First report of a species of Prasiola ( Chlorophyta : Prasiolaceae ) from the Mediterranean Sea ( Lagoon of Venice ) *

The green alga Prasiola (C. Agardh) Meneghini is known from terrestrial, marine and freshwater habitats of Ireland and Britain, the Baltic sea, the Atlantic coasts, Australia, New Zealand, Antarctica and Japan (Womersley, 1984; Burrows, 1991; Rindi et al., 1999; Kovácik, 2001). Species of Prasiola have also been reported for the Black and Azov Seas (Zinova, 1967). According to Athanasiadis (1987), the occurrence of the genus in the Aegean Sea is doubtful (Gallardo et al., 1993). In spring 2002, during studies of the benthic flora of the Lagoon of Venice (North Adriatic Sea, Italy), some small thalli attributable with certainty to the Prasiola genus were found.


INTRODUCTION
The green alga Prasiola (C.Agardh) Meneghini is known from terrestrial, marine and freshwater habitats of Ireland and Britain, the Baltic sea, the Atlantic coasts, Australia, New Zealand, Antarctica and Japan (Womersley, 1984;Burrows, 1991;Rindi et al., 1999;Kovácik, 2001).Species of Prasiola have also been reported for the Black and Azov Seas (Zinova, 1967).According to Athanasiadis (1987), the occurrence of the genus in the Aegean Sea is doubtful (Gallardo et al., 1993).In spring 2002, during studies of the benthic flora of the Lagoon of Venice (North Adriatic Sea, Italy), some small thalli attributable with certainty to the Prasiola genus were found.

STUDY AREA, MATERIAL AND METHODS
The macroalgae communities survey was carried out on 90 sites in a 31,000 ha study area, excluding canals, salt marshes and fish farms but comprising various environmental lagoon typologies like shal- Prasiolaceae) from the Mediterranean Sea (Lagoon of Venice)* low waters, tidal flats and seabeds that were both colonised and not colonised by seagrass communities.Data collection was carried out in spring and autumn to detect seasonal variability, and 5 replicas of 1.5 m 2 each were collected for a total of 7.5 m 2 for each station.Sampling was performed in the midlittoral zone using a bladed net (0.5 mm mesh) to penetrate the first few centimetres of sediment.Sampling sites inside seagrass beds were investigated by collecting 5 shoots for epiphyte analysis.
The collected material was preserved in seawater-formalin (4%) for later study in the laboratory.Permanent slides for microscopic observation were made by mounting the material in a 50% glycerolseawater mixture.Voucher specimens were preserved in 4% formalin seawater and deposited in SELC laboratories.

DESCRIPTION
The thalli of Prasiola are up to 0.5 cm long and consist of bright to dark-green leafy monostromatic blades with a irregularly rounded-ovate shape, sometimes divided and crisped.The blades, devoid of stipe (Fig. 1A, B), are attached to the substratum by means of a rim of the frond or free and are 17-20 µm thick (Fig. 1C).Thalli have a marginal and diffuse growth, with a smooth to irregular margin.In surface view the cells are square or rectangular, 4-5 (7) x 5-8 (9) µm, clustered in groups of 4 or multiple, arranged in regular longitudinal and transverse rows giving the thalli the characteristic grid-like appearance (Fig. 1D); the groups of cells are separated by thick walls of 3.5-5 µm.
The chloroplast is often in a central position, occupying a large area of the cell; it is somewhat lobate with a central pyrenoid.

ECOLOGY AND DISTRIBUTION
The thalli of Prasiola were collected in April and May 2002 on soft substrata in the southern basin of the Lagoon and in November 2002 on leaves of the seagrass Nanozostera noltii (Hornemann) Tomlinson et Posluzny in the northern basin of the Lagoon.In the southern basin all the sites consist of muddy substrata and are located in supralittoral and midlittoral zones, where tide excursion is up to 1 metre, resulting in a continuous wash and dry environment.In the northern basin thalli of Prasiola occurred in midlittoral meadows of N. noltii, on muddy-sandy substrata that are continuously exposed, as reported for the southern basin sites.
In the southern basin the specimens of Prasiola were associated with the green algae Ulva laetevirens Areschoug, Chaetomorpha linum (O.

DISCUSSION
The morphology of the specimens collected is in agreement with the genus Prasiola, in particular with Prasiola crispa (Lightfoot) Kützing, for the following features: a) monostromatic thalli, more or less rounded and crisped, b) lack of a stipe, and c) cells arranged in regular longitudinal and transversal rows (Womersley, 1984;Burrows, 1991;Rindi et al., 1999).P. crispa differs from other species reported for Europe in the habit of the thallus and attachment to the substratum.Prasiola calophylla (Charmichael ex Greville) Kützing is characterised by a slender frond, gradually becoming uniseriate towards the base, attached by a disc-shaped holdfast.Prasiola furfuracea (Mertens ex Hornemann) Kützing has a fan-shaped frond with a short stipe, attached by basal cells or a few rhizoids.Prasiola stipitata Suhr ex Jessen is characterised by a wedgeshaped, fan or hearth-shaped thallus with a long stipe.We never found blades of Prasiola with filaments of Rosenvingiella, as reported by several authors (Kornmann and Sahling, 1974;Edwards, 1975;Rindi et al., 1999), or mixed with uniseriate filamentous specimens ("Hormidium stage") and pluriseriate ribbon-like plants ("Schizogonium stage").P. crispa is primarily a terrestrial alga (Knebel, 1936;Burrows, 1991;Rindi et al., 1999) and we cannot exclude its presence in terrestrial habitats, because we sampled only in the midlittoral zone of the Lagoon.Conversely, the association with bird colonies (Scarton and Valle, 2000) is in accordance with many studies (Kristiansen, 1972;Womersley, 1984).
The identification of our specimens as P. crispa is supported by agreement between the characteristics of our samples and the morphology of this species as described in the literature.Nevertheless, due to the low abundance and very small size of the specimens, we prefer to postpone identification at species-level until further collection is available.
Desmarestia viridis (O.F.Müller) J.V. Lamouroux, Sargassum muticum (Yendo) Fensholt, Undaria pinnatifida (Harvey) Suringar, Antithamnion pectinatum Brauner ex Athanasiadis et Tittley, Polysiphonia morrowii Harvey and Lomentaria hakodatensis Yendo, recently recorded in the Lagoon of Venice (Gargiulo, et al., 1992;Rismondo et al., 1993;Curiel et al., 1996aCuriel et al., , 1996bCuriel et al., , 2002;;Bellemo et al., 2001), were found for the first time at Chioggia Island (southern basin), where several aquaculture farms importing molluscs and fish from other countries are present.Conversely, specimens belonging to the genus of Prasiola and Sorocarpus (Curiel et al., 1999) were found in other areas of the Lagoon.We think that these two genera, already present in Atlantic Europe, have probably reached the Venice Lagoon via a vector other than aquaculture farms.This hypothesis is in agreement with our discovery of the hybrid and sterile form of Graminaceae Spartina x townsendii, which is common in northern Europe, in natural and artificial salt marshes close to the areas where thalli of Prasiola were collected (Scarton and Valle, 2000;Scarton et al. 2003).
In the years following the first occurrence, the genus Prasiola has not been found again either by our study group or by other researchers (Sfriso, pers.comm.), mainly because no more surveys have been carried out in these areas and also because its prop-agation is currently limited only to soft substrata of a specific area of the lagoon.

report of a species of Prasiola (Chlorophyta:
PRASIOLA GENUS IN THE VENICE LAGOON 343 2005 First