Speciation and Infrageneric Classification in the Planktonic Dinoflagellate Tripos (Gonyaulacales, Dinophyceae)



The genus Tripos, formerly known as marine species of Ceratium, is the
dinoflagellate with the greatest number of species and infraspecific taxa (~800) due to the high
morphological intraspecific variability of numerous species. In the past, the species of Tripos were
proposed into distinct genera.



To propose an infrageneric classification and to review the taxonomy and nomenclature
of each taxon in order to establish the correct species and synonymy.



Observations from the Mediterranean Sea, and Atlantic and Pacific Oceans, a review of
the original descriptions and further literature, and the available molecular data.



A re-instatement of this generic split is not justified due to the difficulties to circumscribe
the basal subgenera Amphiceratium and Archaeceratium, and the polyphyletic character of Biceratium.
The subgenus Amphiceratium is dismembered after the classification of the sections Inflata
and partially Fusiformia into Archaeceratium. The subgenus Tripos (autonym) replaces other
names such as Tripoceratium or Orthoceratium. Based on the original descriptions, the records of
T. furca and T. lineatus correspond to T. eugrammus and T. furca, respectively, and T. macroceros
has been reported as T. contrarius. The names T. belone and T. carriensis have been misapplied for
T. pacificus and T. volans, respectively. Tripos arcuatus, T. gracilis, T. inclinatus, T. scapiformis
and T. subcontortus are revived to replace T. euarcuatus, T. declinatus, T. horridus, T. longirostrum
and T. contortus, respectively. The species T. ramakrishnae and T. fusus var. schuettii were
described from individuals infected by endoparasites. Tripos rotundatus comb. nov. is proposed for
C. digitatum var. rotundatum.



Tripos is restricted to 57 correct species, although the speciation and synonymy are
incomplete due to lack of studies in the life cycle and molecular data.



Introduction
The family Ceratiaceae within the Gonyaulacales contains a few freshwater species classified in the genus Ceratium, and numerous marine species classified in Tripos. The species are typically large and robust, often with horns, facilitating the net sampling. In addition, the apparently easy identification using only gross morphology (i.e. outline, including horns) has facilitated abundant information on the distribution, using the species as ecological indicators. Due to the morphological variability, Tripos is the dinoflagellate genus with the greatest number of species and infraspecific taxa (~800) with an embroiled synonymy and unresolved species circumscriptions. The taxonomy is largely reported in the dinoflagellate literature (Balech, 1988;Schiller, 1937;Steidinger and Tangen, 1997;Taylor,  The resolution of the most extended molecular markers for phylogeny of dinoflagellates (SSU-and LSU rRNA gene) is insufficient to solve the relationships in the distal clade. It encompasses most of the species that were classified in the subgeneric names Euceratium, Macroceratium, Orthoceratium or Tripoceratium, and also included some species classified in Biceratium (Fig. 2). The subgeneric name Ceratium must be applied for the freshwater species and the name Euceratium is invalid (article 21.3 of the International Code of Nomenclature, I.C.N.). This distal clade contains the type, T. muelleri, therefore these species ScienceOpen Preprints 8 belong to the subgenus Tripos that is an autonym (I.C.N., art. 22.1) replacing Orthoceratium sensu Sournia or Tripoceratium.
The distinctive characteristic of Macroceratium is the postindentation, with the bases of the antapicals projected more or less posteriorly beyond the midbody. This trait seems to be supported by the molecular data, and these species cluster together, and in a basal position of the distal clade in some phylogenies. However, the support is insufficient to consider Macroceratium as independent from the subgenus Tripos. lineatum are triangular with the apical horn distinct from the rest of the epitheca (Fig. 3a, c), while the epitheca of P. eugrammum is gradually tapering into the apical horn (Fig. 3b). The bases of the hypothecae of P. furca and P. lineatum are almost parallel to the cingulum, while the basis of the hypotheca of P. eugrammum is more inclined. The antapical horns of P.
eugrammum, especially the left one ( Fig. 3b), are more robust than those in P. furca and P. lineatum (Fig. 3a, c). Peridinium lineatum showed shorter antapical horns than P. furca, although this feature may vary intraspecifically as occur in the anterior-most daughter cells.
The lateral contour of the hypotheca is almost perpendicular to the cingulum in P. lineatum ( Fig. 3c), while more inclined in P. furca (Fig. 3a). The comparison of the original illustrations ( Fig. 3a-c) evidences that P. furca described from subarctic waters and P. eugrammum described from a late summer proliferation in the Mediterranean Sea are independent species.
More difficult is to solve the identity of P. lineatum. furca and P. lineatum as synonyms, and they reported an unfortunate illustration of their new combination Ceratium furca. In addition to the excessively thin horns, they figured a chimeric cell with the epitheca of P. furca and the hypotheca with the parallel antapical horns that characterizes P. eugrammum (Fig. 3d showed an individual with divergent antapical horns and the shape of the hypotheca of P. furca. Stein's figures 13-14 showed individuals with very short antapical horns that fit with  (Fig. 3h), and the new species Biceratium debile in his figure 16 (Fig. 3i). Both taxa are synonyms ( Fig. 3h-i). The basionym of the type species of the genus and subgenus Biceratium is the boreal species P. furca, and Vanhöffen (1897) did not report P. eugrammum.

Section Furciformia (including Pentagona)
Tripos furca is the type species of the subgenus Biceratium and the section Furciformia. There is only one available LSU rRNA gene sequence (AF260391) retrieved as T. lineatum and it does not cluster with the basal sequences of P. eugrammus (Fig. 1). With the current available data, T. eugrammus is classified in its own section, and T. furca in the section Furciformia.
We cannot study the relationship among the members of the sections Furciformia and Pentagona based on the LSU rRNA gene sequences because a single sequence is available ( Fig. 1). Based on the SSU rRNA gene phylogeny, the members of Furciformia and Pentagona cluster in distinct clades (Fig. 2). Within this context, the sections Furciformia and Pentagona are merged until further molecular data will clarify this matter.  (1908a). (b) C. lineatum by Kofoid (1908a). (c) C. eugrammum by Kofoid (1908a). (d) C.

Section Candelabra
The general appearance of Tripos eugrammus (Fig. 3O) resembles the freshwater relatives such as Ceratium hirundinella ( Fig. 5a-b), and more especially C. furcoides. The latter taxon was also described from a lake as C. furca var. lacustre. That similarity is more evident between T. candelabrum (not 'candelabrus') ( Fig. 5c) and C. hirundinella ( Fig. 5a-b). Tripos candelabrum is a chain-forming species (Fig. 5d-e), while this feature is rare in T. eugrammus and the freshwater Ceratium. Tripos eugrammus is a eurihaline species that often reaches high abundance in low salinity coastal waters. The species of the freshwater genus Ceratium are characterized by the formation of cysts. Resting cysts have been only reported for T. eugrammus and T. candelabrum (Gómez et al., 2010). The SSU rRNA gene sequences of T.
candelabrum cluster with an unresolved relationship with other taxa (Fig. 2), and LSU rRNA gene sequences are not available. Tripos candelabrum and its section Candelabra remains in the subgenus Biceratium based on the superficial resemblance to T. furca. Other molecular markers of T. candelabrum are also a priority to clarify the evolution of the genus. candelabrum.

Section Lanceolata
The members of the section Poroceratium are characterized by a pore in the epitheca ( Fig. 6ab). The pore has not been reported in the rare species Tripos lanceolatus. Its morphology is intermediate between T. praelongus with a pointed epitheca and a straight cell of T. schroeteri with shorter antapical horns. My observations from the tropical Eastern Pacific (Fig. 6h) fit well in shape and size with the original description of C. lanceolatum by Kofoid (1907a) that described it from the same region (Fig. 6i). Gaarder (1954) identified as C. lanceolatum a large cell (285 μm long) with long and bent antapical horns (Fig. 6j), already illustrated by Schütt (1892) with a pore in the epitheca (Fig. 6k). These observations resembled individuals of T. gravidus var. praelongus with an atypical lanceolate epitheca. Tripos lanceolatus remains in its own section because it is difficult to decide placing it in the sections Poroceratium or Digitata. Other tentative member of Lanceolata could be T. brunellii, only known from the original description by Rampi (1942) (Fig. 6ad).

Section Digitata
Tripos digitatus, classified in the earlier taxonomical schemes in the subgenus Biceratium, is classified in Archaeceratium based on the molecular data ( The classification of T. digitatus var. rotundatus at the species rank is proposed in this study. In the earlier taxonomical schemes, Tripos schroeteri (non T. schroederi) was classified in the subgenus Biceratium based on the superficial resemblance with T. eugrammus. Tripos schroeteri is classified in Archaeceratium because the epitheca is wide and flat, and the antapical horns are unequal, robust and bent . Cleve (1900) described Ceratium belone ( Fig. 6v), further illustrated by Schröder (1906) (Fig. 6w), and  that described it as C. furca var. incisum (Fig. 6x). Cleve's illustrations are sketchy and the description of C. belone was scarcely detailed. The cell shape of Schröder's and Karsten's illustrations were similar to Cleve's taxon, but their illustrations showed more clearly that the left antapical horn is slightly bent. Tripos incisus is a junior synonym of T. belone. However, Jørgensen (1920)

Section of Tripos pacificus (subsection Beloniformia in part)
Karsten (1906) described C. furca var. longum (Fig. 6ae) and Schröder (1906) described the same taxon as C. pacificum (Fig. 6af). Kofoid (1907a) reported a wider cell as C. pacificum, but it is closer to C. belone sensu Cleve (Fig. 6ag). Stüwe (1909) illustrated individuals that fit with the original description of C. pacificum (Fig. 6ah). Jørgensen (1920) under the name C. belone ( Fig. 6ai) figured an elongated cell that corresponded to Schröder's C. pacificum ( Fig. 6af). Jørgensen's interpretation of C. belone was questioned by Pavillard (1931, p. 71), but it has prevailed up to date. Graham and Bronikovsky (1944) admitted a high morphological variability of the epitheca of C. belone, ranging from individuals that fit with T. pacificus to smaller individuals with a thinner epitheca than in the original description of T. belone (Fig. 6aj). There are warm water taxa such as C. furca var. belonoides that can be mistaken for C. belone (Fig. 6ak). Wood (1963) added confusion when he proposed the later isonym C. pacificum (Fig. 6ao) for individuals of T. geniculatus (Fig. 7a-c). The evolutionary origin of T. pacificum can be interpreted in two ways: 1) an elongated cell evolved from a common ancestor with T. eugrammus or T. belone, or 2) a cell with a developed right antapical horn evolved from a common ancestor with T. fusus. Tripos pacificus has a flattened epitheca gradually tapering towards the apex that suggests an affinity with Archaeceratium. Tripos pacificus is here considered a link between Archaeceratium and Amphiceratium. A new section should be proposed for T. pacificus if it is confirmed by the molecular data.

Section Inflata
The support to re-classify the members of the section Inflata and most of the members of clustered with high support with T. digitatus and T. gravidus (Fig. 2). This suggests that T.

Section Fusiformia
Amphiceratium is dismembered after the classification of the section Inflata and most of the species of the section Fusiformia in Archaeceratium. Amphiceratium is here restricted to T.
fusus and very closely allied taxa with an elongated cell body along the antero-posterior axis with almost straight horns, and a reduced or almost invisible right antapical horn (Fig. 8a).
The type species T. fusus, is together with T. eugrammus, one of the more common species in temperate coastal waters. In addition to the individuals under division (Fig. 8b), it can be observed teratogenic or aberrant forms (Fig. 8c-e). Schütt's (1895) illustrated an individual of C. fusus with a wide epitheca (Fig. 8f) that was later reported as the variety C. fusus var.
Most of the authors have considered that they correspond to a single species, placing C. biceps as a synonym of T. eugrammus (=C. furca auct. mult.). As illustrated by Stein (1883), C.
The daughter cell that receives the short right horn from the parent cell has been identified as C. strictum (Fig. 8t-v). Kofoid (1908c) illustrated a cell with short apical and antapical horns (Fig. 8j). This is atypical and Kofoid very likely misidentified an anterior-most daughter cell of T. eugrammus (Fig. 8k) with a life stage of T. extensus. Tripos extensus (Fig. 8m-n) is a thermophilic species unreported for the North Sea, and Claparède and Lachmann (1859) did not observe it (Fig. 8h) in the cold waters of a Norwegian fjord where T. eugrammus is present (Fig. 8i). Ceratium biceps (Fig. 8h) is an immature cell of T. eugrammus (Fig. 8k). The molecular data confirmed that T. extensus is distinct from T. fusus (Gómez et al., 2010), but there are individuals with intermediate characteristics between both species that need research.
The species diagnoses based on the length of the horns are risky because it depends of the degree of maturation after the division or the potential autotomy (Fig. 8r-s). The name T.
longirostrum is available for a species with intermediate characteristics between T. fusus and T. extensus (Fig. 8o-q).  (u-v). The arrow points the right antapical horn. Scale bars: 50 μm.

Subgenus Tripos
Based on the SSU rRNA gene sequence phylogeny, the species of the clade that contains the type, T. muelleri, and distantly related to the sequences of the types of Archaeceratium, Amphiceratium and Biceratium, should be grouped into a single subgenus named Tripos, an autonym (Fig. 2). Most of the authors have split the species of Tripos into four subgenera: Amphiceratium, Archaeceratium (=Poroceratium), Biceratium (=Ceratium, Orthoceratium Meunier) and Tripoceratium (=Euceratium, Orthoceratium Sournia). Kofoid (1909) proposed the subgenus Macroceratium for some species formerly classified in Tripoceratium (C. macroceros, C. gallicum, C. vultur) which "bases of the antapicals projected more or less posteriorly beyond midbody forming a postindentation", while Tripoceratium was restricted to species with "postmagin rounded, no postindentation". Kofoid (1909) was fortunate and the postindentation is a trait supported by the molecular phylogeny, although the molecular divergence is not high to support the classification of Macroceratium as independent from the subgenus Tripos (Fig. 2). The antapical horns of the species of Amphiceratium, Archaeceratium and Biceratium are directed posteriorly, while there is more variability in the species of the subgenus Tripos. In the asexual life cycle, anterior-most daughter or autotomized cells show posteriorly directed antapical horns, while they bent and were anteriorly directed in the mature cells. This feature, together with the variability in the length of the horns due to environmental conditions (i.e., seasonality) are factors that induce a high variability of outlines. Consequently, it is even more difficult to establish the synonymy and species circumscriptions in the subgenus Tripos due to the numerous intermediate morphologies.
macroceras f. armata' (Fig. 9d) figure 29) showed a prominent postindentation and it is not conspecific with his form 'armata'. To the best of my knowledge, nobody has interpreted correctly Ehrenberg's P. macroceros because the diagnosis was not enough detailed, and the illustration could not be accessible. What to do? The basionym of T. macroceros is P. macroceros as described by Ehrenberg (1840), and the diagnosis corresponds to the species figured by him (Fig. 9p), and not further interpretations. Then, C. macroceros auct. mult.
needs other species name. The earlier name at the rank level for this species could be C. gallicum (Fig. 9e). Ceratium deflexum was later ranked at the species level (Fig. 9f). Under the name T. gallicus are included individuals with a progressive curvature of the antapical horns as C. macroceros sensu Claparède and Lachmann (Fig. 9g-j), and individuals with a more abrupt deflection angle of the antapical horns ( Fig. 9k) that fit better with the original description of C. gallicum. Further research is needed to confirm if both forms are conspecific.
Tripos massiliensis is a closely related species to T. gallicus, but with a less prominent postindentation ( Fig. 9l-n). Ehrenberg's P. macroceros (Fig. 9p, u-w) shows a scarce postindentation when compared to C. gallicum and C. massiliense. Junior synonyms of P.
carriense is the anterior-most daughter cell of C. tripos var. typicum, and C. tripos var.
inflexum is an aged cell of the same species that was further described as C. flagelliferum (Fig.   9aa). Gourret used 'typicum' to remark that this form was common in the coasts of Marseilles, and this should not be interpreted as the typical morphology of C. tripos (now T. muelleri).
Based on my experience after one year of daily plankton samplings at the coasts of Marseilles, this species is common and abundant there (Fig. 9ab). Individuals of T. carriensis infected by an endoparasite showed an inflated midbody (Fig. 9ac-ad), and they have been described as C. ramakrishnae (Fig. 9ae). Tripos carriensis is characterized by antapical horns that bent at ScienceOpen Preprints 34 the same height ( Fig. 9x-ae). Tripos trichoceros is more slender and an oceanic species which antapical horns bent at different heights ( Fig. 9af-ah). Gourret (1883) proposed C. carriense for an immature individual and this may induce confusion. Jørgensen (1920) and further authors misinterpreted the identity of C. carriense. Jørgensen illustrated an individual with the antapical horns bent at different heights, with a short curvature of the proximal region of the left antapical horn (Fig. 9ai, al-am). The tentative earlier name for Jørgensen's C.
tergestinum by Schütt (1892). The DNA sequences obtained from the strain CCMP1770 have been identified as T. longipes. There is an imagen available at https://ncma.bigelow.org/ccmp1770, but it shows an aberrant cell that makes difficult to verify the identification.

Section Palmata
Cleve (1897) described C. tripos var. horridum with antapical horns that diverged from the apical horn (Fig. 11h). Based on the original description, C. tripos var. horridum can be considered conspecific with C. longipes ( Fig. 11f-g). Other interpretation is that Cleve (1897) illustrated an immature individual, and the antapical horns will be parallel to the apical horn in the mature individuals. Based on the original description, the name C. horridum which basionym is C. tripos var. horridum is also a synonym of C. longipes (I.C.N. art. 7.3). Gran (1902) and other authors reported individuals with antapical horns parallel to the apical horn for C. horridum (Fig. 11p-q), differing from Cleve's basionym. Ceratium intermedium is a name for C. horridum sensu auct. (Fig. 11k-l). The original illustration of C. buceros is unfortunate because the shape of the hypotheca is unrealistic, probably reported from an aberrant individual (Fig. 11m-o). Jørgensen (1920) reported as C. buceros (Fig. 11n) a cell with the morphology of C. tenue (Fig. 9ak), and he classified C. inclinatum, C. tenuissimum, C. molle and C. denticulatum as varieties of C. buceros. This is unfortunate due the uncertainties on the identity of C. buceros. Tripos intermedius is common in temperate waters.

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Ceratium clavigerum, not 'claviger' is recorded in warmer waters. It exhibited long antapical horns, often inflexed ( Fig. 11r-s), or with a dilated distal end of the antapical horns (Fig. 11t). Tripos platycornis is characterized by the flattening of the distal part of the antapical horns which tips are never pointed (Fig. 12a-n). The main controversy in the identification is when the flattening is missing. The cell body of T. compressus is similar to T. platycornis, but the pointed tips of the antapical horns of Gran's (1902) original illustration (Fig. 12a) are unreported for T. platycornis (Fig. 12b-n). Authors such as Paulsen (1908) or Balech (1988) have considered T. compressus as an independent species, but their illustrations showed blunt tips of the antapical horns as in T. platycornis. These pointed tips of the horns in Gran's illustration is the main obstacle to consider these species as synonyms. A closely related species is T. ranipes (Fig. 12o-x). Tripos platycornis and T. ranipes are the type species of the sections Platycornia and Palmata, respectively. These sections are here merged with T.
intermedius (=C. horridum sensu auct.) and T. clavigerus in a single section, and Palmata is the earlier available name.

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The SSU rRNA gene sequence of Tripos hexacanthus clustered as an independent lineage that could be a link between the section Macroceros and other sections of the subgenus Tripos (Fig. 2). It is difficult to find relatives for T. hexacanthus. This is a peculiar species by characterized with strong surface reticulation of the theca and twisted antapical horns ( Fig.   12y-ab). ranipes by Cleve (1900). (y-ab) Tripos hexancanthus. Scale bars: 50 μm.

Section Tripos (=Rotunda)
The type species of Tripos, T. muelleri, and allied taxa show the typical anchor-like shape lacking the postindentation in the posterior contour (Fig. 13a-e). The species are distinguished based on the relative length and orientation of the antapical horns. When the antapical horns are short, it is not easy to decide whether the morphology is due to autotomy, an immature cell, seasonal variant or a true distinct species. Examples are T. brevis, T. pulchellus, T.
schmidtii among others (Fig. 13f-i). This variability is a nightmare for the species circumscription, but it is interesting from the ecological point of view as it could be interpreted as responses to the environmental conditions. An example is T. egyptiacus that was described from the Canal of Suez (Fig. 13j). This is not an endemism because the Canal was inaugurated in 1869 and there is no time for the evolution of a new species. Then, the extreme and fluctuant environmental conditions (temperature, salinity) in the Canal of Suez induce an anomalous configuration of the antapical horns. The variable morphology of C. egyptiacum could correspond to stressed individuals of T. muelleri or even T. dens, the latter first described from the Red Sea (Fig. 13k). Even more problematic is when the antapical horns are very short or almost absent as observed in T. dens (Fig. 13k) and T. divaricatus (Fig. 13m). Böhm (1931) already reported the morphological variability of T. dens, including the example of a heteromorphic chain (Fig. 13l). One of the daughter cells corresponded to the morphology recently described as T. balechii (Fig. 13o). Lemmermann (1899, p. 345) proposed C. tripos var. divaricatum citing a basionym an illustration by Bergh (1881). That individual from the

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Baltic Sea showed the anterior-most daughter cell of T. muelleri with an incomplete hypotheca ( Fig. 13m). Based on observations near the USA-Mexico border, Kofoid (1908c) ranked C.
Some of the earlier illustrations of new species were too sketchy or were unprecise to represent the real morphology of the species. The illustration of Ceratium tripos var. gracile by Gourret (1883) was unfortunate. Gourret showed a cell with a bent apical horn that is atypical for the members of this section, and the midbody was anomalously elongated along the anterior-posterior axis. An important feature is that the apical horn arose close to the left side of the body and the left side of the epitheca is less convex (Fig. 13q). Gourret (1883) described C. tripos var. gracile, and he cited C. tripos var. gracile Pouchet as synonym. Then, C. tripos var. gracile Gourret is an invalid name because it is a later isonym. The description of C. tripos var. gracile by Pouchet (1883) did not include illustration, and he only reported that the cell showed long and thin double-curved horns. This suggests that it is unrelated to Gourret's taxon. Pavillard (1905) proposed the new species name C. gracile without apparently illustration. He reported that the species corresponded to C. tripos var. gracilis by Schröder (1900, his figures 17b,d,e) (Fig. 13r), and   (Fig. 13s).
gracile (Fig. 13q) corresponded to C. gracile. These illustrations showed the apical horn arising close to the left side of the midbody.  described C. tripos declinatum with individuals showing a distinctive loop in the longer left antapical horn (Fig. 13y).  illustrated the apical horn arising from the middle of the epitheca (Fig. 13y), but it arose closer to the left side in that species (Fig. 13aa). A long left antapical horn with a distal loop is typically observed in oceanic individuals (Fig. 13aa). Ceratium gracile Pavillard 1905 and 'C. tripos declinatum Karsten n. sp. 1907' are synonyms. In this study, C. gracile Pavillard 1905 (Fig. 13t, u-w, aa, ac) is revived because it has the priority over C. declinatum (Fig.   13y). Jørgensen ( , 1920 proposed C. gracile (Gourret) Jørgensen interpreting that C.
Ceratium declinatum f. majus and C. declinatum var. debile were unnecessary varieties because they fit well with the original descriptions of C. gracile (Fig. 13t) and C. tripos var.

Section Limulus
The resolution of the SSU rRNA gene phylogeny is insufficient to resolve the divergence among the species of the subgenus Tripos (Fig. 2), and the LSU rRNA gene phylogeny only that clusters together with moderate support (Fig. 1). The strain CCMP1770 identified as T.
longipes is the only strain with sequences in both SSU-and LSU rRNA gene phylogenies.
Independently of the correct species identification, the LSU rRNA gene sequence of the strain CCMP1770 is distantly related to the clade of the section Tripos. In the SSU rRNA gene phylogeny, the sequence of the strain CCMP1770 clusters with three species with similar morphology retrieved as T. symmetricus, T. arietinus and T. euarcuatus, and more distantly related to representatives of the section Tripos (Fig. 2). This suggests that these three species should not be classified in the section Tripos, and the classification in the section Limulus appears as the first choice.
contortum sensu Cleve and C. concilians are or are not taxonomical (heterotypic) synonyms, these two species are homotypic (nomenclatural) synonyms that share the same basionym. Steidinger and Tangen (1997, p. 474) reported for T. gibberum: "can be confused with C.
concilians which has proximally bent apical horn and rounded right epithecal shoulder. C.
concilians is less reticulate". Observations of the cell division of T. gibberum suggest that reticulation or the bent of the apical horn have little diagnostic value because it depends on the degree of maturation of the cell (Fig. 16i-m). The illustration of C. gibberum var.
contortum sensu Cleve arouse from the left side (Fig. 16g). Cleve's illustration showed a cell with a straight proximal part of the right antapical horn (Fig. 16g), while it is bent since the proximal part in Gourret's taxon (Fig. 16b). Gourret is not closely related to C. karstenii. The available names for Cleve's C. contortum are C. subcontortus (Fig. 16n) and C. saltans (Fig. 16o) by Schröder (1906), or C. longinum ( Fig. 16p) by . Beyond the discussion on the priority of the publication, the illustration of C. longinum (Fig. 16p) does not fit well C. contortum sensu Cleve (Fig. 16g).

List of correct species
About 165 species names of marine species of Ceratium have been proposed. Numerous species (i.e., Tripos muelleri) have tens of described varieties and forms and the genus accounted for a total of about 800 infraspecific taxa. The diagnostic criteria for the species identification such as the relative length and orientation of the antapical horns varies according the cell maturation, autotomy and environmental conditions. There are studies on the sexual life cycle of the freshwater Ceratium, but very few on Tripos (Apstein, 1911). The nature of the gametes will be useful to solve the controversy in the identity of species with short antapical horns such as T. lineatus and T. minutus, and the infraspecific taxa of T. muelleri (Lohmann 1908). The cultures will help investigate the intraspecific variability, but aberrant forms are often observed in culture conditions, and they should not be confused with 'natural' life stages.
We are far to solve the speciation of Tripos based on molecular data with sequences of only 30 and 9 species in the SSU-and LSU rDNA phylogenies, respectively. The species circumscription can be resolved with the sequences of the complete rDNA gene, including more variable regions (i.e., ITS). Currently, there are ITS sequences of only 3 species. For example, there is no a complete of the SSU rDNA sequence of T. muelleri, one of the first described dinoflagellates and abundant next door of the specialized laboratories in molecular phylogeny. The melting of Arctic sea ice has increased the research interest in the region.
However, a documented DNA sequence of T. arcticus is not available. We cannot expect significant advances in a near future, especially for the tropical and deep water species, that are essential to understand the evolution of Tripos.
Tripos is the dinoflagellate genus with more infraspecific taxa, with species with several varieties, and these varieties with several forms. Each author will decide to use a given variety or form, and Gómez (2013)

Subsectio Pentagona Jørgensen 1911
Tripos minutus (Jørgensen 1920 Tripos is largely incomplete due to lack of studies in the life cycle and molecular data.