Patrizia Torricelli

An Ecological Imbalance Induced by a Non-Native Species: The Manila Clam in the Venice Lagoon

Among the 19 non-native species of marine invertebrates which have invaded the Venice Lagoon and have established populations, Ruditapes philippinarum, deliberately introduced in 1983, is surely the most successful species. According to the hypothesis that alien species invasion could be favoured by an altered ecological, chemical or physical state of the system induced by anthropogenic disturbance, R. philippinarum turned out to be ‘the right species at the right moment’. By comparing historical data (1968, 1985, 1990) with 1999 data, changes in macrobenthic community, in particular bivalve molluscs, of the lagoon induced by R. philippinarum introduction and subsequent clam exploiting activity were assessed. It has been possible to describe a sharp reduction, both in terms of distribution area and density, of all other filter feeder bivalves. Moreover, by using the clearance rate of the most abundant bivalve species in 1990 and 1999 (Cerastoderma glaucum and R. philippinarum, respectively), it was possible to estimate that the filtration capacity, expressed as l h−1 m−2, has more than doubled. This has altered the functioning of the ecosystem, resulting in a stronger benthic–pelagic coupling. In this context, R. philippinarum attains control of the system. Considering all this, it is possible to state that the Venice Lagoon ecosystem has entered into a new state, probably more resistant but less resilient, with implications for future management choices.

Sound production during courtship and spawning among freshwater gobiids (pisces, gobiidae)

In this paper the breeding sound production among gobiids is reviewed. Results of recent comparative studies on both pre‐spawning (courtship) and spawning sound production in the three italian freshwater gobiids are reported. A significant production of sounds associated to mating was discovered in two of these species. Furthermore, the quantification of acoustic behaviour across the spawning period (i.e., from the beginning of oviposition until the departure of the female from the males nest) revealed a pattern of spawning sound production similar in the two species.

Spawning vocalizations in male freshwater gobiids (Pisces, Gobiidae)

SynopsisMales of two freshwater Italian gobies, the common goby, Padogobius martensii and the panzarolo goby, Knipowitschia punctatissima, emit trains of low-frequency pulses, i.e. ‘drumming’ sounds, in the presence of a ripe female in the nest. In P, martensii the drumming sound is usually followed by a tonal sound (complex sound). Examination of the pulse structure suggests that these sounds are produced by muscles acting on the swimbladder. Both species exhibited high emission rates of spawning sounds, especially before the beginning of oviposition. Moreover, spawning sound production ceased only after the female abandoned the nest, which always occurred at the end of oviposition. This is the first study reporting the production among fishes of distinct sounds during protracted spawning. Unlike sounds produced just before mating by fishes with planktonic or demersal zygotes, the spawning sound production of these gobies does not function to coordinate mating events in the nest. The presence of a two-part vocalization by male P. martensii even suggests a functional dichotomy of spawning sounds in this species.

Fish assemblages in different shallow water habitats of the Venice Lagoon

The small-sized fish assemblages of the Venice Lagoon were investigated and compared among five shallow subtidal habitats (seagrass beds, sparsely vegetated habitats, unvegetated sand bottoms, mudflats and saltmarsh creeks) in the Northern lagoon basin. Sampling was carried out seasonally (Spring, Summer and Autumn of 2002) in 4–7 stations for each habitat type, by means of a fine-mesh, small beach seine. Two-way analysis of variance was applied to assess the differences in species richness, fish diversity, density and standing stock amongst habitats, whereas fish assemblage composition was investigated by using multivariate analyses (MDS, ANOSIM, SIMPER). The analyses indicated that seagrass beds and saltmarsh creeks are relevant shallow habitats in structuring the small-sized fish assemblages of the Venice Lagoon, supporting specialized and recognizable fish assemblages. Those in seagrass beds, in particular, were characterized by higher species richness and standing stock with respect to all the others. The structuring role of these habitats was discussed in terms of both habitat complexity and degree of confinement. In contrast, sandy bottoms, mudflats and sparsely vegetated habitats were identified as “transition” habitats, with highly variable fish assemblages, influenced by the contribution of the adjacent habitats, and acting probably as both ‘buffer zones’ between the other habitats and migration routes for many fish species in the lagoon.

ANALYSIS OF SOUNDS PRODUCED BY MALE PADOGOBIUS MARTENSI (PISCES, GOBIIDAE) AND FACTORS AFFECTING THEIR STRUCTURAL PROPERTIES

ABSTRACT In this paper a detailed analysis of the physical structure of sounds produced by male Padogobius martensi is reported. Sound production occurs during courtship and inter-male agonistic encounters. Both aggressive and courtship calls are made up of rapidly repeated pulses, with a pulse repetition rate decreasing through the course of the emission. By means of computerized analysis, the pulse repetition rate, its modulation and sound duration were determined. The water temperature was found to exert a marked and significant effect on the above parameters. In particular, the temperature directly affects the pulse rate and its decrease through the course of the emission (i.e. frequency modulation) and inversely affects sound duration. By contrast, size of the calling animal does not significantly influence the sound parameters considered. Aggressive sounds last longer and have a lower pulse repetition rate than the courtship sounds. Moreover, aggressive sounds appear more variable than the courtship...

Breeding ecology and male spawning success in two hill-stream populations of the freshwater goby, Padogobius martensi

SynopsisThis investigation was carried out at two study sites to examine the influence of shelter availability, nest site quality and male body size on spawning success of malePadogobius martensi. At Stirone Stream (first study site) individuals were not randomly distributed with respect to usable stones; larger males and females were consistently found to occupy larger stones. Laboratory experiments showed that the bottom surface area of the shelter was the relevant cue for the choice. The number of nesting males settled in a given area increased with increasing availability of local stone cover; inter-nest distance was directly correlated with male size. At Rivarolo Stream (second study site), which was characterized by lower nest density and a greater shortage of large stones, the pattern of shelter occupation was similar. These results suggest the presence of competition among individuals for the possession of larger-sized shelter sites. At both sites, the number of eggs in the nest increased as stone size increased; however, at the first study site, male body size played a major role in male spawning success as it concerns both the numbers of females mated and the number of eggs, while at the second study site, male body size was not shown to influence male spawning success. Factors responsible for between-site differences in male spawning success are discussed.

The importance of breeding vocalizations for mate attraction in a freshwater goby with a composite sound repertoire

Male gobies, Padogobius martensii, emit ‘tonal’ sounds during courtship, and ‘drumming’ and ‘complex’ sounds during spawning. The complex sound is a two-part vocalization consisting of a drumming sound followed without pause by a tonal sound. In the laboratory, the playback of both the tonal sound and the drumming sound to ripe females P. martensii determined oriented approaching and increased the time spent by the subject within the loudspeaker area. Response levels to the sound playback (drumming sounds) tended to be correlated with degree of ripeness of the female. Functional aspects of the female response to both types of acoustic stimuli are discussed.

Male Courtship Sounds in a Teleost with Alternative Reproductive Tactics, the Grass Goby, Zosterisessor ophiocephalus

Male grass gobies show two alternative breeding tactics, territorial and sneaker, distinguished by body size and difference in ray elongation on the second dorsal fin. The larger males, with elongated fins, are territorial and emit sounds during courtship. Smaller males, without elongated fins, act as sneakers. Both large and small males produce sounds in the presence of a ripe female. Males produce a grunt, lasting about 300 ms, made up of pulses repeated at a low rate (22–68 pps). Pulse duration, number, and repetition rate, did not differ between the two male types, but dominant frequency and sound amplitude did. Dominant frequency had a strong, inverse relationship with body size, whereas sound amplitude showed a weak positive relation to body size. Male size, and not the particular reproductive male tactic employed, is the most important correlate of sound properties in this species.

A quantitative analysis of the occurrence of visual and acoustic displays during the courtship in the freshwater goby, Padogobius martensi (Günther, 1961) (Pisces, Gobiidae)

Abstract Seventeen pairs of Padogobius martensi were observed to compare the relative occurrence of the acoustic and visual displays performed by the male during courtship. The results show that the visual display, which gradually decreases when the male approaches the nest, is the main signal to orient and attract the female towards the males nest; sound production, on the other hand, becomes more frequent when the male approaches and enters his nest. So, the acoustic signal has an important role when the visual one becomes ineffectual, when the male is hidden in the shelter. Visual and acoustic signals seem to have a complementary role and the male P.martensi uses the most effective signal in each particular situation.

A habitat-specific fish-based approach to assess the ecological status of Mediterranean coastal lagoons.

A habitat approach was promoted in the framework of ecological status assessment of transitional waters, assuming the importance of habitat heterogeneity to the overall system status. The approach was applied to the use of fish-based multimetric indices by adapting them to seagrass and marsh habitats in the Venice lagoon, Italy, through selection of appropriate metrics and reference conditions. While for marsh habitats, no clear patterns resulted, the index response for seagrass was consistent with the habitat degradation and loss recorded in the lagoon between 2002 and 2005 and with the higher habitat disturbance in southern and central lagoon sub-basins. The assessment of individual habitats is presented as a first step in the process of evaluating the overall condition of a Mediterranean lagoon environment, which should also take account of the diversity of habitats and their availability within the system to properly define an overall index of ecological status.