Marine Flora and Fauna of the Northeastern 

 United States. Tardigrada 



LELANDW. POLLOCK' 



ABSTRACT 



The manual includes an introduction to the general biology, an Ulustrated key, an annotated 

 systematic list, a selected bibliography, and an index to the Tardigrada of the marine coastal areas of 

 the world to a depth of 5,000 m. 



INTRODUCTION 



The Tardigrada (tardus. L. slow; gradus, L. step) 

 comprise a phylum of microscopic metazoa (usually less than 

 1 mm in length) of uncertain phylogenetic placement. Con- 

 siderations of their status have been based on 1) their 

 growth by molting; 2) absence of ciliated epithelium: 3) 

 presence of a spacious pseudocoelom in adults; 4) muscu- 

 lature in bandlike bundles; 5) metameric, or at least 

 repetitive, arrangement of unjointed legs, as well as of 

 portions of the ventral nervous system and muscular 

 system; 6) the presence of coelomocytes; 7) the absence of 

 circular muscles; 8) a tripartite foregut; 9) a nonchitinous 

 cuticle; and 10) the occurrence of eutely or cell constancy 

 (although this recently has been disputed, Bertolani 1970). 

 Most of these characteristics suggest an organizational 

 complexity approaching that of the aschelminth phyla, 

 especially the Rotifera and Nematoda. Characters suggest- 

 ing relationship with the Arthropoda include the first six 

 characters listed above; in addition, their "ladder-type" 

 ventral nervous sytem recalls the annelid-arthropod line. 

 Tardigradan embryology however apparently includes a 

 total but irregular cleavage pattern and enterocoelous 

 formation of a series of eoelomic pouches, of which only the 

 gonocoel is retained in the adult. While this pattern of 

 development is unlike any other known group, it is most 

 similar to that of the deuterostomous invertebrates. 



Lack of clarity regarding their phylogenetic affinities 

 suggests that the Tardigrada are far removed from their 

 nearest phyletic neighbor. Apparently they are an old group 

 which has become highly specialized for life in peculiar 

 habitats, such as the water films surrounding lower plants 

 and lining interstitial spaces between grains of sand. 

 Morphological diversity among marine tardigrades attests 

 to their age. On the other hand, the comparative uniformity 

 in appearance and simplicity in morphological characters of 

 freshwater forms supports the hypothesis that marine 

 tardigrades are primitive. There are 43 described species of 

 marine tardigrades included in 17 genera. Most are 

 members of the interstitial meiofauna of sandy sediments. 



Since one-half of these are in mono- or ditypic genera, and 

 two thirds have been discovered since 1950, it is likely that 

 many more species will be described in the future. 



Marine tardigrades rarely exceed 0.5 mm in length and 

 are all similar in general body plan (Fig. 1). They possess as 

 many as 11 cephalic appendages, including lateral cirri (a), 

 clavae (cl), external cephalic cirri (ec), internal cephalic cirri 

 (ic), and a median cephalic cirrus (mc). Their bodies usually 

 are cylindrical, with four pairs of legs which terminate in 

 claws, toes, or both. These terminal appendages, the spines 

 or papillae on the legs, and the conformation of the caudal 

 appendage (if present) are important taxonomically. 

 Likewise, the presence and location of somatic cirri, 

 especially posterior- lateral cirrus (e) (Fig. 1), can be of 

 taxonomic significance. In the order Eutardigrada, the 



cephalic 

 appendages 



toe & claw 



somatic 

 spine 



cirrus e 



'Department of Zoology, Drew University, Madison, NJ 07940. 



gonopore 

 anus 

 caudal 

 appendage 



Figure 1 . — Diagrammatic illustration of a composite marine tardigrade. 

 Cephalic appendages: a - lateral cephalic cirrus: cl - clava: ec - external 

 cephalic cirrus; ic - internal cephalic cirrus: mc ■ median cephalic cirrus. 



