fraction in the region 10^11 –10^13 if we consider, for example, the size range
that includes most ciliate species. But ‘rarity’ is a relative term that is strongly
influenced by organism size. A million perch in the same pond would never be
considered ‘rare’, but a protist population of a million individuals would prob-
ably never be detected – even after years of sampling. For most of the relatively
rare protist species (those that are orders of magnitude scarcer than the abun-
dant species; Finlay, Esteban & Fenchel,1998 ), undersampling is inevitable using
typical sampling procedures. For that reason alone, ‘endemic’ species are not
found elsewhere (see also Fenchel & Finlay,2003 ). However (see above), a grow-
ing body of evidence indicates that selective-enrichment culture techniques can
reveal the rare or cryptic diversity of protists (Fenchelet al., 1997 ), such as
freshwater ciliates in hypersaline ponds (Esteban & Finlay,2003 ), but the use of
such methods is not common practice in the search for species that may have
escaped from their ‘endemic’ ranges. Figure9.7 illustrates this point.Species concepts
Is the resolution of morphospecies poorer for smaller than for larger organisms?
For protists with complex and varied morphologies, such as the ciliated proto-
zoa, this is probably not the case. However, the species taxonomy of the smaller
organisms depends on a proportionately higher optical resolution. The species
systematics of chrysomonad flagellates, for example, is necessarily based on
electron microscopy. If protists in general demonstrated the degree of morpho-
logical richness normally perceived in the naked amoebae, we would have a
major problem with the morphospecies concept. But naked amoebae were not
included in our analysis for the detection of the cosmopolitan-biogeography
transition (see above), and the morphological richness that can be perceived in
the protists we did use – including the ciliates, diatoms, foraminifera, chryso-
monads, desmids, heliozoans, choanoflagellates and testate amoebae, as well as
microbial metazoans such as the rotifers, gastrotrichs and microcrustaceans –
indicates that microfauna, meiofauna and macrofauna may share a similar
degree of morphological richness.
Do ‘cryptic species’ or ‘sibling species’ (i.e. species that are morphologically
similar but reproductively isolated) show any large-scale geographical patterns
in their distribution. This has been investigated in some protists, especially the
ciliates, and the evidence indicates that members of sibling species complexes
have worldwide distribution. Mating experiments with isolates of nominal
species from different continents have also shown that the isolates are inter-
fertile (for discussion and references, see Finlay,2002; Finlayet al., 1996b).Cosmopolitan genotypes
Recent molecular studies of isolates of the small ciliate morphospeciesCyclidium
glaucoma from across the world have disclosed multiple, distinct (rDNA)178 B. J. FINLAY AND G. F. ESTEBAN