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Interactive Key to the Species of Chaetognatha from the South Atlantic Bight and northern Gulf of Mexico

adapted from:
McLelland, J.A. 1989. An illustrated key to the Chaetognatha of the northern Gulf of Mexico with notes on their distribution. Gulf Research Reports. 8(2):145-172.

Michel, H.B. 1984. Chaetognatha of the Caribbean Sea and adjacent areas. NOAA Technical Reports NMFS 15, United States Department of Commerce. 33 pp.

How to use this key:
This key is is a dichotomous key and is designed to facilitate easy use of couplets via links. Each couplet frame has a bauplan button to bring up a general chaetognath body design for reference (bottom left of window), and a return to start option (bottom right of window). Diagnoses are available as pop up windows once the species is determined. It is possible to navigate directly to family without using the key by using the Quick Links To Family buttons.

start key

Quick Links to Families

Spadellidae

Eukrohniidae

Krohnittidae

Important notes on the classification and morphology of Chaetognatha

 

General diagram (bauplan) of a chaetognath


1.a. One pair of lateral fins.....................................................................2

Ventral view of Pterosagitta draco showing single lateral fin. Scale = 1 mm

b. Two pairs of lateral fins.................................................................11

Ventral view of Flaccisagitta enflata (bottom) showing two lateral fins. Scale = 1mm.

 

 

 

 

 

 

 





2. a. Lateral fins extending onto trunk segment; one set of teeth...........3


Eukrohnia calliops, ventral view, showing lateral fin extension. Scale = 1 mm.

b. Lateral fins limited to tail segment; two sets of teeth.......................
    .............................................................................genus Pterosagitta

     
Krohn, 1853
     diagnosis


Pterosagitta draco
, whole animal, ventral view. Scale = 1 mm


Eye pigment from right eye, scale = 0.01 mm.























3. a.With a more or less prominent collarette; benthic animals of the genus Spadella.........................................................................25

 





 

 









 

4. a. Lateral fins extending to central ganglion - genus Eukronia........5

Eukrohnia calliops, ventral view, showing lateral fin extension to central ganglion. Scale = 1.0 mm

b. Lateral fins not extending to central ganglion - genus Krohnitta..10


Krohnitta subtilis, ventral view, showing lateral fin extension posterior to central ganglion. Scale = 1mm.

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5. a. Eyes with pigment........................................................................6

b. Eyes without pigment.......................................................................8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6. a. Apical gland-cell complex bilobate and protruding, causing head to appear pointed; hook tips bent inward at 45-90° angles; transverse musculature extending past posterior edge of ventral ganglion...........7

dorsal view of head - Eukrohnia calliops (left); Eukrohnia proboscidea (right). Scale = 0.5 mm

hook tip variations - Eukrohnia calliops (left); Eukrohnia proboscidea (right). Scale = 0.1 mm

b. Apical gland not prominent, a single lobed mass; hook tips straight,     transverse musculature even with posterior edge of ganglion
........................................................... Eukrohnia fowleri Ritter-Zahoney, 1909

diagnosis

Eukronia fowleri, immature specimen, ventral view. Scale = 1 mm

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Eukrohnia fowleri
(left) dorsal view of head (scale = 1 mm); (middle) eyes (scale = 0.1 mm); (right) hook tip variations
(scale = 0.05 mm)

 

 

 

 

 

 

 

 

 

 

 

 

 



 

7. a. Eye pigment small, elongate or "U" shaped, in posterior region of eye...............................Eukrohnia proboscidea Furnestin and Ducret, 1965

diagnosis


Eukrohnia proboscidea (top) whole animal, dorsal view; head, (bottom left) dorsal view (scale = 0.5 mm); (bottom right) left eye (scale = 0.1 mm).

b. Eye pigment large, lunate, encompassing most of median portion of eye..............................................................Eukrohnia calliops

diagnosis

Eukrohnia calliops, whole specimen, ventral view. Scale = 1 mm.

Eukrohnia calliops head, dorsal view (left) (scale = 0.5mm); eyes (right) (scale = 0.1 mm)

 

 

 

 

 

 

 

 

 



 

8. a. Number of hooks greater than 11, hook tips straight...................... .........................................................Eukrohnia bathyantarctica David ,1958

diagnosis

Eukrohnia bathyantarctica, whole animal, dorsal view.

b. Number of hooks less than 11, hook tips bent inward........................9

 














































9. a. Hooks stout, nearly straight; tail less than 25% of body length........ ...............................................................Eukrohnia hamata (Mobius, 1875)

diagnosis

Eukrohnia hamata whole specimen, dorsal view.

 

b. Hooks slender, gently curved; tail usually greater than 25% of body length ..............................................Eukrohnia bathypelagica Alvarino, 1962

diagnosis

Eukrohnia bathypelagica, whole animal, dorsal view.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



10.a. Outer margins of hooks obtusely angled; mature ovaries elongate,  may extend past edge of lateral fins............Krohnitta pacifica (Aida, 1897)

diagnosis

Krohnitta pacifica: (top) whole animal, ventral view (scale = 1mm); (bottom left) right eye (scale = 0.05 mm); (bottom right) hooks (scale = 0.1 mm).

10b. Outer margin of hooks evenly rounded; mature ovaries compact, not extending past edge of lateral fins.........Krohnitta subtilis (Grassi, 1881)

diagnosis

      

Krohnitta subtilis: (top) whole animal, ventral view (scale = 1mm); (bottom left) right eye (scale = 0.05 mm); (bottom right) hooks (scale = 0.1 mm).

 

 

 

 

 

 

 

 

 

 

 

 




 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



12a. Anterior fins long, inserted close to ventral ganglion, connected to posterior fins by raised portion of body cuticle; caudal fin bilobate
................................................................... Flaccisagitta lyra (Krohn, 1853)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Flaccisagitta in McLelland 1989; as Pseudosagitta in Bieri, 1991)

diagnosis

Ventral views of Flaccisagitta lyra whole animal (top) and head (bottom). Scale = 1mm.

b. Anterior fin short, rounded, well separated from ventral ganglion and posterior fins; caudal fin not bilobate ..................................................13

Flaccisagitta enflata, ventral view. Scale = 1 mm.

 

 

 

 

 

 

 

 

 

 

 

 




13.a. Two to four elongate, thin anterior teeth which sometimes protrude anteriorly .........................Flaccisagitta hexaptera (d'Orbigny, 1843)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Flaccisagitta in McLelland 1989 and Bieri, 1991)

diagnosis

Flaccisagitta hexaptera whole animal (top), ventral view; head (bottom left) and anterior teeth, dorsal view (scales = 1 mm); right eye (bottom right) (scale = 0.015 mm)

b. Four to eight short, wide, overlapping anterior teeth............................ .................................................................Flaccisagitta enflata (Grassi, 1881)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Flaccisagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Flaccisagitta enflata whole animal (top), ventral view (scale = 1 mm); right eye pigments (bottom)(scale = 0.015 mm).

 

 

 

 

 

 

 

 

 

 

 

 

 

14. a. Hooks finely serrate on inner margins as seen under 100x magnification ........................... Serratosagitta serratodentata (Krohn, 1853)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Serratosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Serratosagitta serratodentata whole animal, ventral view (top)(scale = 1 mm);
hook and hook detail (middle)(scale = 0.1 mm); right eye pigment (bottom)(scale = 0.01mm)

b. hooks not serrate..............................................................................15

 

 

 

 

 

 

 

 

 

 

 

15. a. Collarette absent or indistinct.....................................................16

Caecosagitta macrocephala head, ventral view, showing lack of collarette. Scale = 1.0 mm.

b. Collarette distinct..............................................................................19

Solidosagitta planctonis head, ventral view, showing distinct collarette.

 


chaetognath bauplan

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16. a. Eyes without pigment; anterior fins entirely rayed, emerging at a point well separated from ventral ganglion
..................................................Caecosagitta macrocephala (Fowler, 1905)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Caecosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Caecosagitta macrocephala (top) immature specimen, ventral view; (bottom) head, ventral view. Scales = 1 mm.

b. Eyes with pigment; anterior fins with anterior rayless zone, emerging at or near ventral ganglion................genus Mesosagitta.......................17

 


chaetognath bauplan

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17. a. Tail segment less than 22% of total body length; mature ovaries compact, with three to five large ova......... Mesosagitta minima (Grassi, 1881)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Mesosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Mesosagitta minima, ventral view. Scale = 1 mm.

 

b. Tail segment ususally greater than 22% of total body length; mature ovaries elongate with numerous small ova..........................................18

 


chaetognath bauplan

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18. a. Seminal vesicles located approximately equal distance from posterior fins and caudal fin; maximum body length at maturity less than 15 mm ..........................................Mesosagitta decipiens (Fowler ,1905)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Mesosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Mesosagitta decipiens posterior end of body, ventral view, showing position of seminal vesicles. Scale = 1 mm.

 

b. Seminal vesicles adjacent to caudal fin and well separated from posterior fins; maximum body length at maturity may reach 20mm
............................................................... Mesosagitta sibogae (Fowler, 1906)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Mesosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Mesosagitta sibogae (top) whole animal, ventral view; (bottom) head, dorsal view. Scales = 1 mm.

 


chaetognath bauplan

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19.a. Gut diverticulae present* ............................................................20

Solidosagitta planctonis (left) and Ferosagitta hispida (right) showing presence of gut diverticulae.

* gut diverticulae are often undeveloped and difficult to see in immature specimens.

b. Gut diverticulae absent................................genus Sagitta..........21

Sagitta megalophthalma showing lack of gut diverticulae.

 


chaetognath bauplan

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20.a. Posterior fins triangular with prominent rayless zone; walls of gut tube lined with large vacuolar cells
........................................................ Solidosagitta planctonis (Steinhaus, 1896)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Solidosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Solidisagitta planctonis (top) whole animal, ventral view (scale = 1 mm); (bottom left) head, dorsal view (scale = 0.5 mm); (bottom right) right eye (scale = 0.1 mm).

b. Posterior fins rounded, completely rayed; no large vacuolar cells associated with gut tube ..........................Ferosagitta hispida (Conant, 1895)

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Ferosagitta in McLelland, 1989 and Bieri, 1991)

diagnosis

Ferosagitta hispida whole animal, ventral view. Scale = 1 mm.

 


chaetognath bauplan

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21.a. Posterior fins well separated from seminal vesicles....................22

 

Sagitta megalophthalma whole animal, ventral view, showing position of seminal vesicles. Scale = 1 mm.

b. Posterior fins contacing the seminal vesicles....................................23

 

Sagitta helenae, whole animal, ventral view, showing position of seminal vesicles. Scale = 1 mm.

 


chaetognath bauplan

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22. a. Anterior fins emerging posterior to ventral ganglion by a distance of about half the length of the ganglion; large vacuolar cells lining middle third of gut tube............Sagitta megalophthalma Dallot and Ducret, 1969

(this genus is referred to as Sagitta in Michel, 1984 and Casanova, 1999; as Sagitta in McLelland, 1989; as Parasagitta in Bieri, 1991)

diagnosis

Sagitta megalophthalma (top) whole animal, dorsal view (scale = 1 mm); (bottom left) head, dorsal view (scale = 0.5 mm); (bottom right) left eye (scale = 0.1 mm)

b. Anterior fins emerging level with posterior edge of ventral ganglion; no large vacuolar cells associated with gut tube
....................................................... Sagitta bipunctata (Quoy and Gaimard, 1827)

diagnosis

Sagitta bipunctata, whole animal, ventral view. Scale = 1 mm.

 


chaetognath bauplan

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23. a. Anterior teeth numerous (8-18), elongate, protruding outward in an overlapping, fan-shaped arrangement
................................................................Sagitta helenae Ritter-Zahony, 1910

diagnosis

Sagitta helenae (top) whole animal, ventral view; (bottom left) head, dorsal view; (bottom right) anterior head, ventral view. Scales = 1 mm.

b. Anterior teeth less numerous (6-9), short, lying flat against head....24

 


chaetognath bauplan

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24. a. Mature ovaries with large ova which are few in number and arranged in one row.............................................Sagitta tenuis Conant, 1896

(this genus is referred to as Sagitta in Michel, 1984, Casanova, 1999 and McLelland, 1989; as Parasagitta in Bieri, 1991)

diagnosis

 

Sagitta tenuis (top) whole animal, ventral view; (bottom) lateral view of ovaries. Scales = 1 mm.

b. Mature ovaries with small, numerous ova arranged in double rows ........................................................Sagitta friderici Ritter-Zahony, 1911

(this genus is referred to as Sagitta in Michel, 1984, Casanova, 1999 and McLelland, 1989; as Parasagitta in Bieri, 1991)

diagnosis

Sagitta friderici (top) whole animal, ventral view; (bottom) lateral view of ovaries. Scales = 1 mm.

 


chaetognath bauplan

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25.a. With digitate or irregularly shaped adhesive structures on tail .....................................................genus Paraspadella........26

Paraspadella schizoptera, dorsal view, showing digitate tail structures.

 

b. Without adhesive structures but with adhesive glands on the ventral surface of the tail......................... Spadella cephaloptera (Busch, 1851)

diagnosis

Spadella cephaloptera, dorsal view.

 


chaetognath bauplan

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26. a. Adhesive structures originate from the posterior edge of the seminal vesicles, are attached laterally, and are confluent with the caudal fin ............................................Paraspadella pulchella Owre, 1963

(this genus is referred to as Spadella in Michel, 1984)

diagnosis

Spadella puchella whole animal, dorsal view, showing position of adhesive structures.

 

b. Adhesive structures extend from the ventral body wall or the posterior portion of the lateral fins or both, not from the posterior edge of the seminal vesicles ..........................................................................27

Paraspadella schizoptera, whole animal, dorsal view, showing position of adhesive structures.

 


chaetognath bauplan

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27. a. Each adhesive structure is divided into four to six slender processes .........................................Paraspadella schizoptera Conant, 1895

(this genus is referred to as Spadella in Michel, 1984)

diagnosis

Spadella schizoptera, whole animal, dorsal view.

 

b. Each adhesive structure is divided into two slender processes, or, in very small specimens, the structure may be undivided
......................................................................Paraspadella nana Owre, 1963

(this genus is referred to as Spadella in Michel, 1984)

diagnosis

Spadella nana, whole animal, dorsal view.

 


chaetognath bauplan

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The Classification of the Chaetognatha

This interactive key is an adaptation of the work of McLelland (1989) and Michel (1984) and covers the species known from the northwestern Atlantic. Since the publication of these papers, there have been many disagreements amongst workers regarding the classification of the genera within the Chaetognatha. However, without a cladistic examination of this group, the classification of its species and genera remains uncertain. For the purposes of this guide we follow the classification used by McLelland (1989) and Michel (1984) (as updated by Salvini-Plawen, 1986) At each terminal couplet, we include notes on the different terminologies used by more recent authors.

The family Sagittidae has been subjected to tremendous revision during the last several decades as a result of the morphological grouping of species within this family. Tokioka (1965) proposed that the genus Sagitta be broken down into 9 smaller genera (seven of which are used in this key: Serratosagitta, Parasagitta, Ferosagitta, Mesosagitta, Solidosagitta, Caecosagitta, and Flaccisagitta). His revision was not retained in his later work but was used by McLelland (1989) and in a revised form by Bieri (1991). In his review of south Atlantic chaetognaths, Casanova (1999) argued against the use of Tokioko's revision based on what he perceived as a lack of cladistical support for the genera as well as the high degree of morphological variability seen in juveniles. Please refer to Bieri (1991) and Casanova (1999) for more rigorous reviews of the taxonomy of the family Sagittidae.

The family Spadellidae was recently divided into multiple genera. Species of Spadella were included within the Caribbean key to chaetognaths (Michel, 1984) but were absent from McLelland’s (1989) Gulf of Mexico review. They are included here because it is likely that their distribution may extend into South Atlantic Bight epibenthic and benthic communities. The appropriate taxonomic changes have been made to this key to accommodate the updated classifications erected by Salvini-Plawen (1986) and Bowman and Bieri (1989), however it is important to note that the due to the benthic nature of some spadellids, their distributions and morphological variability are poorly known.

The classification of Eukrohnia, Krohnitta and Pterosagitta species used in this key has remained static in past and current literature.

Classification of the Chaetognatha of the SAB and Gulf of Mexico

Phylum Chaetognatha (Leukardt, 1894)
     Class Sagittoidea Claus and Grobben, 1905
          Order Monophragmophora Casanova, 1985
               Family Spadellidae Tokioka, 1965
                    Spadella Langerhans, 1880
                    Paraspadella Salvini-Plawen, 1986
                    Hemispadella Casanova, 1996
               Family Eukrohniidae Tokioka, 1965
                    Eukrohnia Ritter-Zahoney, 1909
          Order Aphragmophora Tokioka, 1965
               Family Krohnittidae Tokioka, 1965
                    Krohnitta Ritter-Zahony, 1910
               Family Pterosagittidae Tokioka, 1965
                    Pterosagitta Costa, 1869
               Family Sagittidae Claus and Grobben, 1905
                    Caecosagitta Tokioka, 1965
                    Ferosagitta Kassatkina, 1971
                    Flaccisagita Tokioka, 1965
                    Mesosagitta Tokioka, 1965
                    Sagitta Quoy and Gaimard, 1827
                    Serratosagitta Tokioka and Pathansali, 1963
                    Solidosagitta Tokioka, 1965


Morphological Variation and the Taxonomy of Chaetognaths

Chaetognaths exhibit a high degree of morphological variation as a function of their maturity and environment. This aspect has, in part, caused the classification of the Chaetognatha to be currently in a state of flux. It is important to note some of the more common variants for identification purposes (from Bieri 1991):

size of mature individuals
relative length of the anterior fin
ratio of length of posterior fin on the trunk to the length of posterior fin on the tail segment
formation of the fin rays (accelerated by maturation)
shape of the tail fin
more fully developed alveolar tissue at colder temperatures or at lower salinities
increasing and decreasing number of hooks with age
increasing number of anterior and posterior teeth with age

References:

Bieri R. (1991) Systematics of the Chaetognatha. In: The Biology of Chaetognaths. Q Bone, H Knapp, AC Pierrot-Bults, eds. Oxford Scientific Publishing. pp 122-136.

Bowman, TE and Bieri, R (1989) Paraspadella anops, new species, from Sagittarius Cave Grand Bahama Island, the second troglobitic chaetognath. Proceedings of the Biological Society of Washington 102(3):586-589.

Casanova, J. (1999) Chatognatha. In: South Atlantic Zooplankton. D. Boltovskoy, ed. Backhuys Publishers. pp. 1353-1374.

McLelland, JA. (1989) An illustrated key to the Chaetognatha of the northern Gulf of Mexico with notes on their distribution. Gulf Research Reports 8(2):145-172.

Michel, HA. (1984) Chaetognatha of the Caribbean Sea and adjacent areas. NOAA Technical Report NMFS 15. US Department of Commerce. 33 p.

Salvini-Plawen, LC. (1986). Systematic notes on Spadella and on the Chaetognatha in general. Zeitschrift fur zoologische Systematik und Evolutionsforschung 24(2): 122-128.

Tokioka, T. (1965) The taxonomical outline of Chaetognatha. Publications of the Seto Marine Biological Laboratory 12:335-357.


Citation

Please cite page as:

Southeastern Regional Taxonomic Center [SERTC]. 2004 December. Chaetognaths of the South Atlantic Bight and the Northern Gulf of Mexico. SERTC Taxonomic Information and Educational Resources web page. <http://www.dnr.sc.gov/marine/sertc/Chaetognath%20key/Chaetognath%20key.htm>. Accessed Yr Mon da.