Crustaceana 85 (8) 987-991 THE FIRST RECORD OF GAMMARUS AEQUICAUDA (MARTYNOV, 1931) (AMPHIPODA, GAMMARIDEA) IN THE ESTUARY OF THE RIVER JADRO (ADRIATIC SEA, CROATIA) BY B. RAÐA1,3) and T. RAÐA2) 1) Faculty of Science, Department of Biology, University of Split, Teslina 12/III, HR-21000 Split, Croatia 2) Speleological Society “Špiljar”, Varaždinska 54, HR-21000 Split, Croatia ABSTRACT Gammarus aequicauda (Martynov, 1931) (Amphipoda, Gammaridea) is known as an euryhaline and euryvalent species in warm temperate waters, including lagoon systems and shallow coastal waters. The estuary of the River Jadro is a part of Kaštela Bay (Adriatic Sea), and after construction works on the downstream watercourse of the river, some ecological conditions have changed. A concrete barrier that was built decreases the influence of fresh water, which resulted in an increased salinity and a raised level of dissolved organic compounds. Such conditions were favourable for macroalgae, especially Enteromorpha sp., thus overgrowing riversides and providing amphipods with food and protection against predators. There are no earlier published data confirming the presence of G. aequicauda in Kaštela Bay. This is the first record of G. aequicauda from the River Jadro estuary as well as the first record of the species for Croatian rivers in general. RÉSUMÉ Gammarus aequicauda (Martynov, 1931) (Amphipoda, Gammaridea) est connue comme une espèce euryhaline et euryvalente des eaux tempérées chaudes, qui incluent les systèmes lagunaires et les eaux côtières peu profondes. L’estuaire de la rivière Jadro forme une partie de la baie de Kaštela (mer Adriatique), et après des travaux de construction sur le cours aval de la rivière, certains paramètres écologiques ont été modifiés. Un mur de béton qui a été construit diminue l’influence de l’eau douce, d’où résulte une salinité augmentée et un niveau plus élevé des composés organiques dissous. De telles conditions ont été favorables au développement des macro-algues, en particulier Enteromorpha sp., qui ont envahi les rives de la rivière et fourni aux amphipodes de la nourriture et une protection contre les prédateurs. Il n’y a pas de données publiées antérieures confirmant la présence de G. aequicauda dans la baie de Kaštela. C’est ainsi la première mention de G. aequicauda de l’estuaire de la rivière Jadro, et aussi des rivières croates en général. 3) Corresponding author; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2012 DOI:10.1163/156854012X651691 988 B. RAÐA & T. RAÐA INTRODUCTION The river Jadro is a part of the Dinaric Karst and covers about 50% of the Croatian territory. The area consists of carbonate rocks and karst phenomena along the Adriatic coast (carbonate forms of the Mesozoic and later eras) (Kuhta, 2002). The Jadro has a typical Mediterranean hydrological regime: the hydrological year is presented by two periods, a period of low water levels from May to October, and a period of high water levels from November to April) (Bonacci, 1987). The spring of the river is located at an elevation of 30 meters above sea level. The total length of the river is 4.5 km. It discharges into the Kaštela Bay in the Adriatic Sea. The catchment area to which it belongs is the river Cetina catchment (approx. 260 km2). The physicochemical characteristics of the Jadro have been studied by Štambuk- Giljanovic´ (2005) and Rada¯ (2006, 2008, 2010). The minimum and maximum discharge range from 2 (July 1995) to 100 m3/s (December 2002). Also, studies have been carried out on the phytobenthos (Vukovic,´ 1981) and macrozoobenthos (Rada,¯ 2002, 2006, 2008, 2010). These investigations provided data about the flora and fauna, in which Enteromorpha flexuosa (Wolfen ex Roth) J. Agardh, 1883 and Gammarus aequicauda (Martynov, 1931) were not recorded. However, after the construction of a concrete barrier by the government in 1997, the fresh water could no longer influence the downstream water regime, which consequently changed, and this resulted in ecological conditions becoming favourable for other species. Enteromorpha flexuosa is a very common alga found in places where there is freshwater influence, such as streams discharging into the sea. It is often associated with coastal areas rich in nutrients, like areas with human residence and/or industrial development. It is considered an invasive nuisance species. Because of its high reproductive ability, E. flexuosa is an excellent pioneer species, quickly occupying available substrates. Its density increases dramatically following a disturbance. It is often found in communities with Ulva fasciata Delile, 1813. Both are organisms associated with industrial pollution. Ercegovic´ (1980) confirmed the presence of these species in the Adriatic coastal region. The biology and ecology of G. aequicauda are well known and have been de- scribed by several authors (Stoke, 1969; Chassany de Casabianca, 1979; Diviacco, 1983; Porcu & Tagliasacchi Masala, 1983). This species is widely distributed along the Mediterranean and Black Sea coasts. The species penetrates brackish waters and it is abundant in shallow waters between macroalgae (Prato & Biandolino, 2009). Thus far, there were no data about the presence of G. aequicauda in Croat- ian estuaries..