J. Mar. Biol. Ass. U.K. (2005), 85, 375^382 Printed in the United Kingdom Life cycle, population dynamics and productivity of Ventrosia maritima in the Evros Delta (northern Aegean Sea) Theodoros Kevrekidis*P and Thomas WilkeO *Democritus University of Thrace, Laboratory of Environmental Research and Education, GR-68100, Alexandroupolis, Greece. OAnimal Ecology & Systematics, Justus Liebig University Giessen, Heinrich-Bu¡-Ring 26^32 (IFZ), D^35392, Giessen, Germany. PCorresponding author, e-mail: [email protected] Life cycle, population dynamics and productivity of the larviparous mudsnail species Ventrosia maritima were investigated at low salinities (0.3^6 psu) in di¡erentiated parts of a Mediterranean lagoon (Monolimni Lagoon). Monthly samples were collected during the period from February 1998 to February 1999 in both parts of the lagoon.Ventrosia maritima displayed an annual life cycle. Recruitment occurred in summer and autumn at the outer part of the lagoon and additionally in late winter at the innermost part. A positive correlation was found between the percentages of small individuals and salinity or sediment organic matter at the outer part. Growth practically ceased in winter. The mudsnail displayed remarkable densities and an increase in growth in spring at 51psu indicating that it is highly tolerant to extremely low salinities. Population density showed a signi¢cant seasonal variation; it increased from early summer to autumn (30,000^40,000 individuals m72) following the summer and autumn recruitment. No signi¢cant correlation between the density of V.maritima and several examined physicochemical variables was found; a negative correlation was observed between the density of the mudsnail and that of the co-occurring poly- chaete Streblospio shrubsolii. Secondary production calculated by the size^frequency method gave a mean annual density (N) of 9740 ind m72, a mean biomass (B) of 1.66 g ash-free dry weight (AFDW) m72 y71, a production (P) of 4.51g m72 y71 and a P:B ratio of 2.72 at the outer part of the lagoon and a N of 14,570 ind m72, a B of 3.2 g AFDW m72 y71, a P of 9.9 g m72 y71 and a P:B ratio of 3.09 at the innermost part. At the innermost part of the lagoon, where the seawater renewal rate and hydro-dynamism were lower and the sediment ¢ner and organically richer, V. maritima displayed more recruitment pulses, a larger body size and a denser and more productive population than the one at the outer part. Our ¢ndings are compared to published data for the direct-developing congeners V.ventrosa and V.truncata. INTRODUCTION body size and population density of mudsnail species (Forbes & Lopez, 1990; Grudemo & Johannesson, 1999), The amphi-Atlantic mudsnail species of the subfamily the study of life cycle, population dynamics and produc- Hydrobiinae (Rissooidea: Hydrobiidae) are frequently tivity of hydrobiinids, in relation to the degree of contact important elements of coastal habitats and a major source of the lagoonal habitat with the open sea, emerges as one of food for birds, ¢sh and other predators. Because of their of the research priorities in this highly £uctuating environ- signi¢cant role in brackish-water ecosystems, they have ment. become one of the most intensely studied molluscan The mudsnail species Ventrosia maritima (Milaschewich, genera in the world. Research topics involve parasitism, 1916) was, until very recently, only known from the Black energetics, deposit feeding and nutrition, reproductive Sea region. However, by applying mitochondrial DNA e¡ort and food chain resources, migration patterns and sequencing and phylogenetic tools, the occurrence of this dispersal, as well as population genetics and speciation. species was ascertained in the Evros Delta, northern However, some aspects of their biology and ecology, such Aegean. Additionally, the ¢rst detailed data concerning as life history, population dynamics and productivity, its ecology and distribution in a lagoon system were given have been studied only in few selected species. The direct- (Kevrekidis et al., 2005). Ventrosia maritima proved to be a developing Ventrosia ventrosa (Montagu, 1803), a typical ‘typical’ lagoonal species displaying its highest abundances lagoonal species and the larviparous Peringia ulvae in the innermost and isolated lagoonal parts. However, to (Pennant, 1777), a species of marine origin, are our knowledge, life cycle, population dynamics and examples (Siegismund, 1982; Bachelet & Yacine-Kassab, productivity of this species have not been described 1987; Barnes, 1994; Drake & Arias, 1995; Probst et al., yet. Although some information is available from the 2000). Most of these studies deal with mudsnail local Russian, Ukrainian, Romanian and Bulgarian populations from north-western Europe and north- literature (Chukhchin, 1976), it is not possible to assign eastern America. However, little is known about the these data to certain mudsnail species. Part of the biology and ecology of the mudsnail species at low problem is that numerous, often indistinguishable muds- salinities (55 psu). As the habitat type may well a¡ect nail species, have been reported from the Black Sea Journal of the Marine Biological Association of the United Kingdom (2005) 376 T. Kevrekidis and T.Wilke Biology and ecology of Ventrosia maritima Table 1. The range of several abiotic variables of water and sediment (after Kevrekidis, 2004) and Spearman’s rank correlation coe⁄cient values between these variables and the percentage of Ventrosia maritima individuals having a shell length 41.1 mm (r1) or density of V. maritima (r2)atStationsI1 and B2. Station I1 Station B2 Variable Range r1 r2 N Range r1 r2 N Water Depth (cm) 50^85 0.023 n.s. 70.234 n.s. 13 30^55 0.599 n.s. 0.209 n.s. 11 Salinity (psu) 0.3^5.6 0.824 ** 0.413 n.s. 13 0.3^5.7 70.407 n.s. 0.459 n.s. 13 71 Dissolved O2 (mg l ) 6.05^14.7 70.276 n.s. 0.465 n.s. 11 9.78^18.0 0.045 n.s. 70.036 n.s. 11 O2 saturation (%) 74^122 70.284 n.s. 0.364 n.s. 11 101^220 0.100 n.s. 0.018 n.s. 11 pH 7.4^9.1 70.279 n.s. 0.041 n.s. 13 7.45^9.32 0.501 n.s. 0.113 n.s. 13 Temperature (8C) 1.8^26.7 0.259 n.s. 70.511 n.s. 13 4.2^28.5 70.077 n.s. 70.203 n.s. 13 Sediment Temperature at 1 cm (8C) 2.1^26.6 0.283 n.s. 70.470 n.s. 13 3.7^27.0 70.025 n.s. 70.160 n.s. 13 Temperature at 5 cm (8C) 1.9^26.5 0.283 n.s. 70.470 n.s. 13 3.5^28.6 70.033 n.s. 70.198 n.s. 13 Median diameter (Md) (mm) 143^176 0.156 n.s. 0.161 n.s. 13 94^129 70.244 n.s. 0.050 n.s. 13 Organic matter (%) 0.15^1.73 0.868 *** 0.498 n.s. 13 0.48^2.20 0.220 n.s. 0.330 n.s. 13 n.s., not signi¢cant; **, P50.01; ***, P50.001. region. Moreover, di¡erent names are used in di¡erent MATERIALS AND METHODS littoral states. Study area Preliminary genetic data (Thomas Wilke, unpublished data; Kevrekidis et al., 2005) indicate that the actual The Evros Delta is located at the north-eastern part of number of mudsnail taxa in the Black Sea region might the Aegean Sea. Fresh water £ows into the delta area both be low. In fact, all populations studied so far from the through the eastern and western branches of Evros River Black, Asov and Aegean Seas appear to be conspeci¢c as well as through the streams Mikri Maritsa and and belong to V. maritima (based on comparative genetic Loutron. Three islets and some lagoons have been formed studies of topotypes from Sevastopol, Ukraine). It should in the delta area. Monolimni (or Paloukia) Lagoon occu- be noted that V. maritima is a larviparous taxon character- pying an area of about 1.12 km2 communicates with the sea ized by a small initial whorl, *175 mm in diameter mainly through a 15m wide opening at its north-west end. and a planktotrophic protoconch 2 comprising about In July 1997, water salinity near the bottom varied 1.5 whorls (Anders Ware¤ n & Thomas Wilke, between 30.0 and 31.5 psu overall in Monolimni Lagoon. unpublished data). All other known Ventrosia species are An increased freshwater in£ow during winter and spring direct-developing. Wilke & Davis (2000) suggested that of 1998 following a period of intense rainfall on the river di¡erent life styles (i.e. direct developing species versus catchment area resulted in a sharp decline in salinity species with pelagic larvae) may have signi¢cant e¡ects values during February 1998 to February 1999. The on population structures of those species. With this in monthly variation in the values of water and sediment mind, one would expect that V. maritima may di¡er in physicochemical parameters during February 1998 to important ecological features from its direct-developing February 1999 at Station I1 (located at the outer southern congeners. part of the lagoon) and at Station B2 (located at the inner- The main goals of this study are: (1) to describe the most northern part of the lagoon and where seawater biological and ecological characteristics of the mudsnail renewal rate and the hydrodynamism are lower) has been taxon V. maritima in the Evros Delta (Aegean Sea) such as given by Kevrekidis (2004). Salinity and temperature life cycle, growth, population dynamics and productivity; showed similar values at both stations throughout the (2) to investigate the variability of those characteristics in annual cycle.