some distinguishing characteristics. Zoantharia exhibit a great deal of variability in polyp shape, polyp size, internal anatomy, mouth shape, number of tentacle rows . . . the list is a long one. Basically, Zoantharia are described as exhibiting characteristics that octocorals do not (Hyman, 1940), which kind of opens things up a bit! One trait that is a clear distinguishing feature is the number of tentacles. As we mentioned in volume one, Zoantharia have six tentacles or multiples of six while octocorals have eight or multiples thereof. This arrangement is mirrored internally where the body cavity is divided into six, or a multiple of six, compartments by an equal number of septa.
Of course mother nature has decided to throw a few curves at us. The order to which zoantliids belong, Zoanthidea, exhibits a few oddball traits that set it apart from all other Zoantharia. Zoanthids lack a pedal base, their ends are stalked or wedge-shaped, and their internal arrangement of septa is unlike any other anthozoan and most closely resembles that of the extinct order Tetracorallia (Hyman, 1940). It is because of this septal arrangement that zoanthids are classified into two suborders (figure 2.1). Those that have an incomplete fifth septum (i.e. it does not extend to the actinopharynx) are termed brachycnemic, while those with a complete fifth septum are termed macrocnemic.
Figure 2.1 After Hyman 1940
A. Cross section of Epizoan-thus with macrocnemous septa arrangement.
B. Cross Section of Zoanthus showing brachycnemous septa arrangement..
1. Fourth and fifth septa
2. Dorsal directives
There has recently been a reorganization in zoanthid systematics as reflected in Table 2.1. Throughout this book we have attempted to use Protopalythoa in place of Palytkoa, however, it is not yet
Systematics of the order Zoanthidea
* We are tentatively placing Acro-zoanthus in this family pending further taxonomic clarification.
Subclass Zoantharia (Hexacorallia) Order Zoanthidea
Suborder Brachycnemina Family Neozoanthidae
Genus: Neozoanthus Family Zoanthidae
Genera: Acrozoanthus* Isaurus, Palythoa, Protopalythoa, Sphenopus, Zoanthus Suborder Macrocnemina Family Epizoanthidae
Genera: Epizoanlhus, Thoracactis Family Parazoanthidae
Genera: Gerardia, Isozoanthus, Parazoanthus possible to reassign species names to the new genus with confidence. Therefore, unless we are discussing Palythoa caesia, P. caribaeorum. P. mammillosa or P. tuberculosa, then all other refer-
ences to Palythoa species refers to Protopalythoa. See chapter eight for a description of the differences between these two genera.
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