3.4 Cryptic Species
What is a Cryptic Species?
two or more morphologically indistinguishable biological groups that are incapable of interbreeding (Merriam-Webster)
A species which looks very similar to another, but which is reproductively isolated from it (Collins)
more and more being discovered with molecular work
Do cryptic species occupy the same habitats? Are they spatially variable? Are they always segregated across depths? How is variation across depths compared with across-shelf locations (onshore/offshore)?
Patterns of cryptic species have varied dramatically, but multiple studies have found two distinct causes:
Depth segregated: cryptic lineages are segregated by depth, with one or more lineage specifically found along a light gradient or in different parts of the reef. This may be caused by sympatric reproductive isolation associated with the time that gametes are released into the water. It is also likely due to fixed traits in each lineage that have made them specialized to their habitat. In this case, spawning and recruitment would occur and the lineages would mix along the light gradient, but the likelihood of survival to adulthood would be based on the parent's native habitat. Prada & Hellberg 2013 found depth segregated lineages in octocoral E. flexuosa. Rippe et al. 2020 also found depth segregated cryptic lineages.
Spawning segregated: sympatric lineages become reproductively isolated by individual colonies reproducing at different times of year (Rosser 2015). This prevents gametes from mixing and over multiple generations this can cause adaptations associated with the different spawning populations.
Geographic segregation: Most studies have not found genetic variation across geographical locations. As organisms that release their recruits or gametes into the water, there is usually high dispersal across geographic area. Pipithukal et al. 2021 found geographically isolated cryptic lineages in Japan.
Are they often locally adapted? What is the level of plasticity?
Yes, there is local adaptation associated with cryptic lineages. Rippe et al. 2021 found that juveniles were less likely to survive to adulthood when they were in the 'wrong' habitat.
I don't have any evidence on the level of plasticity
Are they morphologically different? Describe both micromorphology and macromorphology examples of cryptics segregated across habitats.
Typically, cryptic species are not drastically morphologically different, or their morphological range overlaps, making them indistinguishable. However, some studies have been able to find differences in morphology associated with genetic differences.
Budd et al. 2012 found that MCAV had high levels of polymorphism especially in morphology that was not associated with cryptic lineages.
Rosser 2015 also found morphological differences associated with the cryptic lineages.
Are they physiologically differentiated? What is the evidence?
Physiological differences occur on a case-by-case basis, and few studies have specifically investigated coral physiology associated with cryptic lineages.
However, they should exhibit physiological differences associated with their life history.
reproductive isolation in different habitats would result in natural selection or other processes of evolution that would result in the fixation of traits (SNPs). While these traits may not be visible to the human eye, it is likely that there are non-visible traits that are different
ex. heat stress acclimation
Johnson et al. 2022 found that the haplotypes they identified were constrained my environmental variables, suggesting that there are physiological reuqirements associated with the cryptic lineages.
Do they associate with different symbionts? Different microbiota?
Yes, see symbionts.
Prada et al. 2022 found that microbiome changed with depth but symbiont association change with species
Bongaerts et al. 2010 found cryptic lineages along a depth gradient and that their symbionts co-varied along the gradient as well (codiversification)
Is there evidence of heat-stress variation? Variation in pH sensitivity? Oxygen?
Gomez-corrales & Prada 2020 found that cryptic lineage association determined stress response to bleaching.
References
Prada & Hellberg 2013
Reproductive isolation in an octocoral species led to depth-segregated cryptic lineages. The lineages changed more along depth gradients than geographically, across the Caribbean basin.
Prada et al. 2022
The authors conducted reciprocal transplants on OFRA and OANN in panama and measured changes in symbiont type through ITS2 and microbiome. They found that microbiome is affected by depth but not by species and symbiont community is affected by species by not depth. They did not test the scleractinian for cryptic lineages.
Pipithukal et al. 2021
The authors investigated the population structure of Acropora pruinosa in Japan, finding two cryptic populations segregated by geography. There was very low genetic diversity within each population, and very low connectivity between them.
Rippe et al. 2021
Authors found cryptic lineages in MCAV and Siderastrea siderea along a depth gradient. The two species had notably similar genetic structures, with two lineages in shallow and two lineages in deep each. There was minimal gene flow between the two lineages at the same depth, and high site faithfulness from juveniles to adults. This work provides support against the deep reef refugia hypothesis.
Rosser 2015
Two cryptic lineages were idntified in Acropora samoensis associated with seasonal differences in spawning time (spring and autumn). This caused sympatric reproductive isolation, and led to two very distinct genetic groups that also had morphological differences.
Johnston et al. 2022
Genetic diversity analysis of ~650 samples of Pocillopora spp. in Mo'orea found 14 unique haplotypes which included 6 previously identified species. The haplotypes varied by depth more than geographical location. Additionally, many haplotypes were constrained by specific environmental parameters such as PAR and temperature.
Gomez-Corrales & Prada 2020
Reanalyzed previous data to reveal that OFAV exhibited multiple cryptic lineages. There was no trend in environment nor symbiont type with the lineages. Emphasizes the importance of understanding cryptic diversity for understanding other patterns in morphology & physiology, accurately quantifying population sizes and genetic diversity. Found trends in stress tolerance associated with the lineages.
Bongaerts et al. 2010
The authors evaluated the genetic diversity of a coral species and its symbionts in three sites along a depth gradient. They found genetic partitioning of both coral host & symbiont across depths, but not sampling location. There was codiversification of the symbiont and host to specific environments.
Budd et al. 2012
High levels of polymorphism and gene flow in MCAV were not associated with cryptic lineage. The authors found two morphotypes in modern MCAV and four in the fossil record. Molecular analysis did not find any evidence of lineages varying by site or depth.
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