INTRODUCTION

Hagfishes comprise a primitive group of jaw less, cartilaginous and anguilliform, strictly ma rine, benthic fishes with a global distribution (^{Martini, 1998}). They have important roles in the functioning of benthic ecosystems; as hag fishes are fossorial scavengers and/or detriti vores, the effect of excavation and feeding is as sumed to significantly impact substrate turnover and nutrient cycling (Martini, 1998; ^{Knapp et al., 2011}).

All 78 described species of hagfishes are cur rently grouped into a single family (Myxinidae), which is divided into three subfamilies (Rubicundinae, Eptatretinae and Myxininae), and six genera (Rubicundus^{Fernholm, 2013}; Eptatretus^{Cloquet, 1819}; Myxine^{Linnaeus, 1758}; Notomyxine^{Nani & Gneri, 1951}; Neomyxine^{Richardson, 1953}; and Nemamyxine Richardson, 1958) (Fernholm et al., 2013). In the southern region of South America (Argentina and Chile), there are records of 14 species of hagfishes, com prising the second most diverse area for this group in the world. However, it is the area with the greatest gaps of biological information on these species (^{Knapp et al., 2011}). The Argentine shelf and continental slope are the most diverse region of hagfishes in the southwestern Atlantic (Knapp et al., 2011) with seven species: Myxine affinis^{Günther, 1870}; Myxine australis^{Jenyns, 1842}; Myxine debueni^{Wisner & McMillan, 1995}; Myxine fernholmi Wisner & McMillan, 1995; Myxine knappi Wisner & McMillan, 1995; Nemamyxine kreffti McMillan & Wisner, 1982; and Notomyxine tridentiger (^{Garman, 1899}) (^{Norman, 1937}; Nani & Gneri, 1951; Wisner & McMillan, 1995; ^{Góngora et al., 2009}; Knapp et al., 2011; ^{Bovcon et al., 2013}; ^{Figueroa, 2019}; ^{Cousseau et al., 2020}; ^{Delpiani et al., 2020}; ^{Mabragaña & Cousseau, 2021}). However, in central Patagonia, only two species have been recorded to date, M. australis and N. tridentiger (Góngora et al., 2009; Bovcon et al., 2013).

According to ^{Mincarone (2013}a), the known distribution of M. affinis includes the southern most coasts of Argentina and Chile, in the Straits of Magellan, Beagle Channel and other channel systems around Tierra del Fuego, including Isla de Los Estados and Cape Horn. Catches of M. af finis are associated with muddy and sandy bot toms between 3 and 146 m depth (Mincarone, 2013a), and recently was recorded in Burdwood Bank (^{Delpiani et al., 2020}). Information about M. knappi is limited. So far, only 11 specimens are known to be preserved in collections (Mincarone, 2013b). Its distribution comprises the southern end of the Argentine continental shelf, includ ing Cape Horn, Malvinas Islands, and Burdwood Bank, approximately between latitudes 48°S and 55°S (Mincarone, 2013b). This study aims to re port two new records of M. affinis and one of M. knappi in the waters of the San Jorge gulf (central Patagonia, Argentina), extending the knowl edge of their distributions northward by 850 km and 220 km, respectively.

MATERIAL AND METHODS

The records reported here come from two sources: i) The Fishery Observer Program of Fisheries Secretariat of the Chubut Province (POBCh, spanish acronyms of Programa de Observadores a Bordo de la Provincia del Chubut) for the monitoring of the fishing fleet target ing the Patagonian shrimp Pleoticus muelleri^{Bate, 1888}, which in 2010 collected an inciden tally caught specimen of M. affinis (46°52’10” S, 66°12’46” W; 58 m depth; UNPSJB-ICT-2010/33), and ii) a research cruise of the “Pampa Azul” ini tiative conducted by the Oceanographic vessel ARA Puerto Deseado, which in 2016 captured the second specimen of M. affinis (46°38’24” S, 65°47’29” W; 65 m depth; UNPSJB-ICT-2016/15; Fig. 1A) and a specimen of M. knappi (45°51’33” S, 65°05’27” W; 82 m depth; UNPSJB-ICT-2016/16; Fig. 1B). All catches were done with bottom trawl nets.

Fig. 1 Specimens deposited in the UNPSJB-ICT collection: (A) Myxine affinis UNPJSB-ICT 2016/15, 358 mm and (B) Myxine knappi UNPSJB-ICT 2016/16, 303 mm. Scales = 20 mm. 

Specimens were identified to the specific level according to the characters proposed by ^{Wisner & McMillan (1995}) and ^{Figueroa (2019}), and they were deposited in the ichthyological collection of the Instituto de Investigación de Hidrobiología of the Facultad de Ciencias Naturales y Ciencias de la Salud of the Universidad Nacional de la Patagonia San Juan Bosco in Trelew city (UNPSJB-ICT). As diagnostic characteristics of the species M. affinis, the following were con sidered: multicusps of two cusps on anterior and posterior rows, a total of cusps ranging between 38 and 46, absence of gill pores, and a narrow whitish band ventrally limited to below lateral slime pores. As diagnostic characteristics of the species M. knappi, the following were considered: multicusps of two cusps on anterior and posteri or rows, a total of cusps ranging between 34 and 40, absence of gill pores, absence of the ventral whitish band, and a tail length between 12 and 13 percentage of total length. The measurement of the morphometric and meristic variables fol lowed ^{Fernholm & Hubbs (1981}), McMillan & Wisner (1984), and Wisner & McMillan (1995). Morphometric variables and weights were re corded with a caliper of 0.02 mm and a digital scale of 0.01 g accuracy, respectively.

RESULTS

Both specimens of M. affinis identified pre sented 42 dental cusps and narrow whitish band ventrally limited to below the line of lateral pores (Fig. 2A). The first specimen of M. affinis, collected in 2010, a hermaphrodite with 190 oo cytes (size < 1 mm), 61.22 mm long testis and the height of the ventral finfold was 2.12 mm (see Tab. 1). The second specimen of M. affinis was a female, collected in 2016, with 12 oocytes (size < 0.5 mm) (see Tab. 1), and the ventral fin fold height was 0.67 mm. The unique specimen of M. knappi collected in 2016 had 35 total dental cusps, absence of the ventral whitish band (Fig. 2B), and a tail length of 13.2% of TL (see Tab. 1). The ventral finfold height was 2.54 mm. Sex was not determined in this specimen because it was deposited intact in the UNPSJB-ICT collection. The morphometric and meristic variables re corded for M. affinis and M. knappi (Tab. 1), are within the ranges reported by ^{Mincarone (2007}) for the species.

Fig. 2 Ventral view (A) Myxine affinis UNPJSB-ICT 2016/15, narrow whitish band ventrally limited to below the lateral pore line, and (B) Myxine knappi UNPSJB-ICT 2016/16, absence of the ventral whitish band. Scales = 20 mm. 

Table 1 Morphometric and meristic measurements of the specimens of Myxine affinis and Myxine knappi. 

DISCUSSION

In recent decades, the known distribution of some species of hagfishes have expanded north ward in the Southwestern Atlantic, as is the case of M. australis in the southern Brazilian waters (^{Mincarone & Soto, 2001}) and N. tridentiger in waters of Uruguay (^{Racz Lorenz et al., 2014}). These records may be attributed to two factors: (i) an expansion of this cold-water group north ward; or (ii) an increase in the scientific interest in this group, which made possible the detection of specimens in sites where the species are rare because they are at the limit of their distribution. In this paper we report new records of M. affinis and M. knappi in waters of the San Jorge gulf, be tween Dos Bahías cape (44°55’ S, 65°32’ W) and Tres Puntas cape (47°06’ S, 65°52’ W), in central Patagonia, Argentina (Fig. 3). This extends the knowledge of the distribution of M. affinis from its known distribution in the Straits of Magellan to the waters of San Jorge gulf, approximately 850 km and 7° northward. Similarly, the distribu tion of M. knappi, is extended by approximately 220 km and 2° in the same direction, from 48° S to the mouth of the San Jorge gulf. The present study added two new species to the fish diversity in the waters of central Patagonia. It is impor tant to note that the finding of M. knappi is the twelfth historical record for the species.

Fig. 3 Known distribution for Myxine affinis (red area) and Myxine knappi (green area) according to ^{Mincarone (2013}a,b). New records of M. affinis (red point) and M. knappi (green point). 

The records presented here are located in the San Jorge gulf, a semi-enclosed basin of approxi mately 230 km latitudinal opening and 150 km longitudinal width, with an area of 40000 km², being the largest gulf of the Argentinean coast (^{Dans et al., 2021}). The gulf is influenced by the presence of two frontal systems, the Northern Patagonian Frontal System seasonally influ ences the northern region, whilst the southern region is characterized by the permanent pres ence of the Southern Patagonian Frontal System (^{Sabatini & Martos, 2002}; ^{Bogazzi et al., 2005}). Both frontal systems are areas of high primary productivity, constituting sites with high concen tration of marine organisms (^{Yorio, 2009}). One of the main environmental factors limiting the distribution of hagfish species is the low tempera ture (^{Martini, 1998}). The records of the presence of M. affinis and M. knappi in the waters of San Jorge gulf may be associated with the permanent occurrence of the Southern Patagonia Frontal System, which extends from south to north from the Strait of Magellan, with the influence of cold water masses, to the southern end of the San Jorge gulf and the central outer area of its mouth (Bogazzi et al., 2005).

Studies of hagfishes in Argentinean waters are scarce; there are only two published works, ^{Nani & Gneri (1951}) and ^{Wisner & McMillan (1995}), both of great relevance for the study of these fishes at the global level. Despite the scarce scientific interest in the group, in northern and central Patagonia, hagfishes are part of the diet of top predators, like the sea lion Otaria flave scens Shaw, 1800 (^{Koen Alonso et al., 2000}), and the shark Notorynchus cepedianus (^{Péron, 1807}) (^{Crespi-Abril et al., 2003}). In this same area, N. tridentiger and M. australis are identified in the bycatch of the Patagonian shrimp P. muelleri and common hake Merluccius hubbsi^{Marini, 1933} fisheries (^{Góngora et al., 2009}; ^{Bovcon et al., 2013}), with mean Frequencies of Occurrence (FO) of 0.06% and 0.28%, respectively (Góngora et al., 2009). The low FO in the San Jorge gulf could be due to the low vulnerability of these species to the bottom trawl nets. According to ^{Martini (1998}), hagfishes have a fossorial life style, so the use of bottom trawls could gener ate underestimates of abundance. In addition, ^{Gorbman et al., (1990}) point out that identify ing hagfishes at the specific level is a challenging task. Similarly, the observers of the POBCh, ex pressed this difficulty in identifying species due to the great mobility they have and the mucus they secrete when handled. In addition, as the presence of M. affinis and M. knappi in central Patagonia was unknown until now, observers were not instructed in their identification and distinguishing features. Based on the aforemen tioned: i) underestimates of abundance by gear selectivity, ii) difficulty in identification on board and iii) unknown presence of the species in the area, it is likely that M. affinis and M. knappi could have a higher abundance and are not spe cies of exceptional occurrence in the area.