Missing in the Middle: A Review of Equatorially Disjunct Marine Taxa

Abstract

Disjunct distributions of flora and fauna have intrigued biogeographers for centuries and have been central to debates over the roles that dispersal and vicariance have in speciation and evolutionary biology. One of the most notable disjunct distributions is when a single species or closely related taxa are present in temperate or polar regions of both hemispheres, but absent near the equator. Generally referred to as an anti-tropical distribution, this pattern is prevalent in marine taxa, occurring across the Tree of Life from single-celled bacteria and radiolarians, to algae, many invertebrates, fishes and marine mammals. The abundance of different species that exhibit this distribution pattern has led to a large number of theories on how it is formed and maintained. Here, seven central hypotheses that recur throughout the literature regarding the mechanisms responsible for this pattern are described and discussed in light of both historic and contemporary data. Recent advances in paleoclimate modeling and phylogenetic approaches have revealed that three of these hypotheses lack support. However, these data also highlight that a single driver of anti-tropicality is unlikely and that multiple mechanisms are responsible for this observed distribution pattern. The importance of using a multifaceted approach in future studies that incorporates phylogenetic, physiological, life history and ecological data is discussed, as there is a gap in our knowledge regarding adult and larval behavior, as well as thermal tolerances of many anti-tropical species. While recent studies have helped reveal some commonalities between anti-tropical groups, only by using an integrative approach will we be able to more fully understand which mechanisms are most prevalent in driving this pattern across the oceans.

The rise of predation in Jurassic lampreys

“…It would be not surprising if Yanliaomyzon and Mesomyzon had wider distributions because they all had a much longer dorsal fin and hence higher hydromechanical efficiency than the pouched lamprey and Pacific lamprey. As extant predatory lampreys, e.g., the pouched lamprey and the Pacific lamprey, may follow the movements of fish shoals over considerable distances 5 and in varying water depth (commonly 0–500 m, maximum record 1485 m) 54 , in this ‘Isothermal Submergence’ pattern of dispersal hypotheses for anti-tropicality biogeography 55 , the diversification and global distribution of the potential host fishes from the Late Jurassic to Late Cretaceous periods 44 could have also led lampreys’ trans-equatorial dispersals.…”

Potential global distribution of a temperate marine coastal predator: The role of barriers and dispersal corridors on subpopulation connectivity

“…In this sense, the case of Gal apagos Islands highlights the limitations of modeling a 3D environment in two dimensions. This model limitation may also be an underlying cause of connectivity underprediction between the Gal apagos Islands and the Northeast Pacific subpopulation, which is relevant considering N. cepedianus' anti-tropical distribution (Ludt 2021). The advent of promising tools for 3D environmental niche modeling in recent years (Bentlage et al 2013) may provide avenues for better understanding habitat use of other demersal species widely distributed across the vertical dimension.…”

Genetic diversity of the Southern Black Drum Pogonias courbina (Teleostei: Sciaenidae) from Río de la Plata and Atlantic Ocean coasts

“…Trachurus symmetricus, Sardinops, Scomber japonicus, Scomber australasicus, Sebastomus and West Pacific Engraulis), for which the distribution in the Northern and Southern Hemispheres had been attributed as a result of equatorial crossing during Pleistocene climate changes (Burridge 2002). The mechanism associated with this distribution pattern in the Pleistocene epoch is called glacial dispersal (Ludt 2021). In this epoch occurred repeated glacial cycles, which altered the temperature and sea level (Ludt 2021).…”

“…The mechanism associated with this distribution pattern in the Pleistocene epoch is called glacial dispersal (Ludt 2021). In this epoch occurred repeated glacial cycles, which altered the temperature and sea level (Ludt 2021). The glacial dispersal hypothesis considers that there are organisms which are unable to inhabit tropical water at the present time.…”

“…The glacial dispersal hypothesis considers that there are organisms which are unable to inhabit tropical water at the present time. However, during glacial maxima, the decrease in sea surface temperature could have allowed the dispersal from one hemisphere to the other of currently antitropical species (Ludt 2021). Ludt & Myers (2021) using ecological niche models could demonstrate that several anti-tropical species do not have a suitable abiotic habitat in the tropics, however, during the Last Glacial Maximum they found dispersal corridors between hemispheres.…”