Métodos

Colony morphology can be measured as a quantitative trait using surface index (hereafter SI; Dahl, 1973) which is the ratio between the surface area of the colony (in cm2) and planar colony area (also in cm2). SI is different from most traditional rugosity measurements since it is an area ratio rather than a length ratio. The rugosity of a coral colony is the distance ratio of the colony contour and the bisecting planar area (see Figure 2.1). While SI and rugosity are clearly related, SI is preferable since it considers the colony as a three-dimensional object.

CH2: Review of Traits Calculating the SI requires accurate measurement of the three-dimensional surface area of the coral colony. Techniques previously used for calculating surface area in order of increasing complexity are: simple and advanced geometry, foil wrapping, wax coating, planar projection photography, computer tomography and 3-D surface reconstruction, 3-D laser scanning (Raz-Bahat et al., 2009) and X-ray computer tomography (CT) scanning. Recent studies comparing the accuracy of these techniques (Naumann et al., 2009, Veal et al., 2010) have shown that accuracy depends on the morphology of the colony; certain techniques work better for certain colony morphologies.

Creating a species-level database of SI indices for use in trait-based studies and for estimation of reef surface area could be done using the techniques listed above in combination with museum collections of coral skeletons available worldwide. As with colony morphology, SI is likely to be environmentally plastic. Therefore the plasticity of the SI of each species should be quantified by looking at skeletons collected along environmental gradients (i.e. depth, flow, light) if possible. Also, since the surface area is relative to the scale at which a coral colony is measured (colony, corallite, cell, atom etc.) and scale of measurement attainable is dependant on the technique used, care must be taken in comparing surface areas obtained by different techniques.

Building a species or genus level SI database would allow us to estimate 3-D surface area of coral tissue on a reef knowing only the 2-D coverage of each coral species (or genus) since:

SI = 3D colony surface area / 2D colony surface area and,

3D colony surface area = 2D colony surface area x SI

This estimate of the biologically active surface area of a reef could be further refined by using the trait’s corallite spacing and polyp surface area (discussed later) as follows.

CH2: Review of Traits Number of polyps on the reef = 3D colony surface area x corallite spacing Total polyp surface area of reef = # of polyps on reef x polyp surface area However until species-level SI data accumulates, incorporating SI indices is limited to the level of major growth forms (See Table 2.2 adapted from Holmes, 2008).

Table 2.5 Surface indices for six major types of coral colony morphology based on 158 coral skeletons from more than 25 genera and up to 75 cm in diameter (adapted from Holmes, 2008)

Colony morphology Surface index

Massive 3.2 Sub-massive 5.9 Foliose 3.04 Open branching 6.16 Complex branching 6.43 Tabular 2.47 2.3.1.4. Attachment to reef

Coral attachment to reef is a trait with attributes: 1) obligate free-living corals, 2) facultative free-living corals, and 3) obligate attached corals. Obligate free-living corals are always free-living in their adult state. Facultative free-living corals are sometimes free-living in their adult form but also are commonly attached to the substrate. Obligate attached corals are never found as free-living adults. If a more detailed categorization is preferred facultative corals can further be subdivide into a) bushy coralliths, b) submassive coralliths, c) free-living plates, d) polyp balls, e) cones and f) free-living flabellomeandroid.

No Atlantic obligate free-living corals and only two Atlantic facultative free-living corals are known (Mainicina areolata and Meandrina braziliensis). In comparison, the Red Sea and Indo-pacific contains at least 52 species and 17 genera of obligate free living corals (Veron and Stafford-Smith, 2002).

CH2: Review of Traits Obligate free-living corals commonly have disc, dome, oval or hourglass shaped morphologies. Obligate free-living corals include all eleven genera of the fungiid family, three small hourglass shaped corals (Heteropsammia cochlea, Heterocyathus aequicostatus, and Balanophyllia grandis) and one flabello-meandroid coral (Tachypyllia geoffroyi).

Many faculatative free-living branching and encrusting coral species can form coralliths. Branching species form coralliths asexually via fragmentation while massive species form coralliths sexually by colonizing small pieces of rubble. Free- living plates result from detaching plates. The ability to form either coralliths or plates is a potentially important trait since it allows reefs to expand onto sandy bottoms without relying on free solid substrate (Sheppard, 1981).

While corallith formation is the most common free-living form for facultative free- living corals, other forms also occur. Manicina areolata, and Cynarina lacrymalis occasionally detach from the substrate to form free-living cones. One flabello- meandroid species, Meandrina braziliensis, is known to be a facultative free-living coral. Finally, Gonipora stokesi can develop polyp balls that detach from the main colony and roll away onto nearby soft sediments thereby acting as nuclei for the extension of reef (Sheppard, 1981). Corallith formation may be for the purpose of asexual reproduction. However, sometimes free-living coralliths do not reattach and form large extensive unattached reefs, often in shallow protected habitat (for example Glynn, 1974, Scoffin et al., 1985, Roff, 2007).