Recruitment Ecology
The larval phase of many marine invertebrates is their only dispersal phase. This means that once they decide to settle and metamorphosis they have committed to a place to live for the rest of their lives. This decision of where to settle is critical to survival, thus there is strong selective pressure for larvae to detect and discriminate among benthic habitats. Importantly, recruitment can determine organismal abundance by increasing the number of new individuals that arrive and contribute to a local population.
Recruitment is a complex process that integrates the availability of larvae, their settlement behavior and their post-settlement survival and growth. Each of these life history stages are critical for successful recruitment and many ecological factors contribute to success at each stage. Fundamentally a good habitat is critical to recruitment success. My research focuses on increasing our understanding of each of these life history stages to better predict the trajectory of populations in current and future scenarios.
My recent research has focused on coral recruitment since it remains poorly studied but is a critical process necessary for reef recovery (Ritson-Williams et al., 2009). Corals are the ecosystem engineers of coral reefs but we still don’t know what is “good habitat” for coral recruitment. How can we attempt to manage coral reefs for resilience if we don’t know what is good recruitment habitat?
There are a variety of life history strategies to produce enough larvae to maintain populations. Some marine invertebrates produce larvae that are ready to settle almost immediately and some spawn their eggs and sperm into the water column allowing embryos to develop into larvae in the water column. As modern threats reduce the populations of many marine organisms there is great potential to reduce population density enough so that the organisms that spawn their gametes do not have high enough concentrations of gametes to ensure fertilization success (allee effect). This maybe true for spawning coral species and especially dire for larval production as spawning only occurs once or twice a year. However some coral species (especially the brooders) produce larvae every month. This dichotomy between annual spawning and a bet hedging strategy might be an important adaptation for survival in variable habitats, and is the topic of a journal article in preparation.
The health and condition of parental coral colonies might have an important effect on larval condition and performance. Maternal effects can influence larval phenotypes and have been shown for many marine invertebrates but are only starting to be studied in corals. Recently, in collaboration with Dr. Hollie Putnam, I tested how coral colonies exposed to ocean acidification conditions reproduce. After the adult coral colonies were exposed to OA they produced fewer larvae that had higher larval survival and higher post-settlement growth than the larvae from control adults. This research is very exciting because it shows a novel mechanism for acclimation in corals. It may be that some corals can modify their larval phenotypes to create offspring with greater success. I am conducting further experiments to elucidate the potential for maternal conditions to influence larval phenotypes.
Larvae are typical considered a vulnerable stage of an organism, but this is rarely tested. We tested how competition with benthic algae might inhibit larval settlement. We found that algae and cyanobacteria both inhibit settlement success of coral larvae (Kuffner et al., 2006). Further work with these algae showed that this competition was often driven by natural products found in the algae (Paul et al., 2011). In multiple experiments we found that there was greater mortality in response to algal competition at the larval stage than in juvenile or adult corals (Paul et al., 2011; Olsen et al., 2013).
By measuring antioxidant enzymes, along with Dr. Cliff Ross, I have been able to measure sublethal stress in larvae and juvenile corals. Consistently when larvae are exposed to a stress (we have tested temperature, competition with benthic algae and exposure to red tide dinoflagelates) they experience oxidative stress (Ross et al., 2010; Olsen et al., 2013; Olsen et al., 2014). Larvae exposed to a short term temperature stress survived and settled at a normal rate but had much lower post-settlement survival than the control larvae (Ross et al., 2013). This shows that sublethal stress can have a serious effect on organisms’ survival even if it is not immediately detected. These latent effects could have serious effects on an organisms’ demography, but they are rarely tested.
In addition to benthic habitats that reduce coral recruitment I work with both spawning and brooding coral species to determine what type of habitat will increase larval settlement and post-settlement survival. Using a few species of Caribbean spawning corals (Acropora palmata, A. cervicornis, Diploria strigosa, Orbicella faveolata), along with Dr. Suzy Arnold and Dr. Valerie Paul, I have shown that some species of crustose coralline algae (CCA) can increase the rates of larval settlement. However, not all species of CCA increase coral settlement (Ritson-Williams et al., 2010; Ritson-Williams et al., 2016). This is an important distinction, only a few species of CCA increase coral settlement and these are often rare on modern reefs (Ritson-Williams et al., 2014). Since we rarely see the coral larvae settle directly on the CCA surfaces this research suggests that corals use benthic cues to sense the quality of their “neighborhood”. If a species of CCA that has the same physical requirements (light, water motion, etc.) and can survive at a settlement site then it should be suitable for the coral also. This broad scale question of what habitat do marine organisms require for recruitment is very timely, and needs to be characterized for us to understand how natural processes will be disrupted by local and global stressors.
Working with some Caribbean brooding corals I* found that many of these species will settle in response to microbial biofilms. These brooders also settle at high rates in response to some species of CCA, but are much more likely to settle on a biofilm than the larvae of spawning corals (Ritson-Williams et al., 2016). Thus the brooding corals don't require the same “neighborhood” as the spawners. This difference in settlement habitats among coral species is an important discovery. It means that a degraded habitat that only has microbial biofilms but not facilitating CCA species will be much more likely to be colonized by brooding corals compared to spawning species. We are already seeing signs of differential recruitment among coral species in the Caribbean suggesting that modern reefs are already degraded.
After successful settlement coral larvae metamorphose into a single polyp recruit. This recruit is extremely vulnerable to predation and competition. Again the larvae make a very important decision for an adult when they settle, especially for marine benthic creatures like coral that can not move after settlement. Post-settlement survival and growth is poorly understood in corals but my research again highlights a dichotomy between spawning and brooding corals. In a direct comparison of post-settlement survival I* found that brooding corals had a greater rate of survival than spawning corals (Ritson-Williams et al., 2016). This suggests that reefs of the future will be dominated by these brooding species and the spawning species will be increasingly rare, even though they often are the large habitat forming species. Unless we can manage reef habitats for improved “neighborhoods” we will likely see a great reduction in coral diversity.
Some marine species have very specific settlement habitats. This is especially true of specialist predators that only eat a few species of a prey. With Dr. Valerie Paul and Sonia Shjegstad, I studied Phestilla spp., a group of nudibranchs that eat coral. I found evidence for species specific patterns of host use (Ritson-Williams et al., 2003). Within this genus of nudibranch some species have larvae with very short planktonic durations ranging to one species with planktotrophic larvae that probably disperse for longer than a month (Ritson-Williams et al., 2007). Interestingly the length of dispersal correlated to the diet breadth in these nudibranchs; the species with the shortest planktonic larval duration only ate one or two species of corals but the species with planktotrophic larvae consumed multiple genera of coral within a family. Since these nudibranchs are specialists they could not survive without their host corals, and I showed that they required water-soluble cues from their hosts for settlement and metamorphosis (Ritson-Williams et al., 2009). There must be strong selective pressure for an obligate coupling of settlement cues with their prey species since the nudibranchs’ diets are so specialized.
*along with many great collaborators
Recruitment is a complex process that integrates the availability of larvae, their settlement behavior and their post-settlement survival and growth. Each of these life history stages are critical for successful recruitment and many ecological factors contribute to success at each stage. Fundamentally a good habitat is critical to recruitment success. My research focuses on increasing our understanding of each of these life history stages to better predict the trajectory of populations in current and future scenarios.
My recent research has focused on coral recruitment since it remains poorly studied but is a critical process necessary for reef recovery (Ritson-Williams et al., 2009). Corals are the ecosystem engineers of coral reefs but we still don’t know what is “good habitat” for coral recruitment. How can we attempt to manage coral reefs for resilience if we don’t know what is good recruitment habitat?
There are a variety of life history strategies to produce enough larvae to maintain populations. Some marine invertebrates produce larvae that are ready to settle almost immediately and some spawn their eggs and sperm into the water column allowing embryos to develop into larvae in the water column. As modern threats reduce the populations of many marine organisms there is great potential to reduce population density enough so that the organisms that spawn their gametes do not have high enough concentrations of gametes to ensure fertilization success (allee effect). This maybe true for spawning coral species and especially dire for larval production as spawning only occurs once or twice a year. However some coral species (especially the brooders) produce larvae every month. This dichotomy between annual spawning and a bet hedging strategy might be an important adaptation for survival in variable habitats, and is the topic of a journal article in preparation.
The health and condition of parental coral colonies might have an important effect on larval condition and performance. Maternal effects can influence larval phenotypes and have been shown for many marine invertebrates but are only starting to be studied in corals. Recently, in collaboration with Dr. Hollie Putnam, I tested how coral colonies exposed to ocean acidification conditions reproduce. After the adult coral colonies were exposed to OA they produced fewer larvae that had higher larval survival and higher post-settlement growth than the larvae from control adults. This research is very exciting because it shows a novel mechanism for acclimation in corals. It may be that some corals can modify their larval phenotypes to create offspring with greater success. I am conducting further experiments to elucidate the potential for maternal conditions to influence larval phenotypes.
Larvae are typical considered a vulnerable stage of an organism, but this is rarely tested. We tested how competition with benthic algae might inhibit larval settlement. We found that algae and cyanobacteria both inhibit settlement success of coral larvae (Kuffner et al., 2006). Further work with these algae showed that this competition was often driven by natural products found in the algae (Paul et al., 2011). In multiple experiments we found that there was greater mortality in response to algal competition at the larval stage than in juvenile or adult corals (Paul et al., 2011; Olsen et al., 2013).
By measuring antioxidant enzymes, along with Dr. Cliff Ross, I have been able to measure sublethal stress in larvae and juvenile corals. Consistently when larvae are exposed to a stress (we have tested temperature, competition with benthic algae and exposure to red tide dinoflagelates) they experience oxidative stress (Ross et al., 2010; Olsen et al., 2013; Olsen et al., 2014). Larvae exposed to a short term temperature stress survived and settled at a normal rate but had much lower post-settlement survival than the control larvae (Ross et al., 2013). This shows that sublethal stress can have a serious effect on organisms’ survival even if it is not immediately detected. These latent effects could have serious effects on an organisms’ demography, but they are rarely tested.
In addition to benthic habitats that reduce coral recruitment I work with both spawning and brooding coral species to determine what type of habitat will increase larval settlement and post-settlement survival. Using a few species of Caribbean spawning corals (Acropora palmata, A. cervicornis, Diploria strigosa, Orbicella faveolata), along with Dr. Suzy Arnold and Dr. Valerie Paul, I have shown that some species of crustose coralline algae (CCA) can increase the rates of larval settlement. However, not all species of CCA increase coral settlement (Ritson-Williams et al., 2010; Ritson-Williams et al., 2016). This is an important distinction, only a few species of CCA increase coral settlement and these are often rare on modern reefs (Ritson-Williams et al., 2014). Since we rarely see the coral larvae settle directly on the CCA surfaces this research suggests that corals use benthic cues to sense the quality of their “neighborhood”. If a species of CCA that has the same physical requirements (light, water motion, etc.) and can survive at a settlement site then it should be suitable for the coral also. This broad scale question of what habitat do marine organisms require for recruitment is very timely, and needs to be characterized for us to understand how natural processes will be disrupted by local and global stressors.
Working with some Caribbean brooding corals I* found that many of these species will settle in response to microbial biofilms. These brooders also settle at high rates in response to some species of CCA, but are much more likely to settle on a biofilm than the larvae of spawning corals (Ritson-Williams et al., 2016). Thus the brooding corals don't require the same “neighborhood” as the spawners. This difference in settlement habitats among coral species is an important discovery. It means that a degraded habitat that only has microbial biofilms but not facilitating CCA species will be much more likely to be colonized by brooding corals compared to spawning species. We are already seeing signs of differential recruitment among coral species in the Caribbean suggesting that modern reefs are already degraded.
After successful settlement coral larvae metamorphose into a single polyp recruit. This recruit is extremely vulnerable to predation and competition. Again the larvae make a very important decision for an adult when they settle, especially for marine benthic creatures like coral that can not move after settlement. Post-settlement survival and growth is poorly understood in corals but my research again highlights a dichotomy between spawning and brooding corals. In a direct comparison of post-settlement survival I* found that brooding corals had a greater rate of survival than spawning corals (Ritson-Williams et al., 2016). This suggests that reefs of the future will be dominated by these brooding species and the spawning species will be increasingly rare, even though they often are the large habitat forming species. Unless we can manage reef habitats for improved “neighborhoods” we will likely see a great reduction in coral diversity.
Some marine species have very specific settlement habitats. This is especially true of specialist predators that only eat a few species of a prey. With Dr. Valerie Paul and Sonia Shjegstad, I studied Phestilla spp., a group of nudibranchs that eat coral. I found evidence for species specific patterns of host use (Ritson-Williams et al., 2003). Within this genus of nudibranch some species have larvae with very short planktonic durations ranging to one species with planktotrophic larvae that probably disperse for longer than a month (Ritson-Williams et al., 2007). Interestingly the length of dispersal correlated to the diet breadth in these nudibranchs; the species with the shortest planktonic larval duration only ate one or two species of corals but the species with planktotrophic larvae consumed multiple genera of coral within a family. Since these nudibranchs are specialists they could not survive without their host corals, and I showed that they required water-soluble cues from their hosts for settlement and metamorphosis (Ritson-Williams et al., 2009). There must be strong selective pressure for an obligate coupling of settlement cues with their prey species since the nudibranchs’ diets are so specialized.
*along with many great collaborators