Traits Attribute example* Status Section
2.7. Larval traits
2.7.1.Larval traits with sufficient data
The distance at which a larva settles from its parent is determined to large extent by environmental factors that interact with a suite of species-specific larval traits and this interaction ultimately determines the settlement success, dispersal distance, biogeographic patterns and abundance of Scleractinian corals.
Combining larvae survival times with distance to down-stream reefs and current speed allows for calculation of connectivity between reefs and highlights reefs that rely largely on self-seeding. Larval mortality is often due to starvation (Strathmann, 1985), predation (Thorson, 1950), physiological stress resulting from suboptimal environmental conditions (Pechenik, 1987), disease and genetic abnormalities (Rumrill, 1990). Estimating species-level differences in larval mortality rates in situ may well be impossible, although, parametres associated with larval survival potential can be estimated from laboratory cultures.
Coral larval biology has recently been reviewed (Gleason and Hofmann, 2011). While data is accumulating about the autecology of coral larvae, data paucity for most traits prevents integration into trait-based analysis with a few notable exceptions.
CH2: Review of Traits 2.7.1.1.Larval association with symbionts
In the case of sexual reproduction, coral larvae can acquire symbionts through vertical transmission or by horizontal transmission (eg. Coffroth and Santos, 2005). Transmission mode is well documented (see Digital Supplement 1.1.5 adapted from Baird et al., 2009) with a number of important fitness implications.
Vertical transmission of symbionts occurs in all known brooding corals except the Isoporans, and in all spawning species examined to date for the genera Montipora,
Porites, Pocillopora, and Anacropora, while horizontal transmission occurs in the
remaining spawning corals. Thus generational shifts in symbiont populations present in the host can occur in spawning corals but tend not to occur in brooding corals (LaJeunesse, 2005). There are advantages and disadvantages with both vertical and horizontal transmission strategies. Vertical transmission guarantees that the offspring will establish successful association with Symbiodinium of the appropriate type. On the other hand, vertical transmission may prove metabolically expensive for the larvae to maintain thereby interfering with developmental processes, further, the environmental conditions in which the coral larvae settles may prove sub-optimal for the symbiont genotype (Douglas, 2008).
The advantage of acquiring Symbiodinium from the surrounding environment through horizontal transmission is that a higher Symbiodinium diversity within the coral host can be maintained, which increases the chance that Symbiodinium
populations in the holobiont will be maintained even under adverse environmental conditions. The risk of horizontal transmission is that the coral host may fail to acquire Symbiodinium from the surrounding environment (Genkai-Kato and Yamamura, 1999).
2.7.1.2.Egg and larval size
Both egg and larval length or biomasses reflect the energetic investment that each species makes in each reproductive unit; it may represent a key difference in sexual
CH2: Review of Traits
trait reproductive mode. Generally, egg size is larger in brooding species than in spawning species. The relationship between egg/larva size and reproductive mode remains to be formally summarized using a broad dataset.
Egg/larval size has a number of survival implications beyond the well observed fact that large larva of brooding species are competent for settlement much faster than small larvae from spawning corals (Richmond, 1997). Egg size has been compiled for at least ten Caribbean species (Szmant, 1986) and nearly 50 observations have been compiled for Indo-Pacific (Fadlallah, 1983). Since these studies more egg and larval size data have become available but remain to be compiled. The utility of egg and larval size as a trait will depend on the quantity of trait data available for the species in a particular area.
2.7.1.3.Egg colour
Egg colour reflects pigmentation and possibly the eggs ability to protect itself against harmful radiation. Egg colour is easily observed and has been well documented for corals on the GBR with Babcock et al. (1986) providing a summary of egg colour for nearly 100 species. Therefore egg colour can reasonably be included as a trait for many Indopacific locations with current levels of data availability.
2.7.1.4.Larval motility
Larvae may swim or crawl epibenthically. Larvae from spawned corals swim (I could find no records of spawned larvae that crawl) while larvae from brooded corals have been observed to swim or crawl (Fadlallah and Pearse, 1982, Fadlallah, 1983, Paz-Garcia et al., 2007). The motility mode has implication for dispersal distance. The last large-scale summary of larval motility was nearly three decades ago (Fadlallah, 1983).
2.7.2.Larval traits with data paucity
Three important larval traits with data paucity are larval metabolic constraints, competency period, and position in the water column over time. Between these three
CH2: Review of Traits
trait categories I could only find species-level data for 26 species. Here I briefly discuss the importance of each of these traits:
• Based on laboratory cultures, there appears to be species-level differences in both the median and maximum larval lifetime, in other words, different coral species larvae starve at different rates. Graham et al. (2008) observed large differences in both the 50 percent mortality and maximum survival time for five coral species (all broadcasters) and suggested that larval mortality curves based on metabolic constraints is a potentially important trait.
• Larval competency period is not the same as maximum survival times as
larvae lose their ability to recruit often well before death from starvation. The ratio between maximum survival and maximum competence has been calculated for soft corals (Ben-David-Zaslow and Benayahu, 1998) but not for Scleractinian corals. The time required after spawning or larval release to become competent varies greatly between species with brooding corals often having competent larvae within hours while for spawning corals it often takes days. Likewise the total amount of time larvae can remain in the competent stage varies between coral species.
• Tay et al. (2011) observed a downward shift in the vertical position of three coral species larvae in the water column. They noted that the sinking rate was inconsistent with peak settlement competency periods and suggested that such inconsistencies could have serious implication for the success of settlement in different coral species. A temporal inter-play between competency timing mortality rates and vertical movement are all-important in determining distribution. Therefore these three traits require further investigation.
CH2: Review of Traits 2.8. Conclusion
A comprehensive schema of coral life-history traits has been presented under the five categories: 1) morphological, 2) behavioural, 3) physiological, 4) phenological and 5) larval traits. Data availability varies greatly, as do units of measurement, and environmental plasticity of the traits.
Based on both suitability and availability for the species in Southwest Madagascar, 26 traits were selected to be included in this study; these traits are summarized in Table 2.7.
CH2: Review of Traits
Table 2.7 Summary of traits selected for use in this study