Throughout geological time there has been a succession of dominant nannoplankton species, which later manifested themselves as acme zones in the sediment record. Many of these dominant species disappeared from the nannofossil assemblages after their acme had finished and then seemingly became extinct. However, in the last few decades it has become evident that quite a few of these species (or their descendants) are living in coastal assemblages, perhaps as a result of being outcompeted in offshore communities and/or being forced to modify their ecological preferences. Here we explore the so-called ‘coastal refugia’ hypothesis. Studies on living nannoplankton in coastal assemblages are surprisingly few, however, there is a growing amount of biogeographic data to suggest that these coastal nannoplankton are quite widespread along today’s coasts, particularly in subtropical andtropical localities. For instance, Cruciplacolithus neohelis has been found now incoastal waters off Hawaii, France, Puerto Rico, Japan, Guam, and Palau (e.g., Konno & Jordan, 2006), while Braarudosphaera bigelowii has recently been recorded from Japan (Hagino et al., 2009) and the Bering Sea (Konno et al., 2007). The former species has a benthic life-cycle phase which suggests that it has always been a coastal species. However, the fossil record documents the dominance of this morphospecies in open ocean environments of the early Oligocene (~33-29 million years ago). Few studies have tackled this “paleobiogeographical anomaly” (Kelly et al., 2003), and all lack the crucial consideration of adaptation. Different morphospecies may not only have tracked their environmental optima (staying true to “the present is the key to the past” paradigm of paleoecology), but most likely also have changed their environmental preferences through time. The fossil record strongly supports such plasticity in relation to environmental change, as expressed in a wide variety of morphotypes. Indeed, numerous morphotypes within Gephyrocapsa spp. and Reticulofenestra spp. were previously cosmopolitan, open ocean taxa and only later on did some of them move into coastal environments. Today, G. oceanica inhabits shallow tropical waters, while G.muellerae, G. ericsonii and most Reticulofenestra spp. are often associated with upwelling conditions, although recently a Reticulofenestra sp. was found in a shallow Palauan lagoon. Yet it should also be noted that two species, G. ornata and R. sessilis, still remain in open ocean environments. Here we suggest that there are at least two components of the coastal assemblage, 1) species which have always inhabited coastal waters and may or may not have benthiclife-cycle phases such as Cruciplacolithus, Hymenomonas, Pleurochrysis, Ochrosphaera and Jomonlithus, and 2) species which appear to have taken refuge in coastal habitats after being ‘forced out’ of their open ocean niches such as Gephyrocapsa, Reticulofenestra and possibly Braarudosphaera. There is also the possibility that further genera will be discovered in coastal waters in the near future and that Emiliania huxleyi is destined to become the next coastal refugee in the more distantfuture.
Hagino, K., Takano, Y. & Horiguchi, T. 2009. Pseudo-cryptic speciation in Braarudosphaera bigelowii(Gran and Braarud) Deflandre. Marine Micropaleontology, 72: 210-221.
Kelly, D. C., Norris, R. D. & Zachos, J. 2003. Deciphering the paleoceanographic significance of EarlyOligocene Braarudosphaera chalks in the South Atlantic. Marine Micropaleontology, 49: 49-63.
Konno, S., Harada, N., Narita, H. & Jordan, R.W. 2007. Living Braarudosphaera bigelowii (Gran &Braarud) Deflandre in the Bering Sea. Journal of Nannoplankton Research, 29(2): 78-87.
Konno, S. & Jordan, R.W. 2006. Lagoon coccolithophorids from the Republic of Palau, NW EquatorialPacific. Journal of Nannoplankton Research, 28(2): 95-110.
2010. 66-67 p.