R. HUYS AND S. CONROY-DALTON 



C. hendorffi Poppe, 1890 is a junior subjective synonym of C. scutellata. The doubtful status oiSapphir rostratus Car, 1890, 

 Clytemnestra tenuis Lubbock, 1 860 and C. hendorffi var. quinquesetosa Poppe, 1 890 is discussed. 



The intricate taxonomic history of the family is reviewed, including the nomenclatural confusion surrounding the priority of 

 the family name. The phylogenetic relationships of the Clytemnestridae as well the ontogenetic processes underlying the caudal 

 ramus sexual dimorphism in Clytemnestra are discussed. The taxonomic impediment in marine plankton research caused by the 

 failure to recognize pseudo-sibling or cryptic species is highlighted. 



INTRODUCTION 



The greatest habitat shift performed by copepods was undoubtedly 

 the colonization of the open pelagic environment, covering 71 

 percent of the Earth's surface and providing a volume of 1347 

 million cubic kilometres. This habitat was most successfully ex- 

 ploited by the calanoids which can be regarded as the marine 

 planktonic copepods par excellence (Huys & Boxshall, 1991), and 

 to a lesser extent by the cyclopoids and poecilostomatoids which can 

 be particularly abundant in small mesh net samples. The evolution- 

 ary history of harpacticoid copepods in the marine plankton is less of 

 a success story and is to be viewed as the result of multiple 

 colonization. Only three families are currently considered as exclu- 

 sively holoplanktonic, the Miraciidae, Euterpinidae and 

 Clytemnestridae, and each of them can be regarded as an evolution- 

 ary cul de sac. The Miraciidae contains 4 monotypic genera which 

 are typically associated with marine filamentous Cyanobacteria 

 (Huys & Bottger-Schnack, 1994). The Euterpinidae is represented 

 by a single species Euterpina acutifrons (Dana, 1847) which is often 

 abundant in shallow neritic waters. The Clytemnestridae currently 

 comprises two cosmopolitan species which are primarily found in 

 the epipelagic zone but frequently penetrate into deeper layers. The 

 Aegisthidae, commonly regarded as typical holoplanktonic forms 

 found in the mesopelagic and bathypelagic zones, has recently been 

 shown to be only a secondary offshoot from a hyperbenthic ancestral 

 stock (Conroy-Dalton & Huys, 1999; Lee & Huys, in press). Other 

 pelagic harpacticoids exhibit an essentially benthic biology by their 

 association with 'planktonic' substrata, such as Microsetella spp. 

 which attach themselves to discarded and occupied larvacean houses 

 (Appendicularia) (Ohtsuka et al., 1993), and Parathalestris croni 

 (Kr0yer, 1 846) which is typically associated with floating macroalgal 

 clumps (Ingolfsson & Olafsson, 1997). 



Clytemnestrids have been known since the advent of the pioneer- 

 ing oceanographic expeditions such as the U.S. Explorer Expedition 

 (Dana, 1854) and the Voyage of the H.M.S. Challenger (Brady, 

 1883). They were originally classified as poecilostomatoids until 

 Claus (1891a) demonstrated their harpacticoid identity. Virtually all 

 of the taxonomic literature on this family was published in the 

 second half of the 1 800s and apart from cursory treatment by Lang 

 (1948), Wells (1970) and Boxshall (1979) no significant contribu- 

 tions have been added since. 



MATERIAL AND METHODS 



The descriptive terminology is adopted from Huys et al. (1996). 

 Abbreviations used in the text are: ae, aesthetasc; P1-P6, first to 

 sixth thoracopod; exp(enp)-l(2, 3) to denote the proximal (middle, 

 distal) segment of a ramus. Specimens were dissected in lactic acid 

 and the dissected parts were placed in lactophenol mounting me- 

 dium. Preparations were sealed with glyceel (Gurr®, BDH Chemicals 

 Ltd, Poole, England) or transparent nail varnish. All drawings have 

 been prepared using a camera lucida on a Leitz Dialux or Leitz DMR 

 microscope equipped with differential interference contrast. 



Clytemnestra gracilis and Goniopsyllus clausi were examined 

 with a Philips XL30 scanning electron microscope. Specimens were 

 prepared by dehydration through graded acetone, critical point 

 dried, mounted on stubs and sputter-coated with palladium. 



Citations of articles in the International Code of Zoological 

 Nomenclature (ICZN) refer to the fourth edition published in Aug- 

 ust 1999 and superseding previous editions with effect from 1 

 January 2000. Type series and other material is deposited in the 

 collections of the Natural History Museum, London (BMNH). 



TAXONOMIC HISTORY 



The proliferation of generic names in this family at the end of the 

 19th century marked one of the most virulent episodes in the history 

 of harpacticoid taxonomy. The key players in this debate were the 

 eminent and influential Carl Claus and a cohort of opponents 

 including Wilhelm Giesbrecht, S.A. Poppe and Lazar Car. It is clear 

 that much of the confusion arose from observational errors made by 

 both Dana (1854) and Brady (1883). 



Clytemnestra Dana, 1847 



Dana introduced the genus Clytemnestra in the first part of his 

 'Conspectus Crustaceorum' which was published in 1847 (for dis- 

 cussion of publication dates see Huys & Bottger-Schnack, 1994) 

 and included the families Cyclopidae and Harpactidae. This paper, 

 completely lacking in illustrations, provided a Latin diagnosis for 

 the genus and its only species C. scutellata which was placed in the 

 'Harpactidae' together with Harpacticus Milne Edwards, 1840 and 

 Setella Dana, 1 846. Although no type locality was designated, the 

 author did mention that the species was found near the Gilbert 

 Islands and east of Tuamotu in the Pacific Ocean and in the South 

 China Sea. In his second volume of the Crustacea of the United 

 States Exploring Expedition (Dana, 1854) a more extensive and 

 illustrated description of C. scutellata was given based on speci- 

 mens from the Tuamotu samples. 



Lubbock (1856) added a second species C. atlantica which he 

 described on the basis of a single female from an unspecified locality 

 in the Atlantic. The brief original description included illustrations 

 of the habitus and antenna only. Various authors (Poppe, 1891; 

 Giesbrecht, 1892; Lang, 1948) have questioned this identification 

 and referred the species to the genus Pachos Stebbing in the 

 Poecilostomatoida. Pesta (1909) considered C. atlantica as a syno- 

 nym of Pachos punctatum (Claus). In a later report Lubbock ( 1 860) 

 described C. tenuis, again from a single female, collected east of 

 Mauritius. Lubbock himself had some reservations about the sexual 

 maturity of the specimen, and Poppe (1891) considered the species 

 as unrecognizable. Giesbrecht (1892) listed C. tenuis as a possible 

 synonym of C. rostrata. 



Claus (1863) rejected Clytemnestra as a valid genus by stating 

 that the illustrations were so inadequate that they were worthless for 

 identification purposes. 



