First record of Apocorophium acutum ( Chevreux , 1908 ) ( Amphipoda , Corophiidae , Corophiinae ) from Uruguay , with notes on the biology and distribution

The amphipod Apocorophium acutum (Chevreux, 1908) has a worldwide distribution due to dispersion by ballast water and the hulls of ships. Here we provide a record of this species from Rocha department, Uruguay, which is the first record in the Atlantic South American coast. This record is 5,400 km from the nearest previously known record in Venezuela. Images and morphological characteristics are provided to distinguish from other species of Corophiidae previously recorded in the country.


Introduction
Species of the genus Apocorophium (Bousfield & Hoover, 1997) are mostly tube builders, inhabiting U-shaped tubes and are selective deposit feeders, feeding on bacteria, algae and diatoms adsorbed on the surface of the sediment particles.Apocorophium acutum (Chevreux, 1908) inhabits coastal environments of tropical and subtropical climate, in mud bottoms, on artificial hard substrates (Winfield et al. 2007(Winfield et al. , 2011) ) and on hydrozoans (Gavira-O'Neil et al. 2015), sponges (Winfield et al. 2011) and algae (Zakhama-Sraieb et al. 2006).This species is possibly gregarious and presents parental care of its offspring (Winfield et al. 2011).They are detritivores and also carnivores, preying on interstitial organisms.They have a longevity that varies between 0.5 and 1.5 years (Winfield et al. 2011).
This species has been classified as invasive in Mexico due to the negative effects they might have on native invertebrates, reducing their richness and abundance and altering ecological processes related to food chains (Win-field et al. 2011).This species is also classified as invasive in Brunei (Hossain and Hughes 2016).Currently invasive potential of this species in Uruguay is unknown.The aim of this paper is to report the occurrence of A. acutum for the first time in Uruguay, with description, images and illustrations, referring morphological variation.

Methods
The specimens of Apocorophium acutum recorded from Uruguay were collected during a study of crustacean communities on artificial substrates at the town of La Paloma, Rocha department, Uruguay, in the subtidal coastal zone at a depth of 1.5 m (Fig. 1).Samples were taken bimonthly from March 2013 to March 2014 with 6 artificial substrates by removing the community on the surfaces using quadrants and scraping.The specimens were fixed in 4% formaldehyde, preserved in 70% alcohol, and examined using a Premiere BM 2100 stereomicroscope.A total of 336 females of A. acutum was found, and 10 of them were dissected and drawn.The material was deposited in the Entomological collection of Facultad de Ciencias, Universidad de la República (FCC).The distribution map was made using a SimpleMappr (Shorthouse 2010).The determination of the specimens was done following the keys of Bousfield and Hoover (1997), LeCroy (2004) and Jung and Kim (2007).
Description.Female.Body short and broad, 2.00-2.50mm long.Rostrum short, not exceeding or slightly exceeding lateral ridge of head (Fig. 2), lateral ridge produced.
Gnathopod 1 (Fig. 3A), coxa oval, basis nearly equal in length to carpus, gradually increasing in width apically, posterior margin rounded, ischium short, expanded anterodistally, posterodistal margin surrounded by several long setae, merus as long as ischium, posterodistal end with 1 simple setae, carpus slightly shorter than basis, anterior margin rounded, anterodistal margin with one simple seta, posterior margin with 2 longitudinal rows of plumose setae, propodus slender, slightly shorter than carpus, posterior margin with several setae, palm slightly oblique, slightly rounded posteriorly, with several robust setae, dactylus slightly extending beyond palm, with a weak posterior marginal tooth.

Discussion
The examined specimens coincided with previous descriptions except for the followings differences: female: antenna 1 with less separation between the 2 robust setae in the inner margin of proximal end than description of Bousfield (1997) and Jung and Kim (2007) and less setose than description of LeCroy (2004).Antenna 2 more setose and with 2 robust setae in ventral margin instead of 3 described in Bousfield (1997), LeCroy (2004) and Jung and Kim (2007).Pereopod 3 merus with 6 setae in the anterior margin instead of 3 described in Jung and Kim (2007).Telson slightly more rounded than description of Bousfield (1997) and Jung and Kim (2007).The morphological differences found in this study could be attributed to local variations, as noted by Jung and Kim (2007) for specimens found in Korea.
The presence of the species on artificial substrates in the subtidal area agree with the observed in other parts of  (Chevreux, 1908).Female head (body length: 2.30 mm), La Paloma locality, Rocha, Uruguay (FCC 400).Antenna 1 peduncular segment 1 with 2 distinctive robust setae near proximal end of inner margin (at arrow).(Chevreux, 1908).Female (body length: 2.30 mm), La Paloma, Rocha, Uruguay (FCC400).the world (Winfield et al. 2011).It is highly probable that the introduction of this species in Uruguay was due to hulls and ballast water of ships, as observed for other regions, as there is an international port at La Paloma (Demicheli and Scarabino, 2006) and a major commercial port in Montevideo, the capital of Uruguay (Muniz et al. 2004).The amphipod Monocorophium insidiosum (Crawford, 1937), previously recorded for Uruguay, is very similar to A. acutum, and its co-ocurrence probably accounted for the overlooking of A. acutum.The record of A. acutum from Uruguayan coast, around 5,400 km of distance from the nearest register represent a significant range extension Southward for this species, and his detection and monitoring is relevant due to his invasive potential, previously mentioned for other parts of his distribution area (Winfield et al. 2011, Hossain andHughes 2016).