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First record of Arctodiaptomus wierzejskii (Richard, 1888) (Copepoda Calanoida Diaptomidae) from Malta

Received: 24 September 2020 Accepted for publication: 30 September 2020 Online publication: 18 November 2020 Abstract Diaptomid copepods belonging to the genus Arctodiaptomus Kiefer, 1932 were collected in two temporary water bodies in Malta. The morphological identification of the collected specimens proved that they belong to Arctodiaptomus wierzejskii (Richard, 1888) even if a morphological peculiarity pertaining to the morphology of the male right antennule of the Maltese populations was observed and here briefly discussed. This finding constitutes the first record of a calanoid copepod from the inland waters of the Maltese Islands.


INTRODUCTION
The Maltese Archipelago is formed by three main islands (Malta, Gozo and Comino) and a number of minor islets and small rocks mainly composed of Oligo-Miocene limestones. It is located at the centre of the Mediterranean basin, 96 km south of the Sicilian coast. Overall, the archipelago presents a surface area of about 315 km 2 , with Malta, the largest island, covering an area of about 246 km2. The climate is typically Mediterranean, with a wet season in which about 85% of the total annual rainfall occurs, and a dry period lasting roughly from April to September. The average annual precipitation is about 530 mm and the average temperature is 18.6°C (Schembri, 1993).
In the frame of a wide survey aimed at exploring the inland water copepod fauna of the Central Mediterranean area, some calanoid copepods were collected in Malta whose identity and morphology are hereby discussed.

MATERIALS AND METHODS
Crustacean samples were collected in Malta on 31 st January 2008 in a "rock-pool" located in Wied Es-Sabtah (site MT001, WGS84 geographical coordinates: 35.836060 N, 14.514882 E; 44 m a.s.l.) and in a concrete reservoir near Siġġiewi (site MT002, WGS84 geographical coordinates: 35.847023 N, 14.434342 E; 108 m a.s.l.) (Fig. 1). Sampling was carried out with a 200 µm mesh hand net. The collected specimens were fixed in situ using 90% ethanol and sorted out in laboratory under a stereomicroscope.
Co-occurring branchiopod and ostracod taxa were identified according to Alonso (1996) and Meisch (2000), respectively. Samples of Maltese and Tunisian A. wierzejskii, and co-occurring crustacean taxa, are kept in FM collections at the University of Palermo and are available for loan on request. Moreover, 10 males and 10 females from MT002 were deposited in the collection of the Zoology Section "La Specola", Natural History Museum, University of Florence (Italy) under the collection number MZUF 661.

RESULTS
At the sampling date, the electric conductivity values were 199 µS cm -1 (20°C) in MT001 and 688 µS cm -1 (17°C) in MT002. The co-occurring crustacean fauna (Tab. 1) is richer in MT001, including the species typically found in temporary rock pools, whereas that occurring in MT002 only includes the euryecious daphniid Daphnia magna (see also Marrone & Vecchioni, 2020). Unexpectedly, no cyclopoid copepods were collected in the two sites.
The morphology of the collected diaptomid copepods allowed us to identify them as Arctodiaptomus wierzejskii, although the Maltese specimens slightly differ from the standard morphology of the species in a detail related to male right antennule (see below).
The antennules (A1) of females are symmetrical and consist of 25 segments, with the typical chaetotaxy of the species as described in Kiefer (1978). The right male antennule is modified and geniculated in the fourth last segments. The thirteenth segment of the right antennule has a robust and well-developed spine, while in its antepenultimate segment there is a spiny comb-shaped process ( Fig. 2A). When compared with the available drawings of A. wierzejskii and with A. wierzejskii individuals from Tunisia, the Maltese specimens present a morphological peculiarity. In Tunisian male specimens, the 14 th segment of the right antennule has a short but stout tooth. This feature is constantly represented in all the available drawings of the species. Conversely, in the Maltese individuals the tooth on the 14 th segment is normally absent (Fig. 3). Only in one out of the 35 studied Maltese males the spine is present, although poorly developed (Fig. 3H). Tab. 1 -Crustacean assemblages recorded in the two study sites on January 31 th 2008. / Popolamenti a crostacei osservati nei due siti di studio il 31 gennaio 2008. * Ceriodaphnia quadrangula Müller, 1785 sensu Alonso (1996).

DISCUSSION AND CONCLUSIONS
Arctodiaptomus Kiefer, 1932 is a diverse diaptomid genus currently divided in various subgenera of doubtful validity (e.g. Ranga-Reddy, 1994;Dussart & Defaye, 2002;Boxshall & Halsey, 2004). Its type species, Arctodiaptomus wierzejskii, is a euryecious taxon, which can produce both subitaneous and drought-resistant eggs, and is able to colonise a wide range of different habitat types, ranging from brackish, warm and shallow temporary pools, to tundra pools and large oligotrophic lakes (Gurney, 1931;Kiefer, 1978;Ranga-Reddy, 1994;Błędzki & Rybak, 2016). The species is widely distributed across the Palearctic region including the Maghreb, the whole Europe north to to the Shetlands, and the eastern Palearctic east to Mongolia and northern China (Dussart & Defaye, 2002;Błędzki & Rybak, 2016;Marrone et al., 2017). Oddly enough, although the species is known to occur in peninsular Italy and Sardinia, with few scattered populations (Ruffo & Stoch, 2006), it seems to be absent in Sicily . Conversely, in some relatively close areas, for example the Apulian peninsula, it is the most widespread calanoid copepod in temporary inland waters (Alfonso & Belmonte, 2011), often dominating and characterizing the microcrustacean communities of these ponds (Alfonso et al., 2016).
The existence of morphological differences between different A. wierzejskii populations was already reported in literature; e.g. in the Tunisian, Algerian and Central Asian populations females have the right lobe of the fifth thoracic somite distinctly facing outwards (Gurney, 1931), the males of some Scottish populations present a projection of the antepenultimate segment of the right antennula much shorter than usual (Fryer & Joyce, 1981), and an unusual male and female chaetotaxy was observed in Arctodiaptomus cf. wierzejskii from Mongolia (Marrone et al., 2015). Accordingly, in spite of the slight morphological difference pertaining to male right A1, we are here conservatively attributing the Maltese Arctodiaptomus populations to A. wierzejskii. However, also in the light of the wide distribution area and the great variety of habitats colonized by the species, it cannot be excluded that under the binomen Arctodiaptomus wierzejskii are actually lumped several different evolutionary lineages. The molecular characterization of selected populations is needed in order to understand whether the observed morphological differences are consistent with a clear pattern of genetic diversity, or are rather to be ascribed to a noteworthy intraspecific morphological variability with no taxonomical value. To date no information on the presence of calanoid copepods in Maltese inland waters was available. The present finding thus constitutes the first record of a representative of the family Diaptomidae in the Maltese Archipelago, and contributes to the knowledge of the distribution of freshwater calanoids in the Mediterranean area. However, the finding of an unidentified calanoid specimen in a rock pool in the area of Għallis (Malta; S. Lanfranco, in litteris) suggests that diaptomids could be widespread in the Maltese Archipelago, and that more surveys aimed at getting a better knowledge of its freshwater copepod fauna are desirable.