Pelagic amphipods (Crustacea, Amphipoda, Hyperiidea) from the southern Gulf of Mexico with notes on the distribution of species

Abstract Background Studies referring the amphipod diversity have been mainly focused on the benthic environment. This study aimed to analye the epipelagic amphipod fauna composition in a sector of the southern Gulf of Mexico (GoM). Previous records in the Gulf mainly comprised the oceanic province; our dataset included both oceanic and neritic zones, off several fluvial and lagoon systems. The biological material comprised 485 data records and a total abundance of 3,802 individuals. New information Surveys were conducted at 21 sampling stations around the Veracruz Reef System National Marine Park, a marine protected area in the southern GoM. As a result of this research, we found 16 families, 34 genera and 78 species belonging to the suborder Hyperiidea. Our records include species from the oceanic province (up to 1,200 m depth), such as those from the genus Scinia, members of the infraorder Physosomata. In addition, Lycaeopsiszamboangae were found off the Alvarado Lagoon. Information on the habitat of 78 amphipod species (neritic, oceanic) is provided. The dataset is available at https://www.gbif.org/dataset/af18f3f8-f899-4c97-af47-8a110f856f92


Introduction
Pelagic amphipods encompass a large number of crustacean species from the suborders Amphilochidea, Hyperiidea and Senticaudata, found worldwide. In tropical waters, however, researchers are still finding new records of species (Violante-Huerta et al. 2020, Domínguez-Nava et al. 2021, Violante-Huerta et al. 2021a suggesting the need to improve the knowledge of their distribution around the world. Amongst the pelagic amphipods, the Hyperiidea is the dominant suborder with 292 species described in the world (Burridge et al. 2017). They exhibit greater diversity in the oceanic zone (Bowman and Gruner 1973, Lorz and Peracy 1975, Violante-Huerta 2019, yet various studies show that their abundance is higher in the neritic zone (Gasca 2004, Velázquez-Ramírez 2021. In the vertical plane, they occur from surface to abyssal depths, even in the hadal zone (Vinogradov et al. 1996, Vinogradov 1999. Hyperiids are carnivores and feed on other zooplankton organisms, such as polychaetes, chaetognaths, copepods, small crustaceans and even other amphipods (Bowman 1978, Williams and Robins 1981, Zeidler 1984. Furthermore, several species have been seen in association with gelatinous zooplankton, such as medusae, siphonophores and salps (Harbison et al. 1977, Laval 1980. In the Gulf of Mexico (GoM), 17 families of the Hyperiidea suborder and a total of 119 species have been recorded , LeCroy et al. 2009, Violante-Huerta 2019, Hereu et al. 2020. Hyperiid surveys in the GoM have mostly addressed the oceanic area and refer to new records of species, descriptions of intraspecific morphological variability and/or the analysis of some ecological features (Gasca 2003, Gasca 2004, Violante-Huerta 2019, Hereu et al. 2020, Violante-Huerta et al. 2021b). This study explores the composition of the hyperiid amphipod community in a sector of the southern Gulf that includes neritic and oceanic waters. This area is influenced by the discharge of freshwater outflows and contains the Veracruz Reef System National Marine Park, a protected natural area.

Project description
Title: Pelagic amphipods (Crustacea: Amphipoda: Hyperiidea) from the southern Gulf of Mexico with notes on the distribution of species.

Study area description:
The study area, located in the southern GoM, is included in Marine Ecoregion 14 of North America (Wilkinson et al. 2009). The sampling grid comprised neritic and oceanic zones (Fig. 1). The inner neritic zone is influenced by the discharges of several freshwater outflows, such as the Papaloapan River and Alvarado Lagoon. Surface circulation in the neritic zone exhibits a seasonal variation depending on the wind stress: during the autumn-winter period, circulation is towards the southeast, whereas during the spring-summer, surface waters flow towards the northwest (Zavala-Hidalgo et al. 2003). In the oceanic zone, the circulation pattern is cyclonic throughout the year (Pérez-Brunius et al. 2013, Sanvicente-Añorve et al. 2014.

Sampling methods
Description: The samples were obtained in the GoM between 18.84622 to 19.63814 North latitude and 95.28207 to 96.27735 West longitude, off the Mexican State of Veracruz. This area encompasses oceanic and neritic waters and contains the Veracruz Reef System National Marine Park, a protected natural area. The area is influenced by the discharge of the rivers Actopan, La Antigua, Jamapa, Papaloapan and the Alvarado Lagoon.
Sampling description: Zooplankton samples were taken aboard the vessel "Justo Sierra", during the oceanographic cruise named SAV-I, carried out from 29 November to 2 December 2007. The sampling grid comprised 21 neritic and oceanic oceanographic stations organised in six transects perpendicular to the coastline. Sampling was performed using a Bongo net of 333 and 505 µm mesh size; a flowmeter was placed at the mouth of each net to estimate the volume of filtered water. The net was towed for about 7 to 27 minutes (depending on bottom depth) at a speed of 2-3 knots. Collected samples were fixed in a 4% formaldehyde seawater solution buffered with sodium borate. Sampling depth ranged between 10 to 200 metres, depending on bottom depth.

Quality control:
Positions of sampling stations were georeferenced and displayed on a map using the Google Maps platform. The name of each species was verified using the taxon match tool of WoRMS (World Register of Marine Species).
Step description: Amphipods were sorted from the samples collected with the 505 µm mesh size and preserved in 70% ethanol. The amphipods were identified, based on specialised literature (Stebbing 1888, Shih 1991, Vinogradov et al. 1996, Vinogradov 1999, Zeidler 1990, Zeidler 1992, Zeidler 1999, Zeidler 2000, Zeidler 2003, Zeidler 2004a, Zeidler 2004b, Zeidler 2016 and classified in accordance with Lowry and Myers (2017). Microdissections of taxonomically important structures (such as antennae, gnathopods, pereopods and uropods) were performed under the stereoscopic microscope to identify the hyperiids. The biological material is kept in the Laboratorio de Ecología de Sistemas Pelágicos of the Instituto de Ciencias del Mar y Limnología, Universidad Nacional Autónoma de México.

Geographic coverage
Description: The study area comprised neritic and oceanic waters off the Mexican State of Veracruz, in the southern GoM. Several freshwater outflows influence the area: the rivers Actopan, La Antigua, Jamapa, Papaloapan and the Alvarado Lagoon. A marine protected area, Veracruz Reef System, exists within the explored area ( Fig. 1

Taxonomic coverage
Description: A total of 3,893 amphipods of the suborder Hyperiidea were sorted from the samples, from which 3,802 were identified to a species level. The identified individuals belonged to two infraorders, 16 families, 34 genera and 78 species (Tables 1, 2). Classification was after Lowry and Myers (2017 General taxonomic counts of hyperiid species. Members of the Physocephalata infraorder, accounting for 99.4% of the total abundance, inhabit the epi-and mesopelagic layers and some species are considered characteristic of the tropical epipelagic fauna (Vinogradov et al. 1996, Vinogradov 1999. In contrast, species of the Physosomata infraorder (0.6%) are rare organisms that inhabit deep waters, but some species are frequently found in the upper epipelagic layer (Vinogradov et al. 1996, Gasca 2009). In this study, the Physosomata infraorder was represented by only one family, 10 species and 21 individuals, mostly found in the oceanic zone ( Table 2).
The dominant species was Lestrigonus bengalensis, representing 67.7% (2,575 ind.) of all the identified hyperiids. It was found in all the surveyed sites, with the highest abundance in the neritic zone. This species has a circumtropical distribution in both neritic and oceanic waters, with its major abundance in shallow waters (Bowman 1973, Thurston 1976, Siegel-Causey 1982, Vinogradov et al. 1996. Only seven additional species recorded an abundance percentage higher than 1%: L. macrophthalmus (163 ind.), Lycaeopsis zamboangae (115 ind.), Eupronoe intermedia (77 ind.), Tetrathyrus forcipatus (63 ind.), Anchylomera blossevillei (61 ind.), Brachyscelus globiceps (55 ind.) and Phronimopsis spinifera (54 ind.). Except for the latter species, all these species were found in both oceanic and neritic areas (Table 2). In contrast, P. spinifera showed a clear tendency to occupy the oceanic zone. In particular, L. zamboangae was distributed in neritic and oceanic waters in front of the Alvarado Lagoon; previous studies indicated that the species has an affinity for surface waters of tropical and temperate regions (Siegel-Causey 1982, Zeidler 2004b).
Of the 78 species here recorded, only four were solely found in the neritic zone with one or two individuals: Amphithyrus sculpturatus, A. muratus, Schizoscelus ornatus and Scina lepisma. These species have been recorded in low numbers in oceanic areas (Gasca and Shih 2001, Gasca 2009, Violante-Huerta 2019, Hereu et al. 2020); thus, their presence in the neritic zone is not indicative of their main habitat.

Temporal coverage
Notes: From 29 November to 2 December 2007. decimalLatitude

Collection data
The geographic latitude (in decimal degrees, using the spatial reference system given in geodeticDatum) of the geographic centre of a Location. decimalLongitude The geographic longitude (in decimal degrees, using the spatial reference system given in geodeticDatum) of the geographic centre of a Location.

locality
The original textual description of the place.

country
The name of the country or major administrative unit in which the Location occurs.
countryCode The standard code for the country in which the Location occurs.
geodeticDatum The ellipsoid, geodetic datum or spatial reference system (SRS) upon which the geographic coordinates given in decimalLatitude and decimalLongitude are based.
coordinateUncertaintyInMetres The horizontal distance (in metres) from the given decimalLatitude and decimalLongitude describing the smallest circle containing the whole of the namePublishedIn The four-digit year in which the scientificName was published. kingdom The full scientific name of the kingdom in which the taxon is classified. phylum The full scientific name of the phylum or division in which the taxon is classified. class The full scientific name of the class in which the taxon is classified. order The full scientific name of the order in which the taxon is classified. family The full scientific name of the family in which the taxon is classified. genus The full scientific name of the genus in which the taxon is classified. specificEpithet The name of the first or species epithet of the scientificName. taxonRank The taxonomic rank of the most specific name in the scientificName.
occurrenceStatus A statement about the presence or absence of a Taxon at a Location. basisOfRecord The specific nature of the data record.