﻿Caribbean Amphipoda (Crustacea) of Panama. Part I: parvorder Oedicerotidira

﻿Abstract Amphipods in the parvorder Oedicerotidira are burrowers, furrowers, or surface skimmers. Members of the parvorder share a well-developed posteroventral lobe on coxa 4, an equilobate coxa 5, an immensely elongate pereopod 7 that differs in structure from pereopod 6, and an entire telson. Within the parvorder, only the family Oedicerotidae has been documented from Bocas del Toro, Panama, represented by two species. This research documents a range extension for Hartmanodesnyei (Shoemaker, 1933) and describes a new species of Synchelidium Sars, 1892. An identification key to the species of Caribbean Oedicerotidae of Panama is provided.

Defining characteristics of amphipods in the family Oedicerotidae include having a well-developed antenna 2, reaching at least half the length of antenna 1; a strong down-curved rostrum; well-developed dorsolateral eyes; coxae 1-3 well-developed, each longer than the previous coxa; a subchelate gnathopod 1; article 3 of gnathopod 2 less than 2 × as long as wide; and a distally attenuate pereopod 7 that is longer and more slender than pereopod 6. Amphipods in the family Oedicerotidae differ from those in the families Exoedicerotidae and Paracalliopidae in having separate urosomite segments and lacking the oblique setal row on the maxilla 2 inner plate (Lowry and Myers 2017). Most species of the parvorder burrow into sediment, but little else is known about their ecology.
Two species of oedicerotid amphipods were collected from Bocas del Toro, Panama, one of which is new to science. Both species are diagnosed and the new species is described herein, and an identification key is provided to distinguish between the three species known from the Caribbean waters of Panama.

Materials and methods
Amphipods were collected by hand using a Ziploc bag to scoop up fine sand from Crawl Cay, Bocas del Toro, Panama at depths of 1.5-5.0 m. The sand was elutriated with freshwater to remove amphipods. Live specimens were sorted to morphospecies, placed in clove oil for imaging, and preserved in 99.5% EtOH for later examination. Preserved specimens were transferred to glycerol, measured from the tip of the rostrum to the base of the telson, and dissected under a stereomicroscope. Specimens were illustrated using a Meiji MT5900L phase contrast microscope with an Olympus U-DA drawing tube attached. Illustrations were digitally inked following Coleman (2003) in Adobe Illustrator 2020 using a Wacom Intuos Pro Pen Tablet. Specimens are deposited in the Smithsonian Institution, U.S. National Museum of Natural History (USNM).  Diagnosis. Antenna 1 of female subequal to peduncle of antenna 2; antenna 2 of male much longer than that of female; head, anterodorsal angle broadly subquadrate. Pereopods 3 and 4 propodus subrectangular, dactylus elongate, slender; pereopods 5 and 6 dactylus elongate, subequal to propodus in length; pereopod 7 basis posterior margin with several short setae, carpus and propodus, posterior margin with several spine groups, dactylus elongate, slender.
Ecology. These amphipods burrow into sand in shallow subtidal habitats.
Male (paratype, 2.0 mm). Similar in all aspects to the female with the exception of the following: Eye slightly larger; antenna 2 flagellum elongate, at least 0.5 × body length (broken); gnathopods 1 and 2 bases slightly wider than in female.
Etymology. After the Latin purpur, meaning purple and vitellum, meaning yolk and referring to the striking purple color of the eggs in the brood pouch of females of this species.
Distribution. Panama: Bocas del Toro (present study). Ecology. These amphipods burrow into sand in shallow subtidal habitats. Remarks. Synchelidium purpurivitellum sp. nov. is similar to the geographically close species Americhelidum americanum in many aspects, but differs in having a strongly toothed gnathopod 1 propodal palm (smooth in A. americanum), a broadly rounded posteroventral corner of epimeron 2 (produced in A. americanum), and lacking the posterodistal lobe found on the basis of pereopod 7 in A. americanum.
Within the genus, Synchelidium purpurivitellum sp. nov. is most similar to Synchelidium maculatum Stebbing, 1906, sharing the long gnathopod 1 carpus and toothed

Discussion
The results of this study increase the number of Caribbean oedicerotid amphipods known from Panama to three, with the documentation of a new species and a range extension to include Panama for H. nyei. Hartmanodes nyei has previously been reported from the western Atlantic Ocean, including the Caribbean Sea (Shoemaker 1933;Rakocinski et al. 1993;Thomas 1993;LeCroy 2000), with two questionable records in the Pacific Gulf of California (Barnard 1962;Bousfield and Chevrier 1996). The genus Synchelidium now contains eight species worldwide, with Synchelidium purpurivitellum sp. nov. being the first Caribbean species of the genus. Americhelidium americanum was reported from the Caribbean as Synchelidium americanum before the closely related genus Americhelidum Bousfield & Chevrier, 1996 was erected (Thomas 1993). Bousfield and Chevrier (1996) designated Americhelidium as a North Pacific coastal marine genus, including A. americanum, and Synchelidium as a European Atlantic genus. Despite this designation, Synchelidium purpurivitellum sp. nov. is placed in the genus based on the strongly toothed gnathopod 1 propodus, broadly rounded epimeron 2, and the lack of a posterodistal lobe on the basis of pereopod 7, all of which are the diagnostic characters selected by Bousfield and Chevrier (1996) to distinguish this genus from Americhelidium. It is likely that the genera have a wider distribution, but have yet to be documented properly, given the similarities among the species, which would explain its presence in Caribbean Panama. Future investigation of additional material, including type material will aid in understanding the relationships between species of the Americhelidium and Synchelidium. Documenting these sand burrowing species from Bocas del Toro may allow their inclusion in future applied studies, including studies on trophic interactions, habitat-use, and population or community analyses.

Funding
Funding for this study was provided by a National Science Foundation grant: Collaborative Research: ARTS: Understanding Tropical Invertebrate Diversity Through Integrative Revisionary Systematics and Training (1856421). Publication costs were provided by the Georgia College & State University GC Journeys Program.