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Abstract The ciliates in this class are thought to 
represent the nature of the ancestral ciliate lineage.
Their non-dividing macronuclei make them “karyo-
logical relicts”. They are a strongly supported clade, 
characterized by postciliodesmata arising from the 
somatic kinetids, their non-dividing macronuclei, 
and by robust phylogenetic support based on small 
subunit rRNA gene sequences. The class is divided 
into three orders, based primarily on oral features. 
These ciliate are conspicuous inhabitants of benthic 
marine habitats. Their elongated worm-like bodies 
can be seen crawling between sand grains and detrital
particles. Thus, they are quite contractile and fl exible, 
and also capable of regeneration. The extrusomes 
of this class are also unique with cnidocysts and 
orthonematocysts being found nowhere else in the 
phylum. Oral structures are quite variable, ranging 
from simple circumoral dikinetids to somewhat 
complex adoral ciliature. Stomatogenesis can be 
either parakinetal or buccokinetal, although much 
remains to be done on this aspect of their biology. 
Their non-dividing macronuclei, which arise at 
each cell division from division of a micronucleus, 
are often numerous and typically clustered around 
a micronucleus. Two unusual features of taxa in the 
group are the harvesting of epibiontic bacteria by 
Kentrophoros and the use of mineral crystals in the 
Müller’s vesicle to sense gravity by Loxodes . 
 Keywords Postciliodesma, paradiploid, interstitial
 The ciliates assigned to this class are considered 
by some to represent the nature of the “dawn” or 
 eociliates that first diverged from the alveolate 
lineage. They have been labeled “ karyological 
relicts ”, a term introduced by Grell (1962) and 
publicized by Raikov (1969, 1982, 1985), because 
they exhibit a simple form of nuclear dualism : the 
 macronucleus is paradiploid but non-dividing. They 
have also been labeled “ cortical relicts ” because 
the cortex in some forms is thought to repre-
sent the ancestral condition: Kentrophoros does 
not have differentiated oral ciliature, but it does 
have somatic dikinetids , which are presumed to 
be the ancestral condition for the phylum (Lynn 
& Small, 1981; Small, 1984). There are over 
130 species of these primarily interstitial ciliates, 
commonly found in the sands and sediments of 
marine littoral environments (Foissner, 1998b). 
Intertidal sands are the habitat for “relict” forms of 
various groups of small invertebrates, leading one 
to believe that the psammophilic karyorelicteans 
are also of ancient vintage (Corliss, 1974b, 1975b; 
Raikov, 1969). Finlay and Fenchel (1986) have 
suggested, based on their research on Loxodes , 
which is the only freshwater representative of the 
class, that these ciliates might also be “biochemical 
relicts” because of the odd mitochondrial potential 
of nitrate respiration under low oxygen conditions, 
which are common in interstitial environments. 
 The karyorelicteans are united by two major 
features: the presence of a non-dividing paradiploid
macronucleus or macronuclei; and by postciliodesmata 
in which the microtubules are arranged as 2 + ribbon 
+ 1 in a repeating fashion (see Somatic Structures ).
The class is supported robustly by small subunit 
rRNA gene sequences (Hammerschmidt et al., 
1996; Hirt et al., 1995). The actin of Loxodes is 
quite divergent from other ciliates (Kim, Yura, Go, 
 Chapter 5 
 Subphylum 1. 
KARYORELICTEA – The “Dawn” or 
122 5. Subphylum 1. POSTCILIODESMATOPHORA: Class 1. KARYORELICTEA – The “Dawn” 
& Harumoto, 2004). It is a matter of opinion whether 
this supports the ancestral nature of karyorelicteans
or demonstrates again the unreliability of actin as 
a phylogenetic marker for ciliate evolution (see 
Philippe & Adoutte, 1998). 
 They form a diverse assemblage when one considers 
their oral structures. Some genera are ventrostomous
(e.g., Geleia , Loxodes ); some genera are pros-
tomous (e.g., Trachelocerca , Trachelolophos ); and 
some genera have apparently no differentiated 
oral ciliature (e.g., Kentrophoros ). Bardele and 
Klindworth (1996) have observed that this parallels 
the evolution of oral structures in other groups. 
They argued that Kentrophoros may, in fact, have 
secondarily lost its oral apparatus when it acquired 
the obligatory symbiosis with thiotrophic or sul-
fur bacteria , an interpretation consistent with the 
observations of Foissner (1995a). 
 The distribution of these obligatorily psammobiotic 
species is global, though they are “endemic” with 
respect to their biotope. Means of dispersal remain 
unknown: Corliss and Hartwig (1977) supposed that 
continental drift may have been partially responsible. 
 5.1 Taxonomic Structure 
 We recognize three orders in this class: Order 
 Protostomatida ; Order Loxodida ; and Order 
 Protoheterotrichida . Alternative classifications have 
been proposed: Foissner (1998b) has argued that 
the bristle kinety , which frames the glabrous stripe 
or non-ciliated somatic cortex of protostomatids 
is homologous to that of loxodids , and so he sup-
ports uniting these in the Subclass Trachelocercia 
de Puytorac, Grain, and Mignot, 1987. We remain 
sceptical of this homology until ultrastructural 
investigation has demonstrated clear similarities 
in the kinetid structures or more extensive gene 
sequence data resolves the phylogeny of this class. 
 The Order Protostomatida includes the prostomous
Family Trachelocercidae and the “astomous” 
Family Kentrophoridae (Fig. 5.1). Oral struc-
tures are simple and ingestion may be either at 
the anterior end or along the glabrous stripe (see 
 Oral Structures ). The Order Loxodida includes 
the ventrostomous Families Loxodidae and 
 Cryptopharyngidae . These ciliates typically swim 
on the right surface of their flattened bodies. The 
oral cavity has a simplified ciliature of dikinetids 
(Fig. 5.1). The Order Protoheterotrichida , which 
includes the ventrostomous Family Geleiidae , are 
holotrichously ciliated and contractile, resembling 
their namesakes the heterotrichs (see Chapter 6 ). 
Their non-dividing macronuclei relate them to the 
other karyorelicteans even though their oral struc-
tures are more complex with simple adoral polyki-
netids and unusual paroral polykinetids on the right 
side of the oral region (Fig. 5.1). 
 A number of recent works have provided details 
of the morphology of these taxa: Trachelocercidae 
(Foissner, 1996c, 1997g; Foissner & Dragesco, 1996a, 
1996b), Kentrophoridae (Foissner, 1995a, 1998b), 
Loxodidae (Foissner, 1995/1996, 1996b, 1998b), and 
Geleiidae (Dragesco, 1999), but refer to Chapter 17
for detailed descriptions. 
 5.2 Life History and Ecology 
 These typically elongate and highly contractile 
ciliates are conspicuous consituents of interstitial 
habitats, especially sands and sediments of the 
marine littoral or brackish estuaries. Karyorelicteans 
have been recorded from interstitial habitats, 
often sandy ones in the marine sublittoral, in Africa
(Dragesco, 1965), western and eastern Europe 
(Agamaliev, 1971; Azovsky & Mazei, 2003; 
Dragesco, 1963, 2002; Fernández-Leborans & 
Fernández-Fernández, 1999; Kovaleva & Golemansky, 
1979; Mazei & Burkovsky, 2003), North America 
(Borror, 1963), and the Arabian Gulf (Al-Rasheid 
& Foissner, 1999). The only recorded exception is 
Loxodes , which is found in freshwater sediments 
(Finlay, 1982; Finlay & Berninger, 1984). Most 
species are classified as microaerophilic , restricted 
to sediments because these regions contain reduced 
oxygen concentrations, often becoming anoxic 
within a few centimeters of the sediment-water 
interface. However, Loxodes can move into the 
water column if the interstitial waters of the 
sediments become anoxic (Goulder, 1980). Finlay, 
Fenchel, and Gardener (1986) suggested that cyto-
chrome oxidase is the oxygen receptor for

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