Introduction

Knowledge of gammarid amphipod diversity in Colombia and in particular in the Colombian Pacific is scarce. Gammarids of Gorgona Island (2° 58’ 00” N and 78° 11’ 24” W), a protected area in the Colombian Pacific belonging to the Eastern Tropical Pacific Marine Corridor, have been mainly identified to family level (^{CORTÉS et al., 2012}; ^{VALENCIA et al., 2014}), and only a few studies have reached lower taxonomic levels (^{BARNARD, 1967a}; ^{VALENCIA et al., 2014}). We present a new record for the Colombian Pacific of the gammarid amphipod Hyachelia tortugae Barnard, 1967. The species was originally described for the Galapagos Islands and occurs exclusively as an epibiont of sea turtles (^{BARNARD, 1967b}). Hyachelia tortugae has also been reported for the Hawaiian Islands (^{BALAZS et al., 1987}), Palmyra Atoll (^{YABUT et al., 2014}), and the Pacific coast of Costa Rica (^{ROBINSON et al., 2016}); in all cases, it was found only on green turtles. This amphipod inhabits the buccal cavity of green turtles where it seems to feed on food residues (^{BARNARD, 1967b}), but it has also been recorded on the neck, skin, and skin lesions of turtles (^{AGUIRRE et al., 1998}).

The H. tortugae identified were collected as part of a study that aimed to characterize the intraspecific variation and diet of the green turtles of Gorgona Island (see^{SAMPSON et al., 2017}). Juveniles were manually captured at night over the reefs of La Azufrada and Playa Blanca, bimonthly from February to December 2012. Esophageal lavages were performed on each turtle and contents were preserved in 4% formaldehyde. Amphipods were sorted from the samples, identified, and deposited in the Marine Biology Collection at the Universidad del Valle, Cali, Colombia (CERBMcr-UV).

Of 77 lavaged green turtles, five contained amphipods, including H. tortugae and three species of hyperiids (Table 1). The H. tortugae were likely flushed out from the buccal cavity during the esophageal lavages carried out to analyze the diet of the turtles. Three H. tortugae individuals, one female (approx. 5 mm) and two males (approx. 6 mm) (Table 1), were found in two of the green turtles captured during February and March 2012 (Figure 1). The identified specimens fit the original description by ^{BARNARD (1967b)}. Hyachelia tortugae is a sexually dimorphic amphipod that presents short antennae, with the first one being shorter than the second one. The gnathopods of the males are enlarged and subchelate: gnathopod 1 presents a propodus with an oblique palm and a dactylus that does not fit the palm (Figure 2A); gnathopod 2 presents a stout propodus, longer than broad, and an oblique palm that has spines and two protuberances (Figure 2B). In the pereopods, the propodus forms a subquelate palm and has four stout spines (Figure 2C). Uropods 1 and 2 present large rami, in which the outer ramus is setose, whereas the inner ramus lacks setae (Figure 2D). Uropod 3 has a short peduncle and the telson is cleft.

Table 1 Amphipods collected in esophageal lavages of green turtles (Chelonia mydas) from Gorgona Island, Colombian Pacific. 

* SCL = straight carapace length.

Only two species from the Hyachelia genus have been described: H. tortugae and H. lowry ^{Serejo and Sittrop, 2009}, the latter from Australia. The main characteristics used to distinguish these two species were described by ^{SEREJO & SITTROP (2009)} and more recently by ^{YABUT et al. (2014)}. A characteristic of Hyachelia is that it is the only genus in the family Hyalidae known to present an epibiont lifestyle and to have, as is usual in other epibiont amphipods, pereopods that have been modified as grasping appendages (^{BARNARD, 1967b}) (Figure 1C). Although epibiosis, the settlement by organisms on living surfaces, is a common association in the marine realm due to the need of benthic organisms to find a substrate (^{WAHL, 2009}), the association between amphipods and vertebrates is less common and little is known about the nature of the relationship (^{MOORE, 1995}). In general, the substrate offered by the sea turtle carapace, skin tissue, and buccal cavity creates microenvironments that allow the presence of a great variety of fauna (^{FRICK & PFALLER, 2013}). Due to sea turtles’ characteristic long-range migrations, these types of associations may have important implications for the dispersion of marine invertebrates (^{FRICK & PFALLER, 2013}).

Figure 1 Hyachelia tortugae from Gorgona Island (CAN-UV 1756): a) female, b) male, c) male head. Photos by Ortega, J.F. & Valencia, B. Laboratorio Imágenes Postgrado Ciencias Biología Univalle. 

Figure 2 Hyachelia tortugae from Gorgona Island (male, CAN-UV 1755): a) gnathopod 1, b) gnathopod 2, c) dactylus of pereopod 7, d) uropods and telson. 

In addition to H. tortugae, the hyperiids Parascelus sp., Paralycaea gracilis Claus, 1879, and Schizoscelus ornatus Claus, 1879 were identified from the esophageal lavages of green turtles from Gorgona Island (Table 1). Hyperiids present symbiotic associations with gelatinous zooplankton (^{HARBISON et al., 1977}) and were likely ingested when the turtles were feeding on gelatinous organisms. ^{AMOROCHO & REINA (2007)} reported that tunicates (salps and doliolids) were the main dietary component of green turtles in Gorgona Island, which were also found in some of the turtles lavaged during this study (^{SAMPSON et al., 2017}). The three hyperiids identified in this study have been previously reported by ^{Valencia & Giraldo (2012)} in the area, where P. gracilis was one of the five most abundant species.

Considering that sea turtles use Gorgona Island as a foraging, resting, and/or nesting site (^{AMOROCHO & REINA, 2007}), characterizing the epibiont community associated with each species and understanding the nature of these associations could provide important insights on the habitat use and movements of the turtles, and could also serve as an indicator of their health status. Hence, systematic studies should be specifically designed for this purpose in the future to obtain a better description of the epibionts found on the sea turtles that occur around Gorgona Island and in the Colombian Pacific in general.

This research was conducted under permit PIDB DTPA 018-11 granted by Parques Nacionales Naturales de Colombia (PNN). LS received funding from the Natural Sciences and Engineering Research Council of Canada and a teaching assistantship from the Biology Department at the Universidad del Valle. Turtle surveys were carried out with support from the monitoring program of the Henry von Prahl scientific station of PNN Gorgona and was partially funded by Universidad del Valle, research project: “Variación intraespecífica y ecología trófica de Chelonia mydas en el Parque Nacional Natural Gorgona - CI7914”. Images of H. tortugae were taken at the laboratory of images of the graduate program in Biology, Universidad del Valle by Juan Felipe Ortega. We thank all individuals involved in data collection.