Zooplankton feeding by corals underestimated
Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
| Acropora palmata; a keystone species in trouble |
| Written by Tim Wijgerde |
|
Acropora palmata is one of the dominant reef-building coral species in the Caribbean, next to Acropora cervicornis. It is a so-called keystone species. In the last few decades, populations of A. palmata and A. cervicornis have declined substantially (van Oppen & Gates, 2006; Baums et al, 2005). In the past, several theories have been proposed to address this question. A genetic break-up Recent understanding of coral reef population genetics has shed new light on the fragility of individual reefs. It is now known that Acropora palmata displays little to no genetic exchange between the western and eastern Caribbean (Baums et al, 2005), implying that its larvae are not able to travel over distances of this magnitude. A. palmata is known to spawn every year, in August. This usually occurs after the august full moon, during which all colonies spawn in synchrony (Szmant, 1986). The subsequent larval dispersal is dependent on many factors, of which ocean currents are most influential. Figure 1 shows a surface current plot of the Caribbean, for August. Red lines indicate strong currents, whereas blue lines indicate low ones. In the Caribbean, Mona Island and Puerto Rico form a boundary between the west and east. This region displays an area of ocean current mixing, which is nicely reflected by the genetics of these A. palmata populations (Baums et al, 2005). Statistical analysis showed that the genotyped (by using microsatellite markers , pieces of DNA which vary between individual coral colonies) individuals fell into two clusters, whereas the Puerto Rico and Mona Island could be assigned to either group. No other localities showed the same feature, indicating a west, east and mixed central region.
Figure 1: Average surface currents in the Caribbean in August. Dark red indicates highest energy (fastest currents), and blue indicates lowest. Acropora palmata spawns most often in August. Sampling localities are indicated by ovals. Red ovals, localities in the western Caribbean and the Bahamas: Panama (PA), Mexico (ME), Florida (FL), the Bahamas (BA), Navassa (NA); green ovals, localities in the eastern Caribbean: US Virigin Islands (VI), St Vincent and the Grenadines (SVG), Bonaire (BO) and Curacao (CU). Red/green ovals, localities in an area of mixing (see text): Mona Island (MO) and Puerto Rico (PR). Plot courtesy of Z. Garraffo and E. Chassignet, University of Miami (Adapted from Baums et al., molecular ecology, 2005). Figure 2 shows a statistical analysis using the software program STRUCTURE. This program is able to read the DNA sequences, and categorize individuals in groups. It clearly shows that A. palmata can be subdivided into two genetically distinct populations. The top A panel was calculated without providing STRUCTURE with information about the geographical origin of samples. The lower B panel was generated by asking STRUCTURE to classify the genets (coral colonies which display small differences in their DNA sequences) sampled in Puerto Rico (designated as ‘unknown’) while inserting the geographical origin of all other genets. Two genetically distinct groups can be seen. Fig.2: Geographical subdivision of western and eastern Caribbean populations of Acropora palmata as inferred using STRUCTURE analysis of microsatellite markers. (A) Results when STRUCTURE was run without providing information about the geographical origin of samples. Two clusters are distinguishable, a western cluster with genets from Panama (PA), Mexico (ME), Florida (FL), the Bahamas (BA), Navassa (NA), Mona Island (MO) and Puerto Rico (PR) and an eastern cluster with genets from the US Virigin Islands (VI), St Vincent and the Grenadines (SVD), Bonaire (BO) and Curacao (CU). (B) Results when STRUCTURE was asked to classify the genets sampled in Puerto Rico (designated as ‘unknown’) while having the geographical origin of all other genets designated a priori (Adapted from Baums et al., Molecular ecology, 2005). Given the data provided above, it becomes clear that A. palmata, next to numerous other Scleractinians, is not able to genetically mix western and eastern populations. This is due to the limitations of larval dispersion of subpopulations. Next to A. palmata, many corals display restricted gene flow (Table 1). The markers which are in use today, as depicted in Table 1, vary. Some researchers focus on enzymatic sequences such as allozymes, whereas others choose ITS (internal transcriber region) or microsatellites . The latter two are non-coding DNA sequences, which display subtle nucleotide differences (also called SNPs, or single nucleotide polymorphisms). These sequences have proven to be useful for genotyping. In contrast, allozyme sequences are coding, which is subject to evolutionary selection. This influences the genealogic genotyping on a large time-scale, as selective mutations do not display a linear correlation with time, in contrast to non-coding mutations (the so-called “molecular clock’’). However, as the determination of specific time-points of coral divergence is not the aim of population genetics, it could be argued that allozymes constitute a proper way of genetic identification. Table 1: Summary of the available literature on genetic connectivity in scleractinian corals. Note that the gene flow is low for many species! N.t., not tested; N.a., not applicable; GBR, Great Barrier Reef (Adapted from van Oppen et al., Molecular Ecology, 2006)
Small-scale conservation Given the current large-scale status of coral reef ecoconservation, it is clear that the tides have to be turned by altering this strategy. Conservation should be more focused on smaller-scale preserves, in adequate numbers, to physically link the subpopulations together. Only this will ensure a proper recolonization between subpopulations. References:
|
