The genus Kryptolebias, Costa 2004

Kryptolebias was described by W. Costa (2004) on the basis of osteological differences (of four species previously assigned to Rivulus). In his paper he used the name Cryptolebias, but it was preoccupied by a Miocene fossil fish (from present-day Italy). Thus, Costa (2004a) was compelled to make an amendment, which became Kryptolebias. Vermeulen & Hrbek (2005) subsequently supported the generic status of Kryptolebias with mitochondrial DNA sequence evidence, describing a fifth member K. sepia.
Three of the four species in Costa's description had their habitat in south-east Brazil, not too far from the coastline. The fourth species is a marine hermaphrodite species living along the coast from Florida, the Caribbean Islands (including the easterly coastline of South America) down to the Brazil. The species described by Vermeulen & Hrbek, however, lives far from the other members, in Surinam. It is believed by the senior author that new expeditions in the huge area between the two localities will discover more species belonging to this genus.

Kryptolebias is an older genus than Rivulus. A plausible hypothesis is that Rivulus evolved from coastal/estuarine species, and adapted to fresh waters and inland habitats.

Kryptolebias is a killifish genus (family Rivulidae) composed of seven currently valid species (Berbel-Filho et al. 2022), although the number of species in the genus is likely to change as some taxonomic debates are still ongoing (BerbelFilho et al. 2022; Huber 2016). Phylogenetic analyses have indicated the presence of two distinct monophyletic clades within Kryptolebias, one of them composed of narrowly distributed freshwater species living in temporary streams and pools in South America: K. campelloi (Costa 1990) from North Brazil; K. sepia Vermeulen & Hrbek 2005 from Suriname; K. gracilis Costa 2007, and K. brasiliensis (Valenciennes 1821) from Southeast Brazil. The other clade is composed of three species living in mangrove forests along the tropical and subtropical western Atlantic basin, the 'mangrove killifish clade': K. marmoratus (Poey 1880), K. hermaphroditus sensu Costa 2011, and K. ocellatus (sensu Costa 2011) (Berbel-Filho et al. 2022; Costa, Lima, and Bartolette 2010; Murphy, Thomerson, and Collier 1999; Tatarenkov et al. 2009, 2017). Kryptolebias is a remarkable genus in many aspects. For instance, K. marmoratus and K. hermaphroditus sensu Costa 2011 are the only two vertebrates known to be capable of self-fertilization (Berbel-Filho et al. 2022), whereas K. ocellatus (sensu Costa 2011) is a hermaphroditic but obligate outcrossing species (Berbel-Filho et al. 2020). This variation in mating systems makes Kryptolebias a unique vertebrate system for investigating the genomic, physiological, and behavioral changes involved in the transition from outcrossing to selfing. In addition, K. marmoratus, the most well-studied Kryptolebias species, is considered a highly amphibious fish (Turko, Rossi, Berbel-Filho, Waldir M., Stacy Pirro, Andrew Thompson, Sergio M. Q. Lima, Sofia Consuegra, and Ricardo Betancur. 2023. "The Complete Genome Sequences of Three Species from the Killifish Genus Kryptolebias (Rivulidae, Cyprinodontiformes)." Biodiversity Genomes, May. https://doi.org/10.56179/001c.77448. and Wright 2021), with extreme physiological and behavioral adaptations to live out of water, in some cases for months (Taylor 1990). The amphibious nature of K. marmoratus is also likely to be valid for other Kryptolebias species, providing unique opportunities for studying the phenotypic and genomic changes involved in the transition from aquatic to terrestrial habitats.

 
Kryptolebias brasiliensis - male. Image made by F.Vermeulen.

To avoid long-term taxonomic confusion, we would like to provide some background on the taxonomic status of K. ocellatus (Sensu Costa 2011), whose genome was sequenced here. Due to morphological similarities and syntopy between species, the taxonomic status of the mangrove killifish clade has been historically confusing, particularly in Southeast Brazil. Briefly, Rivulus ocellatus was initially described by Hensel (1868) using a single specimen from Rio de Janeiro, Brazil. Later, Seegers (1984) suggested the existence of two syntopic species in Rio de Janeiro: the hermaphroditic R. ocellatus as in Hensel (1868), and a yet undescribed species composed of hermaphrodites and males, named R. caudomarginatus. After taxonomic revision of the family Rivulidae, Costa (2004) reclassified some previously known Rivulus species (Rivulus brasiliensis, R. campelloi, R. caudomarginatus, R. ocellatus, and R. marmoratus) into a new genus called Kryptolebias. After morphological evaluation of the K. ocellatus holotype by Costa (2011) argued that the species originally described by Hensel as K. ocellatus was in fact K. caudomarginatus (as in Seegers (1984)). Therefore, K. caudomarginatus has become a junior synonym for K. ocellatus. The other syntopic species composed of selfing hermaphrodites was then named as K. hermaphroditus (Costa 2011). However, discussions on the taxonomic nomenclature of these mangrove killifish species are still ongoing (Huber 2016). This taxonomic connudrum is likely to be fully resolved only when the genetic data of the formalin-fixed K. ocellatus holotype, initially described by Hensel (1868), is available. For the genome generated here, we used the currently valid taxonomic classification, with the selfing species occurring from the Caribbean to Southeast Brazil, named K. hermaphroditus sensu Costa 2011, and the androdioceous outcrossing from South and Southeast Brazil, K. ocellatus (sensu Costa 2011) (Berbel-Filho et al. 2020, 2022). Here we provide whole genome sequencing data for the mangrove killifish K. ocellatus (sensu Costa 2011) (Fig. 1a), and two freshwater Kryptolebias species: K. brasiliensis and K. gracilis (Fig. 1b and c, respectively). Although Kryptolebias ocellatus has no current classification of its conservation status, K. brasiliensis and K. gracilis are categorized as endangered and critically endangered species, respectively, by the Brazilian list of threatened fish species (MMA 2022).

Behavior:

Although four of the five members live in fresh water, it is clear that brackish water boosts their reproduction and well-being. Also, the marine hermaphrodite species K. marmoratus (which normally lives in crab holes in mangrove areas) can be kept in brackish-, or even pure-, freshwater, if the change is not too sudden.