Biological Oceanography

(^) When that systematist assigns a name to a set of animals newly recognized as very
similar and also distinct from obviously related sets, she or he provides a detailed
description and (when preservation makes it possible) stores “type” specimens in
some accessible, curated facility, generally a museum. Those practices allow new
workers to determine the identity (or not) of new specimens with named species.
Identification, also subjective to a degree, is central to biogeography. Species
designated and identified in this fashion are termed “typological species”.
(^) In recent decades, most systematists have adopted the philosophy that classification
of organisms should reflect their phylogeny (evolutionary relationships) as closely as
possible. The revolution in molecular genetics promoted this adoption by providing
semi-quantitative estimates of degrees of relationship: similarity (or difference) in
DNA sequences from selected genes. These are eminently suitable for explicit
systems (computable algorithms) that construct phylogenetic trees. At the level where
species names are assigned to related sets of sequenced individuals at the tips of the
branches, this involves deciding what proportion of DNA base pairs (among hundreds
to thousands) might vary among individuals likely to be exchanging genetic material
(something like the biological species definition) versus larger proportions. Once this
decision is made, usually based on within-group versus among-group variation in
previously defined typological species (circularity seems inevitable), it can be applied
to new cases within suitable limits, say just among diatoms or snail fish. Species
defined in this way are termed “phylogenetic species”. They play an increasingly
significant role in marine biogeography, because many morphologically defined
species are being found on the basis of gene differences to be clusters of closely
related, “cryptic” species. Included in this trend has been development of DNA “bar
codes” based on mitochondrial DNA (mtDNA): partly with the goal of identifying
individuals from their “codes”, partly to identify new, particularly cryptic, species
(reviewed by Goetze 2010). Bar coding with mtDNA was the basis of a large-scale
census of marine life (COML), recently completed. It would be premature to evaluate
the results here. Species are not static population entities. On sufficient time scales
they are adapting to changing circumstances, splitting, sometimes hybridizing across
previous mating barriers.
(^) There are not very many species of either algae or animals in the ocean water
column compared to the number on land, and this is particularly true for epipelagic
habitats. Among crustacea for example, extensive cataloguing by Razouls et al.
(2005–2011) found 2454 described species of marine pelagic copepods, the most
diverse group, Vinogradov et al. (1996) listed 233 hyperiid amphipods; Blachowiak-
Samolyk and Angel (2008) listed 217 ostracods, and a catalogue updated to 2011 lists
305 ostracods, some recognized but still to be described (Martin Angel, pers. comm.),
and Baker et al. (1990) listed 86 euphausiids. Some planktonic groups, for example,
copepods and ostracods, remain understudied, with species numbers increasing,