Fauna: invertebrates - Udine Cultura

More documents

Recommendations

Info

74 ■ Sessile fauna Sessile animals are those living permanently attached to the substrate. In the sea, they are quite numerous on rocky bottoms and, more generally, on any substrate hard enough to enable them to adhere. On sandy and muddy seabeds, sessile organisms may be found only on the very small hard substrates (stones, shells, etc.) scattered on sediments. Seagrass leaves and rhizomes offer sessile fauna very particular substrates, which often require special adaptations. In particular, seagrass leaves fluctuate and continually renew themselves and, due to their small size, can only be colonised by minute organisms. Although rhizomes are slightly stabler, they are more selective than the rocky floors which sessile fauna usually inhabits. Due to their adaptations, these organisms living on seagrasses are called epiphytes, but are different from those found on other substrates. This is particularly true of Neptune grass leaves, less for its rhizomes and for other seagrasses, whose fauna is less characteristic. Seagrasses therefore play an important role in the ecology and evolution of sessile fauna. Hard, rocky substrates make up only a small portion of littoral seabeds, which are generally composed of sedimentary rocks, inhospitable habitats for these types of animals. Sessile organisms can colonise these environments only in two ways: by “jumping” from one small island of hard Electra posidoniae, an epiphytic bryozoan exclusive to Posidonia leaves substrate to another – avoiding direct contact with sediments - or by putting up with the hostile habitat and developing adaptations that prevent them from sinking into the sediment. The latter solution is used by large animals, like burrowing sea anemones and sea pens, and the former has been adopted by small, opportunistic species with short life-cycles. This, however, has led to ecological speciation for epiphytes of living organisms, like molluscs (some Epiphytic hydrozoans on Posidonia leaves hydroids, for instance, live exclusively on bivalves, gastropods, or hermit crabs in gastropod shells) or seagrasses. In the Mediterranean, Posidonia oceanica is the seagrass that forms the most extensive and stable meadows, thus offering good opportunities for avoiding sedimentary environments. It is not surprising then, that the only true specialised epiphytes live on Neptune grass. Generally, sessile species living on seagrasses are colonial: colonies are created by budding from a founding individual developing from a single, minute larva colonising the substrate. Colonies represent efficient strategies to monopolise suitable substrates, once they have been located. Seagrass epiphytes usually belong to several taxonomic groups, from the simplest, like protozoans, to the most highly evolved, like chordates. Protozoans are unicellular organisms that were classified as animals in the past and are now ascribed to other kingdoms: they are generally microscopic, although some species are visible to the naked eye. Most of those living on seagrasses belong to the class of forams (foraminiferans). Porifers or sponges are the most primitive animals, almost exclusively sessile and of various sizes and shapes. There are two types living on seagrasses: calcisponges, with a few, tiny species, and demosponges, which include most porifers. Cnidarians are a large group of animals with stinging cells (cnidocysts) and two different body forms: polyp and medusa. Polyps are generally sessile, and medusae, such as jellyfish, are free-swimming. Polyps may live alone or in colonies, according to species. On seagrasses, hydrozoans are the most numerous species (with polyps and, less frequently, with medusae), and there are also some anthozoans (corals). 75
76 Although most polychaete annelids are vagile, some are sessile and live in tubes that they secrete themselves and which secure them to the substrate. Tubes may have mucus, mud, parchment and rubber textures, like those of feather duster worms (sabellids), and may even be calcareous, like those of serpulids and spirorbids. Some of these families are occasionally found on seagrasses. Surprisingly, arthropods, which in the sea include mainly crustaceans, are represented here by sessile species, like barnacles (cirripeds) and the wellknown Poli’s stellate barnacle. They may also be found on seagrasses. Almost all bryozoans (also known as moss animals) are colonial, and each colony houses many tiny individuals called zooids. Colonies are sometimes very large and are typically sessile, encrusting or erect. Seagrasses host species of three orders: cyclostomes, with calcitic, tubular zooids; cheilostomes, with box-shaped calcitic zooids; and ctenostomes, with noncalcified sac-shaped zooids. Even chordates include sessile species: tunicates belonging to the ascidians (sea squirts). There are both solitary or “simple” and colonial species. The minuscule individuals that form their colonies are called ascidiozooids. If they are joined at the base by a single stolon, they are called social ascidians; if they are all enclosed in one tunic, they are known as compound ascidians. Some species colonise seagrasses. Aglaophenia harpago, an epiphytic hydrozoan, on Posidonia leaves Epiphytic fauna on Posidonia oceanica. Although several animals are epiphytic on the leaves of Neptune grass, only cnidarians and bryozoans include species which are so highly specialised that they cannot be found on any other substrate. They are called exclusive characteristic species. Their level of specialisation is such that they dominate over the many other leaf colonisers, and sometimes even make up 95% of the entire epiphytic population. As Neptune grass is a Mediterranean endemic, these species are also endemic, although their distribution is similar throughout the Mediterranean so that, apart from a few exceptions, epiphytic fauna exclusive to Neptune grass is always the same, independently of the geographic area within the Mediterranean. Bryozoans are generally the most numerous animals on the surface of leaves, with the exclusive species Electra posidoniae. This is a cheilostome forming ribbon-shaped, slightly calcitic, encrusting colonies. Other bryozoans exclusive to Neptune grass leaves are cheilostomes that produce encrusting colonies which are, however, round instead of ribbonshaped, like Collarina balzaci, Fenestrulina joannae and Ramphostomellina posidoniae. The last species is so far known to inhabit only the Aegean Sea, and may therefore be an exception to the general rule that epiphytic fauna on Posidonia is found throughout the Mediterranean. Many other bryozoans are found on leaves, although not exclusive to them. Among ctenostomes there is Mimosella verticillata, M. gracilis and Pherusella tubulosa. Cheilostomes are represented by species of the genus Aetea (A. anguina, A. lepadiformis, A. sica, A. truncata), Celleporina caliciformis, Chorizopora brongniartii, Fenestrulina malusii, Haplopoma impressum and Microporella ciliata. Cyclostomes are Disporella hispida, Patinella radiata and Tubulipora plumosa. Hydroids may also be quantitatively and qualitatively very important. Four species are typical and exclusive: Aglaophenia harpago, with feather-like colonies about 15 mm high, Orthopyxis asymmetrica (= Campanularia a.), with stolon-shaped colonies a few millimetres high, Pachycordyle pusilla, which also has stolon-like colonies with naked polyps, and Sertularia perpusilla, with erect colonies up to 7 mm high. A. harpago and P. pusilla are not precisely exclusive, as they have also been found on seahorse grass. There is also a Sertularella species that may be exclusive to Posidonia leaves, but experts still need to describe it formally. From the ecological-evolutionary viewpoint, an interesting case is afforded by Monotheca obliqua. This species has pinnate colonies as high as 40 mm, and is found on various substrates and on Posidonia leaves. Some morphological differences have led experts to distinguish a typica form - ecologically 77
Page 1 and 2: Fauna: invertebrates MARIA CRISTINA
Page 3 and 4: 56 Vagile invertebrate fauna associ
Page 5 and 6: 60 The decapod Hippolyte inermis ca
Page 7 and 8: 64 Vagile invertebrates associated
Page 9 and 10: 68 of the genus Gammarus, with spec
Page 11: 72 level, conditions gradually beco
Page 15 and 16: 80 secundaria. Leaf tip colonisatio
Page 17 and 18: 84 The tubicolous polychaete Sabell
Page 19 and 20: 88 about Nanozostera noltii, very l
Page 21 and 22: Blotched picarel (Spicara maena) se
Page 23 and 24: comber (S. scriba). The blacktailed
Page 25 and 26: 100 The striped red mullet (Mullus
Page 27 and 28: 104 Vertical distribution of main f
Page 29 and 30: seabream, two-banded seabream, annu

Fauna: invertebrates - Udine Cultura

Create successful ePaper yourself

Delete template?

Save as template?