Occurrence of Lophogaster spinosus Ortmann, 1906 (Crustacea, Lophogastrida) in the Gulf of Cadiz (NE Atlantic)

Occurrence of Lophogaster spinosus Ortmann, 1906 (Crustacea, Lophogastrida) in the Gulf of Cadiz (NE Atlantic).— The occurrence of the lophogastrid crustacean Lophogaster spinosus is reported for the Gulf of Cadiz waters, in the North–East Atlantic. This is the first report of the species for the Iberian Atlantic region. Samples were collected by demersal trawling during a fisheries research survey performed in March 2008. A total of four specimens were collected at three sampling sites. Depths of occurrence ranged between 363 and 548 m.


Introduction
Knowledge of lophogastrid crustaceans is relatively scarce, especially when compared to other more diverse and abundant crustacean groups, such as Decapoda, Copepoda or even the phylogenetically close group, Mysida. The previous Mysidacea are nowadays split in two orders, the Lophogastrida and the Mysida (Martin & Davis, 2001;Meland & Willassen, 2007;Wittmann & Ariani, 2010). A total of 51 living species, belonging to eight genera, are currently known within Lophogastrida (Anderson, 2010). The most diverse genus is Lophogaster, that has a total of 20 species. Most Lophogaster species are pelagic or bathypelagic, while some can be considered epibenthic, showing daily vertical migrations, such as L. typicus M. Sars, 1856 or L. eurylepis Bamber & Clark, 2004(Kaartvedt, 1989Bamber & Clark, 2004;Cartes et al., 2011).
We report the occurrence of the lophogastrid crustacean Lophogaster spinosus Ortmann, 1906 in the Gulf of Cadiz, the first report of the species for the Iberian Atlantic region.

Material examined
Samples were obtained during a demersal fisheries research survey (ARSA_0308) run by the Instituto Español de Oceanografía (IEO) in March 2008 on board the R/V 'Cornide de Saavedra', in the Gulf of Cadiz (SW Iberian Peninsula, NE Atlantic Ocean) ( fig. 1). The aim of the survey was to obtain estimates of density, biomass, size, and age structure of the main demersal species of interest to fisheries, and to gather information on the ecosystem structure and dynamics, and environmental data. A demersal trawl gear was used (Silva et al., 2011). All hauls were performed during daytime hours. All morphometric measurements on the specimens were taken with a precision of 0.01 mm. Carapace length (CL) was taken as the reference size (as in shrimp-like decapod crustaceans) given the higher precision that a measurement can be taken on this hard structure versus other measures used previously, such as total length (that includes pleon and telson). CL was measured from the anterior orbital edge to the laterodorsal posterior carapace edge.

Results
The specimens were identified as Lophogaster spinosus Ortmann, 1906 (fig. 2) based on the original description of the species (Ortmann, 1906) and on information provided by Fage (1942) and Tattersall (1951Tattersall ( , 1955. The species occurred in three samples (table 1), with a total of four individuals. The four specimens have been deposited in the Biological Collections of Reference (CBR) of the Institut de Ciències del Mar-CSIC (accession numbers: ICMM_20110705-01 to -04).
Morphological and meristic characteristics of the sampled specimens are provided in table 2. The carapace length of these specimens ranged between 8.08 and 9.77 mm in males, while the only female caught measured 10.95 mm CL.
The diagnostic characters identified are as follows: long rostral spine, extending beyond antennal scale; posterolateral angles of the carapace produced into a long spine reaching the end of abdominal segments 2 (males) or 3 (female); carapace smooth; elongate antennal scale, more so in the female (length/width ratio: 3.8) than in the males (ratio: 2.6-3.3);  Number of spinules between telson apical spines n.a. n.a. n.a. 7 (broken) (broken) (broken) 8-10 teeth on the outer margin of the antennal scales, in addition to the terminal spine; one strong spine on each posterolateral angle of the tergum of the last abdominal somite; lateral margins of the telson with 6-8 spines including the apical spines; 7 spinules between the telson apical spines.

Discussion
The known distribution of L. spinosus comprises the eastern and western North Atlantic between latitudes 40°N and 30°S with an apparent gap in equatorial waters (Tattersall, 1955;Wittmann et al., 2004; Southampton Oceanography Center Discovery Collections Midwater Database, on line). The closest known reports of the species are from deep waters off Portugal at approximately latitude 40°N, longitude 15°W (Southampton Oceanography Center Discovery Collections Midwater Database, on line). The species has been recently reported from the Canary Islands (Dürr & González, 2002;Wittmann et al., 2004).
In the Gulf of Cadiz, the species has been found to live on muddy bottoms of the upper continental slope ( fig. 1), in areas characterised by the occurrence of submarine mud volcanoes which condition the structure and deposition dynamics of the sediments (Díaz del Rio et al., 2003). Depths of collection ranged between 363 and 548 m (table 1). Interestingly, this epibenthic report demonstrates the occurrence of L. spinosus on continental slope bottoms in day-time hours. Tattersall (1951) included this species in the 'deep-water bottom-living species of the continental shelf' as differing from the 'pelagic species', while Tattersall (1955) suggested that this species performs daily vertical migrations in the water column. However, the absence of night-time samples on the bottom and in the water column during the survey does not allow verification of this hypothesis. Most reports of L. spinosus have however been obtained from epipelagic or bathypelagic macroplankton/nekton samples at depths between 260 and 700 m in much deeper waters (Wittmann et al., 2004). Tattersall (1955), taking into account the comments of Fage (1942), previously reported that 'L. spinosus, in contrast to all the other species of the genus, inhabits the open ocean far from land...'. However, some specimens have also been reported from examination of stomach contents of bathyal epibenthic/demersal fish, such as deep-sea Beryx spp. (Dürr & González, 2002).
Concerning the morphology and meristic characters of the specimens studied, we emphasize that males showed a proportionally longer rostral spine with respect to the only female analyzed (table 2), although Fage (1942) stated that was more developed in females. The posterolateral spines of the female, however, were proportionally longer than those of the males, since they reached the end of the second pleonal segment in males, but clearly overreached the third segment in the female. The shape of the antennal scale was more elongate in the female (length/width ratio: 3.8) than in males (length/width ratio: 2.6-3.3). All these morphologic, morphometric, and meristic characters are worthy of study when more specimens become available to assess whether they are real sexual dimorphism characteristics, whether they are size dependent, or whether they are just the product of the low sample size obtained.