Jellyfish comprise a large group of ancient marine invertebrates with an evolutionary history dating back roughly 500 million years, and consequently these organisms have faced extreme environmental changes. In the last decades, the presence of jellyfish has drawn the foci of investigation because there has been a perceived increase in their abundance and distribution. Recently, jellyfish populations have been found in geographical regions where they have not been previously observed.
To face environmental challenges, marine jellyfish have selectively developed adaptations to reach high fecundity rates, as a consequence, under favorable conditions, this may result in rapid populations growth. Also, anthropogenic activities, including species introduction and eutrophication, may induce an increase in the population density of jellyfish. This review describes and discusses the adaptive responses of these fascinating organisms to global warming. Keywords: jellyfish, marine oldest survivor, Scyphozoa, cnidarians 1. Introduction The phylum Cnidaria is an ancient clade of animals comprised by corals, sea pens, sea anemones, jellyfish and hydroids, and includes nearly 10,000 species inhabiting aquatic environments, and since they originated during the Neoproterozoic Era they may be considered as one of the most prosperous living organisms in terms of survival and reproductive success.
Jellyfish are widely distributed in tropical, subtropical, temperate, and boreal coastal waters from surface to bottom . Because of their life cycle, these organisms appear and disappear seasonally, presenting their maximum abundances during spring or summer in most of the species. Since jellyfish large swarms or blooms have been observed in the last years in different parts of the world, it has been suggested that various environmental factors, as temperature, nutrients, precipitation, and winds, maybe promoting their increased abundance . The arrival of jellyfish to the coasts of different parts in the world is a natural phenomenon that has been increasing. As an example, there were no reports of the occurrence of jellyfish in the Berre Lagoon in southeast France until the recent manifestation of blooms of the clinging jellyfish Gonionemus vertens . The new presence of jellyfish in some coasts has increased the interests in the scientific community because the information in this regard is almost null. Jellyfish, intricate environmental challenges, new scientific tools, and novel biological responses are the basis for this review. As long as their large evolutionary history, jellyfish tolerance and physiological abilities to face environment changes seem to be not fully understood to date. The abilities of jellyfish species to survive and reproduce have been especially linked to climate changes.
Thus, new findings of their reproductive abilities, food utilization mechanisms, and chemical energy production and use, are some of the aspects that will be updated in this manuscript. 2. Jellyfish Origin (How much time do they have on the planet? The phylum Cnidaria includes two large phylogenetic groups which mostly comprise all the species: Anthozoa (Corals and anemones) and Medusozoa (jellyfish). The subphylum Medusozoa comprises four classes of organisms: Scyphozoa, Cubozoa, Hydrozoa, and Staurozoa. The phylogenomic relationships between these classes remain uncertain to date. Traditionally, the class Scyphozoa has been divided, based on phylogenetic studies with rDNA , in two monophyletic groups: Coronatae and Discomedusae . Nowadays, the report of Bayha et al., is the most important phylogenetic study dealing with Scyphozoa. Throughout their evolutionary history, jellyfish species of different shapes and pigmentation patterns have evolved. These morphological characteristics have been traditionally used as a base for taxonomic classification. In the last few decades, the availability of genetic tools has been added to the morphological toolbox, and they have been useful to complement, elucidate and sometimes to even clarify morphological ambiguities among jellyfish taxonomic groups. However, the integration of morphological and molecular approaches is required, to have an explicit, objective, and quantitative dissection about the results. Previous studies on the geographic variation of the species from the genera Mastigas and Cassiopea describe morphological differences between species and some differences are so insignificant that it could be cryptic species or sisters . Kramp , proposed that the Cassiopea genus, in particular, may have originated in the Indo-Pacific and dispersed from south of Africa towards the Atlantic, where its expansion ended at the end of the Tertiary period.
Taking into account the ‘rule of thumb’ that suggests that there is a 0.5 to 1.4% divergence for the gene encoding the cytochrome c oxidase subunit 1 (COI) per million years , the results of Holland et al, about speciation on Cassiopeia, suggest that the divergence of species of this genus occurred between 46-8 million years ago, similar to that estimated for Aurelia cryptic species .
Jellyfish ecological role Jellyfish are highly conspicuous aquatic organisms of major importance in coral reef areas . Although they are present in almost all aquatic environments, from microscopic sizes to meters in length, the ecological relevance of these species has been underestimated for a long time as they were considered as ‘dead ends’ in trophic webs, due to an apparent lack of predators, and their suggested low nutritional value . Recently, however, due to its predatory capacity, this group of organisms has been extensively studied including the reports of Alvariño Purcell and Arai, Pauly . These authors suggest that jellyfish are undeniably the largest consumers of plankton production, including holoplankton and meroplankton. In addition, the feeding habits of these organisms include larvae and fish eggs which are essential for the ecosystem stability and human consumption.
