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Stirring, charging, and picking: hunting tactics of potamotrygonid rays in the
upper Paraná River
Domingos Garrone-Neto1 and Ivan Sazima2
Hunting tactics of potamotrygonid freshwater rays remain unreported under natural conditions. Three main foraging tactics of
Potamotrygon falkneri and P. motoro are described here based on underwater observations in the upper Paraná River. Both
species displayed similar behaviors. The most common tactic was to undulate the disc margins close to, or on, the bottom and
thus stirring the substrate and uncovering hidden preys. Another tactic was to charge upon prey concentrated in the shallows.
The least common tactic was to pick out prey adhered to the substrate. The first tactic is widespread in several species of
marine rays in the Dasyatidae, whereas the remainder (especially picking up prey on substrata above water surface) may be
restricted to the Potamotrygonidae.
As táticas de caça de raias potamotrigonídeas permanecem sem registro sob condições naturais. Três táticas de forrageamento
são aqui descritas para Potamotrygon falkneri e P. motoro, com base em observações subaquáticas no curso superior do rio
Paraná. Ambas as espécies apresentaram comportamento semelhante. A tática mais comum foi a de ondular as margens do
disco próximo ao, ou no, fundo e assim perturbando o substrato e revelando presas abrigadas. Outra tática foi a de investir
sobre presas concentradas no raso. A tática menos frequente foi a de apanhar presas aderidas ao substrato. A primeira tática
é comum em diversas espécies de raias marinhas da família Dasyatidae, ao passo que as outras duas (em particular apanhar
presas em substratos acima da superfície da água) podem estar restritas a Potamotrygonidae.
Key words: Potamotrygonidae, Potamotrygon falkneri, Potamotrygon motoro, Variable hunting tactics, Southwestern Brazil.
1Departamento de Zoologia, Caixa Postal 510, Universidade Estadual Paulista, 18618-000 Botucatu, São Paulo, Brazil.
garroneneto@yahoo.com
2Museu de Zoologia, Caixa Postal 6109, Universidade Estadual de Campinas, 13083-970 Campinas, São Paulo, Brazil. Retired and
associated as voluntary researcher. isazima@gmail.com
Freshwater rays of the Potamotrygonidae are restricted
to South America, where they occur in all main river systems
(Rosa, 1985). These rays are often caught for ornamental trade,
and occasionally used as food by riverine people (Castex,
1963; Rosa, 1985; Charvet-Almeida et al., 2002). As any other
elasmobranch, potamotrygonids are carnivorous and prey
on a variety of animals, from aquatic insects and crustaceans
to fishes, feeding behavior of theses rays being inferred from
gut analyses (Castex, 1963; Achenbach & Achenbach, 1976;
Lasso et al., 1996; Lonardoni et al., 2006; Garrone Neto et al.,
2007; Silva & Uieda, 2007).
We describe here the variable hunting tactics of two
species of Potamotrygon based on records made under
natural conditions in the upper Paraná River, and compare
these to hunting tactics of some species of marine rays.
The study area is in the vicinity of Castilho and Três
Lagoas (about 20°47’S, 51°37’W) on the border of São Paulo
and Mato Grosso do Sul States, Southwestern Brazil. We
recorded the behavior of 132 individuals identified in the field
as Potamotrygon falkneri (n= 62) and Potamotrygon motoro
(n= 70). Hunting tactics were recorded underwater while
snorkeling or scuba-diving (Sabino, 1999) at day and night
hours during the dry season from June to October in 2006
and 2007. A total of 112 h of underwater observation was
made through the study, most of them with snorkeling (97 h
at depths of 0.5 to 12 m) and the remainder time with scubadiving
(15 h at depths of 8-18 m). “Ad libitum” and “behaviour”
sampling rules (Martin & Bateson, 1986) were used throughout
114 Hunting tactics of potamotrygonid rays
the observational sessions, mostly recorded on a plastic slate.
Nocturnal observations were made with use of waterproof
spotlights covered with red plastic filter or indirect
illumination, both of which apparently disturb the fishes less
than yellow light or direct illumination (pers. obs.).
Additionally, digital photographs and video-records were
taken to check visual observations. Two specimens of each
of the two field-identified species, as well as images and
extracts from video-tapes are stored in the Fish Collection of
the Museu de Zoologia “Prof. Dr. Adão José Cardoso” (ZUEC
6331 – P. falkneri and 6332 – P. motoro) at the Universidade
Estadual de Campinas. A picture of P. orbignyi from another
locality is used here to demonstrate an unusual hunting tactic
we recorded for the two species described in this paper.
Both species displayed similar hunting tactics. The most
common one (n= 113 records) was a ray to hover close to the
bottom or settle on it, and undulate the disc margins (Figs. 1a
and 2a). The undulating movement stirred the substrate and
uncovered hidden small invertebrates (mostly larval insects,
snails, and crabs), apparently trapped under the ray’s disc.
The body of the ray assumed a convex shape and expansions
and retractions of the orobranchial chamber were clearly
visible, along with sediment being expelled through the gill
slits and spiracle. This behavior was recorded both during
the day (n= 35) and at night (n= 78) for rays ranging about 15-
70 cm in disc diameter and in depths ranging 0.2-16 m. Another
tactic was recorded while a ray slowly approached the
shallows and charged at prey concentrated there (Fig. 2b).
The prey likely was trapped or stunned under the ray’s disc.
Prey types obtained during this hunting behavior were mostly
freshwater shrimps (Palaemonidae) and small fishes (mostly
Characiformes). The charging behavior was recorded at night
only (n= 13) for rays ranging about 25-40 cm in disc diameter
and in depths ranging 0.2-0.5 m. The third tactic we recorded
was while a ray approached vertical or inclined surfaces such
as tree stumps, boulders, and concrete slabs and walls. The
ray often exposed the anterior part of the disc above the
water surface and picked out prey adhered to the substrate
(Figs. 1b and 2c). Prey types obtained this way were mostly
snails (Ampulariidae and Hydrobiidae). This behavior was
recorded during the day only, and was the least common (n=
6) for rays ranging about 30-40 cm in disc diameter and in
depths ranging 0.4-2.2 m. We suppose that larger individuals
would need to exert greater effort to maintain the vertical
position needed to pick prey with use of this technique.
Additionally, larger individuals gradually include small fishes
in their diet (Silva & Uieda, 2007, our pers. obs.).
The first hunting tactic, namely “undulate disc and stir
substrate”, here recorded for Potamotrygon falkneri and P.
motoro seems a preying mode widespread in marine species
of the Dasyatidae such as Dasyatis americana (Aguiar, 2005;
Sazima et al., 2007), D. centroura (I. Sazima, pers. obs.), D.
crysonota (Ebert & Cowley, 2003), and Taeniura lymma
(Fishelson, 1977). This hunting tactic is likely found in most if
not all dasyatid rays while foraging over unconsolidated
substrata. A similar hunting tactic seems to be used by some
species of myliobatid rays such as Rhinoptera bonasus (Smith
& Merriner, 1985), although the also myliobatid Aetobatus
narinari mostly uses its spade-like snout to dig out snail and
other preys while hovering close to sandy substrate (see an
instructive picture in DeLoach, 1999).
The two remainder hunting tactics, namely “charging in
the shallows” and “picking up prey on substrata above water
surface” (this latter a variant of “picking up prey on vertical
and inclined substrata”) seems restricted to the
Fig. 1. Two hunting behaviors of potamotrygonid rays.
Potamotrygon falkneri undulating its disc close to the
bottom, stirring the substrate and uncovering hidden prey
(a), and Potamotrygon orbignyi approaching a tree stump to
pick snails adhered above water surface (not visible on the
photograph) (b). The former species is from the study area
mentioned in this paper, whereas the latter species was
observed in the Maranhão River in Goiás State, Central Brazil.
D. Garrone-Neto & I. Sazima 115
Potamotrygonidae among Myliobatoidei as far as we are
aware. The versatile hunting behavior reported here for the
two species of Potamotrygon likely contributed to their recent
and successful colonization of new areas and habitats in the
upper Paraná River (see Garrone-Neto et al., 2007 and
references therein).
Acknowledgments
We greatly thank to Cláudia Eiko Yoshida and Paulo José
Pyles Cicchi for valuable help during fieldwork and the
photographs; Ottilie Carolina Forster e Laura Furlan Luvisoto
for help during fieldwork at the Três Lagoas area; Marcos
Teixeira da Silveira, diving fisherman of the Colônia Z-3/MS
gave us valuable information about the rays and the study
sites; logistic support at the Universidade Federal de Mato
Grosso do Sul - Campus Três Lagoas was possible through the
kindness of Maria José Alencar Vilela; part of financial support
went through the kindness of Virgínia Sanches Uieda (PROAPCAPES/
UNESP); both authors are grantees of the CNPq.
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Fig. 2. Hunting tactics of Potamotrygon falkneri and Potamotrygon motoro at the Paraná River. Settling close to the bottom,
undulating the disc to stir the substrate, and engulfing uncovered prey trapped under the disc (a); approaching the shallows,
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Accepted November 2008
Published March 31, 2009