditions (Table 1). The substantial phenotypic divergences of fish in lotic and limnic habitats may directly relate to hydrology, water flow (Table 1), food availability and predators. In the following paragraphs, we will discuss in details how Pterygoplichthys spp. have evolved in phenotypic divergences in growth and reproduction, which are critical for their invasion in lotic and limnic habitats in Vietnam.
Table 1
Parameters of sampling locations
Parameters | Dinh River | Suoi Trau Reservoir |
Geographic Coordinates (N, E) | 12° 29.740' 109° 7.686' | 12° 30.302' 109° 2.694' |
Type | lotic | limnic |
Length (km) | 53 | - |
Square - Volume | 916 km2 | (1.42–9.8) x 106 m3 |
Altitude (m) | 2 | 22 |
Depth (m) | 3–4 | 4–9 |
Water speed (m/s) | 0.1–0.7 | 0 |
Temperature (o C) | 25.4–32.9 | 25.4–32.9 |
pH | 6.75–7.58 | 7.09–8.5 |
Salinity (‰) | 0.04–0.05 | 0.0 |
TDS, ppm | 48–60 | 71–100 |
Total P (µg/l) | 45.9–75.9 | 49.7–65.3 |
Total N (µg/l) | 769–985 | 826–869 |
Atmospheric pressure (mmHg) | 757–758 | 760–761 |
Both males and females of Pterygoplichthys spp. collected from the Dinh River (lotic habitat) and Suoi Trau Reservoir (limnic habitats) in this study had negative allometric growth (b = 2.66–2.89), indicating that these fish were likely to be slimmer when they grow bigger. This finding was consistent with the growth pattern of P. pardalis from Langat river, Malaysia (b = 2.538 ± 0.039) (Samat et al. 2008) and Pterygoplichthys spp. from six water bodies in Guangdong Province, China (b = 2.515–2.973) (Wei et al. 2017).
Another important line of evidence for the strong phenotypic divergence in Pterygoplichthys spp. are the size, maximal size and age of both males and females of from Dinh River (lotic habitat) were about 45–50% longer than those from Suoi Trau Reservoir (limnic habitat). Indeed, the maximum length and lifespan of Pterygoplichthys spp. from Dinh River were 50 cm and 4.2 years old, and those from Suoi Trau were 34.6 cm and 2.9 years old. The differences between the two types of water bodies (Table 1) may result in variation in food, dissolved oxygen, territory, or other interspecific competition and subsequently affect the size of the fish. In general, Pterrygoplicthys spp. have rapid growth (10 cm/year) and a relatively short lifespan (ca. 5 years) (Gibbs et al. 2013; Gibbs et al. 2017). The lifespan of Pterrygoplicthys spp. in both Dinh river and Suoi Trau Reservoir were less than the estimated lifetime of this species in Volusia Blue Spring, Florida, USA (5.25 years) (Gibbs et al. 2013, Gibbs et al. 2017) whereas they had a similar length (L∞ = 50–52 cm). The shorter lifetime may relate to the higher average temperature in Vietnamese habitats than in Floridan habitats, as lifetime and temperature typically correlate negatively (Truong et al. 2020).
In this study, six stages of gonadal development of Pterygoplichthys caught from Dinh River and Suoi Trau Reservoir were described and similar to Jumawan and Herrera (2014). However, the weight of stage I gonad in Pterygoplichthys spp. in this study was 10-fold smaller than that of fish collected at Marikina river, Philippines (Jumawan and Herrera 2014), which may relate to the difference in size at maturity. Furthermore, this fish species can alter its reproductive strategies (either annual single spawner or annually repeated spawner) and its GSI index in adults varied greatly, which may explain the difference.
Interestingly, the size at 50% maturity of females of Pterygoplichthys spp. in Suoi Trau Reservoir and Dinh River was 13 cm Ls, 15.91 cm Ls, respectively; which is much smaller than in their native habitats. In South America, the native Loricariids began to breed at a size of 25 cm, unknown Ls or Lt (Mendoza 2009). In North America, the 50 percent sexual maturity of P. disjunctivus was 19.2 cm Ls in El Infiernillo Reservoir, Mexico (Rueda-Jasso et al. 2013) and 26 cm Ls in Volusia Blue Spring Florida, USA (Gibbs et al. 2008; Gibbs et al. 2017). In Asia, the 50 percent sexual maturity was 24 cm Lt of P. disjunctivus in East Kolkata Wetlands, Indian (Suresh et al. 2019) and Taiwan was 25 cm Ls of P. multiradiatus from Kaoping river (Liang et al. 2005), and was 26 cm Ls of P. disjunctivus in Marikina river, Philippines (Jumawan and Herrera 2014). In our study, the size at first maturation of fish is smaller than fish in native and other studies (similar to P. pardalis in Peninsular river, Malaysia, 13 cm Ls, Samat et al. 2016). The smaller size at maturity may be a response to the exploitation pressure, removing the majority of large-size, matured animals from the populations (de Roos el al. 2006). This is unlikely the case for suckermouth armored catfishes as they are not the primary target for fish exploitation. More likely, the smaller size at maturation is one of the universal patterns of ectothermic animals in response to elevated temperature (Daufresne et al. 2009; Cheung et al. 2013); the development outpaces the growth when temperature increases until the upper thermal optimum (Forster et al. 2011). Corroborating to this, the average water temperature in both Dinh River and Suoi Trau Reservoir varies between 25.4–32.9 oC, higher than in Florida, U.S.A., around 23 oC (Gibbs et al. 2017). The smaller size at maturity of Vietnamese animals in reponse to elevated temperature has been observed in aquatic invertebrates of the same location (Doan et al. 2019), but none have documented the evidence for fish in Vietnam.
In this study, Pterygoplichthys was found to breed all year round, as indicated by the monthly changes in GSI and the percentage of fish with matured stages. The main reproductive season would be from April to October, with a peak in July-August as indicated by the highest GSI values of fish collected from both Dinh River and Suoi Trau Reservoir during this period. This finding was consistent with a previous study which reported that Pterygoplichthys in Dinh River was fertile from October to December (GSI: 6.59–15.77%) and recovered in January (Zworykin and Budaev 2012). These results also agree with the observation of the main breeding season of Pterygoplichthys in the summer regardless of invading habitats worldwide. For example, in Mexico and the United States of America Pterygoplichthys reproduces in summer from May to October or November (Rueda-Jasso et al. 2013; Gibbs el al. 2008; Gibbs et al. 2017). P. multiradiatus in Kaoping River, Taiwan, breed from March to September with a peak from June to August (Liang et al. 2005). In other geographical areas with only two seasons (rainy and dry seasons), the main reproductive season of Pterygoplichthys occurs in the rainy season. For example, in Malaysia, the P. pardalis reproduce from September to January (Samat et al. 2016), the P. disjunctivus from June to September in The Philippines (Jumawan and Herrera 2014) and from July to November in India (Suresh et al. 2019).
P. disjunctivus could change their reproductive strategy to adapt to the local environment (Gibbs et al. 2017). The reproductive strategy of Pterygoplichthys spp. from southern Vietnam was diverse. Pterygoplichthys spp. can be the single spawner with only one time reproduction per year as indicated by the index Fb/Ft = 100% or multiple spawners which could reproduce several times a year as indicated by Fb/Ft index varying from 7.9% to nearly 100% (Table 4). In India and The Philippines, P. disjunctivus are multiple spawners only (Suresh et al. 2019; Jumawan and Herrera 2014). The alteration in their reproductive strategies may be interpreted as an adaptation to variations in environmental, biological or geographical conditions (Gibbs et al. 2017), which need to be clarified in freshwater habitats in southern Vietnam.
One important finding of this study was that the absolute fecundity of Pterygoplichys spp. was five times higher in Dinh River than in Suoi Trau Reservoir, although the distance between the two sampling sites is only 8 km, and with the same weather conditions. The difference in absolute fecundity of Pterygoplichys spp. was associated with the larger body size of fish collected from these two habitats, as confirmed by the positive correlations between these two parameters (Fig. 9). Indeed, the difference was no longer present when the fecundity was normalized for the body size as there was no significant difference in relative fecundity of Pterygoplichys spp. from lotic and limnic habitats. Furthermore, the type of water bodies or water quality parameters (Table 1) may also contribute to this difference. The relative fecundity of Pterygoplichys spp. in this study was comparable to P. disjunctivus from El Infiernillo Reservoir, Mexico (RFt = 12.5 eggs/g) (Rueda et al. 2013) and East Kolkata Wetlands, Indian (RFt = 8–22 egg/g) (Suresh et al. 2019).
The average diameter of the top size class eggs in Pterygoplichys spp. from Dinh River was moderately larger (~ 13%) than that of fish from Suoi Trau Reservoir, which may relate to the adaptive responses to different habitat conditions. Generally, the smaller eggs size in reservoirs may result from unfavourable environmental conditions such as high water temperature variations, higher water level changes and fluctuations, food shortages accompanied by a long dry season. Indeed, the size of fish from Dinh River were larger than in Suoi Trau Reservoir (Table 3) and fish obtained from Dinh River had a higher abundance of food in the digestive tract (benthic, planktonic, amphibiontic, terrestrial food items) than in Suoi Trau Reservoir (only benthic, planktonic) (Stolbunov and Gusakov et al. 2021). This leads to the larger diameter of the top size class eggs in Dinh River than in Suoi Trau Reservoir. Other studies in lotic habitat such as: in Volusia Blue Spring, Florida, U.S.A (latitude ~ 28º N), average diameter top decile class size of period 2005–2007 was 3.44 mm, period 2012–2014 was 3.01 mm (Gibbs et al. 2008; Gibbs et al. 2017) and the diameter of the top size class eggs was 3–4 mm for fish from Marikina River, The Philippines (latitude ~ 14° N) (Jumawan and Herrera 2014) are larger than the top decile class size in Dinh River, Viet Nam (latitude ~ 12° N). It seems that, the size of the egg smaller toward the lower latitudes which needs further investigations.
Overall, the study results showed that the growth, lifespan, fecundity and egg diameter of suckermouth-amored catfishes in limnic showed smaller values than in the lotic habitat, suggesting a strong phenotypic diversgence. A generally short lifespan of Pterygoplichthys spp. together with the small size at 50% maturation, high fecundity, being single and multiple spawners and ready to reproduce at any months of the years, all of these important characteristics suggest that Pterygoplichthys spp. are highly opportunistic to any local conditions and habitats in Vietnam where they invade. It is still unknown whether these phenotypic divergences have a genetic basis, which may be revealed by a common-garden experiment. Irrespective of the mechanisms, our results are the first and shed light on how Pterygoplichthys spp. are thriving in typical freshwater habitats in Vietnam.