MARINE AND FISHERY SCIENCES 34 (2): 269-274 (2021)

https://doi.org/10.47193/mafis.3422021010604

ABSTRACT. Seahorses (Genus Hippocampus) are rarely reported as prey items. Their low pop-

ulation numbers, their ability to blend with the environment and their low palatability are the fea-

tures making them an unsuitable option for predators. Five individuals of the Patagonian seahorse

Hippocampus patagonicus were found in the stomach of a single narrownose smooth-hound shark

Mustelus schmitti from a sample of 959 stomachs analyzed. This rare record in a single stomach out

of hundreds analyzed suggests that these individuals might have clustered in a group which was

detected by the opportunistic M. schmitti.

Key words: Crypsis, diet, ‘El Rincón’, Syngnathidae, Triakidae.

Registro inusual de depredación múltiple del caballito de mar patagónico Hippocampus pata-

gonicus por el tiburón gatuzo Mustelus schmitti en aguas costeras argentinas

RESUMEN. Los caballitos de mar (Género Hippocampus) raramente son reportados como pre-

sas. Su baja densidad poblacional, su habilidad para confundirse entre su entorno y su baja palata-

bilidad son características que los hacen una opción poco atractiva para depredadores. Cinco indi-

viduos del caballito de mar patagónico Hippocampus patagonicus fueron encontrados en un único

estómago del análisis de una muestra de 959 estómagos del tiburón gatuzo Mustelus schmitti. Este

inusual registro en un solo estómago de los cientos analizados sugiere que los caballitos de mar se

encontraban agrupados cuando fueron detectados por el tiburón de hábitos oportunistas M. schmitti.

Palabras clave: Cripsis, dieta, “El Rincón”, Syngnathidae, Triakidae.

Sharks often play a role in the transfer of energy between trophic levels

within marine ecosystems. The diet composition of sharks has been studied

in order to understand the natural history of a particular species, their role in

marine ecosystems, the impact of shark predation on economically valuable

or endangered prey, among other reasons (Wetherbee and Cortés 2004).

269

*Correspondence:

diegoluzzatto@conicet.gov.ar

Received: 9 December 2020

Accepted: 9 February 2021

ISSN 2683-7595 (print)

ISSN 2683-7951 (online)

https://ojs.inidep.edu.ar

Journal of the Instituto Nacional de

Investigación y Desarrollo Pesquero

(INIDEP)

This work is licensed under a Creative

Commons Attribution-

NonCommercial-ShareAlike 4.0

International License

Marine and

Fishery Sciences

MAFIS

NOTE

Unusual record of a multiple predation of the Patagonian seahorse

Hippocampus patagonicus by the Narrownose smooth-hound Mustelus

schmitti in Argentine coastal waters

MAURO BELLEGGIA1, 2, 3, MARÍA G. PUJOL4, MARÍA L. ESTALLES5, DANIEL E. FIGUEROA2, 6 and

DIEGO C. LUZZATTO6, *

1Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP), Paseo Victoria Ocampo Nº 1, Escollera Norte, B7602HSA - Mar del

Plata, Argentina. 2Laboratorio de Biología de Peces, Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata

(UNMdP), Funes 3350, B7602AYL - Mar del Plata, Argentina. 3Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET),

Argentina. 4Museo Municipal de Ciencias Naturales “Lorenzo Scaglia”, Libertad 3099, B7600 - Mar del Plata, Argentina. 5Instituto de

Formación Docente Continua - San Antonio Oeste, Islas Malvinas 1190, R8520 - San Antonio Oeste, Argentina. 6Instituto de Investigaciones

Marinas y Costeras (IIMyC-CONICET), Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Mar del Plata (UNMdP),

Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina

On their side, seahorses are rarely found as

prey items (Kleiber et al. 2011). Morphological

and ecological characteristics might contribute to

predator avoidance. These species exhibit a

remarkable capacity to be cryptic, they can

change their coloration pattern and some species

have long skin filaments or encrusting organisms

on the skin enabling them to blend with their

environments. They can remain virtually immo-

bile for a long time and occur at low densities,

which further complicates their localization (Fos-

ter and Vincent 2004). Their bony plates, spines

and low energetic value make them an unpalat-

able option for predators. In addition, in all the

cases recording seahorses as prey items, the pred-

ator has opportunistic and generalist habits

(Kleiber et al. 2011).

The Narrownose smooth-hound Mustelus

schmitti (Springer, 1939) is an opportunistic and

generalist benthic mesopredator. Its diet varies

along geographical locations and ontogeny and it

is mainly composed of fishes, crabs, isopods,

polychaetes, and mollusks (Belleggia et al. 2012).

The geographical range of this shark largely over-

laps with that of the Patagonian seahorse Hip-

pocampus patagonicus Piacentino and Luzzatto

2004 (Figure 1 A). Both species inhabit the coastal

areas of the Southwestern Atlantic Ocean extend-

ing from Rio de Janeiro (22° S, Brazil). However,

their southern limit differs. While M. schmitti

extends as south as Ria Puerto Deseado (47° 45′ S,

Argentina) mainly at depths below 140 m (Chiara-

monte and Pettovello 2000), H. patagonicus

extends up to Puerto Madryn (42° 47′ S, Argenti-

na) and has been described as occupying patches

in restricted areas at low depths mainly <20 m

(Piacentino and Luzzatto 2004; Luzzatto et al.

2012; Silveira et al. 2014).

The Narrownose smooth-hound is the shark

most commonly caught by Argentinean fleets

(Sánchez et al. 2012). Heavy exploitation over its

entire geographical range (including nursery

areas), its decreasing biomass, declines in land-

ings, and reduction of size at first breeding have

all led to this species being considered Critical

Endangered by the International Union for Con-

servation of Nature (IUCN) Red List (Pollom et

al. 2020). The Patagonian seahorse is also a

threatened species and it is catalogued as Vulner-

able due to habitat degradation and overexploita-

tion (Wei et al. 2017). The present study reports

and describes a rare record of a multiple preda-

tion of the seahorse H. patagonicus by the shark

M. schmitti.

Sharks analyzed were collected during six bot-

tom-trawl research cruises conducted by the

research vessels ‘Doctor Eduardo L. Holmberg’,

‘Capitán Cánepa’, and ‘Capitán Oca Balda’ as

part of a study led by the Instituto Nacional de

Investigación y Desarrollo Pesquero (INIDEP)

during 2008-2011 (Belleggia 2012). These

research cruises were performed using a random

stratified sampling scheme (stratified on latitude

and depth; Figure 1 B). Surveys have been regu-

larly conducted by INIDEP in order to estimate

abundance. Fishing was conducted during day-

light hours (07:00-19:00), at 3 to 4 knots for 30

min at each sampling site using an Engel bottom

trawl (200 mm mesh in the wings, 103 mm in the

cod end, 4 m vertical opening and 15 m horizon-

tal opening). As part of a trophic ecology study,

we undertook research into the diet of M. schmitti

by sampling 959 individuals. Specimens of M.

schmitti were sexed and total length (TL) was

measured to the nearest mm from tip of the snout

to the posterior end of the dorsal caudal lobe in its

natural position. Maturity stage was recorded

according to Walker (2005) and Colonello et al.

(2011). Stomachs were excised, frozen and ana-

lyzed at the laboratory.

Prey items were identified to the lowest possi-

ble taxonomic level, counted and weighed to the

nearest 0.01 g. Diet composition was quantified

for each prey by calculating the percentage fre-

quency of occurrence (%F, total number of stom-

achs in which a given prey was found expressed

as percentage of total number of stomachs with

food), the percentage by number (%N, total num-

270 MARINE AND FISHERY SCIENCES 34 (2): 269-274 (2021)

ber of a given prey as percentage of total number

of prey found), percentage by mass (%M, mass of

a given prey as percentage of total mass of prey

found) and index of relative importance: IRI =

%F ×(%N + %M) (Pinkas et al. 1971), expressed

as a percentage (%IRI; Cortés 1997).

A M. schmitti mature male of 652 mm TL and

1,052 g weight caught on research cruise ‘Capitán

Oca Balda’ in ‘El Rincón’ area (40.139° S-

61.436° W; 20 m depth) in April 2011 (Figure 1

B) contained five individuals of H. patagonicus

in different stages of digestion in its stomach.

One of the seahorses remained almost intact,

allowing several morphological measurements

(Figure 2 A). Another individual was partially

digested and some measurements were performed

(Figure 2 B). Both individuals were deposited in

the Syngnathid Collection of the Museum of Nat-

ural History ‘Lorenzo Scaglia’ (MMPEAA)

(Table 1). The other three individuals were

inferred by fragments of their tails and thoraxes.

No morphological measurements could be per-

formed on them.

Out of 959 specimens of M. schmitti analyzed

and 944 (98.43%) containing prey items, only

one contained seahorses. According to the index-

es estimated to quantify the diet composition of

M. schmitti, percentage of frequency of occur-

rence (%F) of seahorses in the diet was 0.01, per-

centage by number (%N) was 0.05, percentage by

weight (%W) was 0.02, and percentage of index

of relative importance (%IRI) was <0.001.

Together with the five seahorses identified, this

stomach also contained other frequent prey items

for M. schmitti: shrimp Artemesia longinaris

(Bate, 1888) (0.04 g), brachyuran crabs Armases

rubripes Rathbun, 1897 (0.13 g) and Collodes

rostratus Edwards, 1878 (1.31 g, 14 mm), and

seven polychaetes of the families Ampharetidae

(0.33 g) and Onuphidae (0.21 g).

271

BELLEGGIA ET AL.: PREDATION OF THE PATAGONIAN SEAHORSE

Figure 1. A) Geographical range of Mustelus schmitti (blue) and Hippocampus patagonicus (red) in the Southwestern Atlantic.

Green square represents the study area. B) Study area indicating the position of bottom trawl samplings per date: October

2008 open dots, November 2008 bold dots, May 2009 open squares, September 2010 bold squares, April 2011 open tri-

angles, June 2011 bold triangles. Red asterisk indicates the sampling site (40.139° S- 61.436° W; 20 m depth) where the

specimen of M. schmitti presenting five individuals of H. patagonicus in its stomach was recorded.

Argentina

Uruguay

62° 61° 60° 59° 58° 57° 56° 55° 54° 53° 52° 51°

50 m 100 m 200 m

42°

41°

40°

39°

38°

37°

36°

35°

Atlantic Ocean

Southwest Atlantic

The record of patches of H. patagonicus at

shallow and intermediate depths (0-20 m) along

the coastal area of north Patagonia and Buenos

Aires Province and its record at deep waters (60

m) in the Uruguayan and Argentine Common

Fishing Zone (Luzzatto et al. 2012, 2014) indicat-

ed the presence of seahorses over a large depth

range. Juvenile specimens of H. patagonicus

drifting attached to floating debris or swimming

freely may interconnect the patches and facilitate

their ability to disperse to deep waters (Luzzatto

et al. 2013). The record presented here together

with that provided by Estalles et al. (2017) near

Monte Hermoso allow to infer the presence of the

species in an extended coastal area (Figure 1 B).

As it was indicated above, M. schmitti is an

opportunistic mesopredator shark. Its teeth are

similar in shape, arranged in semi-pavement like

crushing plates dentition, which makes it possible

for them to eat crabs, gastropods, bivalves and

other species protected by hard carapaces (Be-

lleggia et al. 2014). Bony plates, spines and other

morphological features of seahorses do not seem

to make them an unpalatable option for the shark.

However, this finding is the first record of sea-

horses in the diet composition of a Mustelus spp.

The predation event reported here is in line

with the general findings of seahorses as prey

items: their presence in the diet composition of

marine predators is infrequent and mainly con-

sisted of a single record. The presence of more

than one seahorse individual in a single stomach

is even rarer (Kleiber et al. 2011). The presence

of five seahorses in a single stomach out of hun-

272 MARINE AND FISHERY SCIENCES 34 (2): 269-274 (2021)

Figure 2. Measurable individuals of Hippocampus patagonicus obtained from the stomach of Mustelus schmitti. MMPEAA 36

(A) and MMPEAA 49 (B). Scale bar indicates 2 cm.

dreds analyzed suggests that these individuals

might have clustered in a group that was detected

by the opportunistic M. schmitti.

INIDEP contribution no 2240.

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