Invertebrates

In comparison to epiphytic macroalgae, there are markedly fewer studies of the distribution and abundance of epiphytic macroinvertebrates. Unlike algal

Table 2. Contribution of epiphytic algae to total above-ground biomass as a percentage of seagrass + epiphytes in different seagrass meadows.

Seagrass

Range (%)

Source

Posidonia sinuosa

0-16

McMahon et al. (1997)

10-32

Kirkman and Manning (1993)

Posidonia coriacea

0.3-42

Unpublished

Thalassia hemprichii

29-42

Moncreiff et al. (1992)

9

Jagtap (1998)

Thalassia testudinum

31-44

Wear etal. (1999)

Amphibolis griffithii

7-75

Unpublished

Amphibolis antarctica

41-47

Kirkman and Manning (1993)

Syringodium isoetifolium

31

Heijs (1985b)

29-95

Mukai and Ishijima (1995)

Syringodium filiformis

37-41

Wear etal. (1999)

Cymodocea rotundata

44

Heijs (1985b)

Cymodocea serrulata

53

Heijs (1985b)

Heterozostera tasmanica

0-8.2

Unpublished

Zostera marina

1-36

Moncreiff et al. (1992)

Halodule uninervis

49

Heijs (1985b)

Halodule wrightii

19-68

Moncreiff et al. (1992)

Halodule wrightii

44-52

Wear etal. (1999)

Thalassodendron ciliatum

68.5

Bandeira (2002)

Heterozostera tasmanica/Zostera

49.5

Ierodiaconou and Laurenson (2002)

marina/Ruppia megacarpa marina/Ruppia megacarpa epiphytes where there is no clear evidence that any of the algae are obligate seagrass epiphytes, there are several reports of obligate invertebrate epiphytes of seagrasses (e.g. Hughes et al., 1991a). As with epiphytic algae, there are distinct spatial patterns within plants, such as between the stems and leaves on species of Amphibolis spp. (Borowitzka et al., 1990; Edgar and Robertson, 1992) and along leaves in other species such as P. oceanica (Casola et al., 1987). There is a general inverse relationship between the abundance of epiphytic invertebrates and algae, with the algae more abundant near the plant apex, whereas the invertebrates are most abundant on the lower parts of the plant leaves or stems. Bry-ozoans, hydroids, and ascidians tend to be more common as epiphytes on seagrasses with long-lived parts, such as Amphibolis spp. (Borowitzka et al., 1999) or the rhizomes of P. oceanica (Colmenero and Lizaso, 1999). On P. sinuosa and P. australis leaves, hydrozoans were more abundant on the lower part of leaves, other taxa showed no strong trend in distribution along leaves (species of bryozoa, porifera, and foraminifera) and a of spirorbid polychaete was more abundant near the apex (Trautman and Borowitzka, 1999). A similar distributional pattern was observed on Z. marina leaves (Nagle, 1968). In addition, some epiphytic invertebrates, as with algae, showed a preference for the concave side of P. sinuosa leaves, in cluding a species of porifera and a hydrozoan. The reasons for these spatial patterns are not clear, but may be related, in some cases, to hydrodynamics around the leaf surface (Trautman and Borowitzka, 1999).

Substratum availability is not the sole determinant of epiphytic invertebrate abundance. The density of tunicates on Z. marina is influenced by the particular species of amphipod grazers on seagrasses (Duffy and Harvilicz, 2001). Light availability also has been suggested as a factor negatively influencing the abundance of epiphytic invertebrates. For example, P. oceanica assemblages tend to be dominated by epiphytic algae at shallow depth (10 m) but in deeper waters (20-30 m) epiphytic invertebrates dominate, with their contribution to biomass increasing from about one-third to >50% of the total epiphytic biomass (Lepoint et al., 1999).

The motile epifauna has received much more attention than the attached epiphytic invertebrates. Jer-nakoff et al. (1996) have reviewed the literature comparing the motile epifauna between different types of seagrass and concluded that there were few differences in the composition of motile epiphytic grazers that could be clearly related to the form of the sea-grass host, though there were differences in the abundances. Within a species of seagrass host, density of the seagrass does appear to affect motile epifau-nal abundance; for example, Edgar and Robertson (1992) noted that more open stands of Amphibolis spp. were relatively depauperate in motile epifauna compared with dense stands.

The abundance and distribution of epiphytic macroinvertebrate grazers is strongly influenced by the abundance and distribution of the epiphytic algae or periphyton on which they graze (Bologna and Heck, 1999; Fong et al., 2000; Valentine and Duffy, Chapter 20, Sections V to VIII). The density of grazers is a function of both seagrass habitat structure and the trophic attraction of the habitat. The relative importance of these two factors appears variable, with different studies weighting each factor differently. Artificial seagrass has been used to examine the relative roles of these two factors and showed that heterogeneity of a habitat was not, alone, the major determinant of epiphytic grazer biomass; trophic attractiveness of the habitat appears to be more important (Bologna and Heck, 1999; Bostrom and Mattila, 1999). However, it seems that different grazers respond to different factors and a comparison of fauna on A. griffithii and P. sinuosa found that the abundance of amphipod and gastropod grazers correlated with food availability on P. sinuosa and with the biomass of leaves on A. griffithii, suggesting that in A. griffithii provision of cover was more important (Jernakoff and Nielsen, 1998).

Was this article helpful?

0 0

Post a comment