STUDIES IN THE MORPHOLOGY, TAXONOMY, AND ECOLOGY OF THE PERIDINIALES 



By Herbert William Graham 



Abstract 



Although the Peridineae (Dinoflagellata) have been 

 rather extensively studied, there is still very little 

 known about many aspects of this interesting group of 

 the Protista. The purpose of this paper is to contribute 

 to our knowledge of the morphology, taxonomy, and ecol- 

 ogy of this group. 



It was the author's privilege to have the opportunity 

 to study the Peridineae of the plankton samples collect- 

 ed on the last world cruise of the nonmagnetic research 

 vessel Carnegie of the Department of Terrestrial Mag- 

 netism, Carnegie Institution of Washington. The exten- 

 sive nature of this collection not only permitted a de- 

 tailed taxonomic analysis of a large variety of species, 

 but also afforded a good opportunity to make a study of 

 the geographic distribution of these forms throughout 

 the Pacific and North Atlantic oceans, based on compar- 

 able material. The simultaneous collecting of biologi- 

 cal samples and hydrographic data also offered a rare 

 opportunity to correlate the established distributions 

 with oceanographic conditions. 



The present paper deals with twenty-nine repre- 

 sentatives of the order Peridiniales. They are distrib- 

 uted among the families of the order as follows: one in 

 Goniodomaceae; seven in Ceratocoryaceae; four in Gon- 

 yaulacaceae; sixteen in Peridiniaceae; and one in Cer- 

 atiaceae. They include six new species, three new vari- 

 eties, and six new forms. Two old specific names were 

 replaced and one subgenus raised to the rank of genus. 

 A monographic treatment was given all the species 

 (seven) of the family Ceratocoryaceae. 



The species selected were subjected to a morpho- 

 logical analysis with the following objects in mind: (1) 

 to establish a standard for the analysis and recording 

 of the skeletal features of peridinian species; (2) to ac- 

 quire a knowledge of the detailed skeletal morphology 

 of certain species; (3) to study variation within the 

 group by subjecting a few forms to intensive analysis; 

 and (4) to contribute to the concepts of the relationships 

 of the genera of the peridiniales. In addition to this 

 morphological and taxonomic treatment, each species 

 was studied from the standpoint of geographic distribu- 

 tion and relationships to the following hydrographic 

 conditions: temperature, salinity, hydrogen-ion concen- 

 tration, and phosphate content. 



Much of the confusion among systematists in the 

 past regarding the Peridineae has been due to inade- 

 quate descriptions and figures. Since type specimens of 

 peridinians usually cannot be preserved, it is impera- 

 tive that the descriptions and figures of the species be 

 not only complete but standard in nature so that ade- 

 quate comparisons with other material can be made. 

 Standard methods of measurement and of expression of 

 body shape were introduced in the case of each genus 

 studied. 



The skeletal features of the forms treated were 

 very carefully analyzed by microdissection. Particular 

 attention was given the ventral area, a region of the 

 theca of the Peridiniales which was almost unknown be- 

 fore these investigations. A great diversity in the 

 structure of the ventral areas of the various genera was 

 found, although a common basic plan was evident. The 



area is composed of from five to seven external plates 

 and, sometimes, one internal plate. A nomenclature 

 was devised for these plates which would serve to iden- 

 tify them and at the same time express homologies. 



In some families of the Peridineae, species are 

 well defined and there is comparatively little difficulty 

 in determining them. In other groups, however,. specific 

 variability is so striking that no investigator has yet 

 attempted a comprehensive taxonomic treatment. In the 

 Peridiniaceae, for instance, the plate pattern is so var- 

 iable that it cannot be successfully used for the differ- 

 entiation of genera, and the body shape so variable that 

 all attempts at specific segregation have failed. For this 

 reason, a statistical method was applied to certain rep- 

 resentatives of Peridlnium in order to determine its 

 efficacy in the delimitation of species. A large number 

 of specimens was measured; the body shapes were ex- 

 pressed numerically; and the frequency of the various 

 shapes was plotted. By this method groupings were ob- 

 tained which indicated the delimitation of species and at 

 the same time demonstrated the range of variation of 

 each species. 



On the basis of the comparative study of the ventral 

 areas of the various genera it was concluded that Goni - 

 odoma is the most primitive genus; that Ceratocorys is 

 closely related to Goniodoma but somewhat more ad- 

 vanced; that Gonyaulax . Acanthogonyaulax , and Spiraulax 

 are decidedly more advanced and in addition are closely 

 related to each other; and that the genera Peridinlum 

 and Ceratium are highly specialized and, although relat- 

 ed, are developed along divergent lines. The compari- 

 sons of the girdle, hypotheca, and epithecal plates of 

 these genera corroborate the conclusions arrived at 

 from the study of the ventral area itself although these 

 results could not have been obtained by the study of 

 these last series of plates alone. 



Some of the marine peridinians are useful in trac- 

 ing the movements of oceanic water masses. The dis- 

 tributions of the species treated in this report were ex- 

 amined in relation to hydrographic conditions in order 

 to ascertain the value of each species as an oceano- 

 graphic "indicator." Of the species treated, Ceratoco - 

 rys horrida and Goniodoma polyedricum were found to 

 be the most valuable indicators of hydrographic condi- 

 tions. Their distributions are closely correlated with 

 the temperature of the water and the species are suffi- 

 ciently common to make negative records of occurrence 

 significant. 



Ceratocorys horrida was found throughout the trop- 

 ics and warm-temperate regions. Its limits of distrib- 

 ution were sharply marked and these limits coincided 

 with the 19° C isotherm. Thus, in an intensive oceano- 

 graphic investigation, an occasional record of this spe- 

 cies in water with temperature less than 19° would in- 

 dicate an intrusion of tropical water or of water mixed 

 with water of tropical origin. 



Goniodoma polyedricum . on the other hand, is a very 

 common tropical species which can endure transfer into 

 regions of much cooler water. It occurred at practical- 

 ly all the Carnegie tropical stations (where the temper- 

 atures ranged from 20 to 30° C). It was carried into 



