This key is best used in conjunction with the figures of Tokioka (in Yamaji's Nihon Kaiyo Purankuton Zukan), Alvariño (Naga Report series) and the recent papers by J.-P. Casanova.
Two pairs of lateral fins: Sagittidae (A) One pair of lateral fins: Eukrohniidae (B) or Heterokrohniidae (C)
(A) Sagittidae
1. Small (less than 14 mm) sagittid chaetognaths may be epipelagic contaminants. (most common off California are Serratosagitta bierii and Parasagitta euneritica). The common midwater small sagittid is Decipisagitta decipiens.
2. Large (up to 50 mm), weakly muscled, flaccid body: Flaccisagitta (I) or Pseudosagitta (II).
3. Very large head in relation to the body (up to 27 mm), gut has orange pigmentation: Caecosagitta macrocephala. Habitat depth = 1000-5000 m
4. Very robust body (up to 25 mm) with large number of ammonia-filled buoyancy sacs in body cavity (adds to opacity), anterior and posterior lateral fins are connected by very thin extension of posterior fin tissue; seminal vesicles may be blue: Solidosagitta zetesios. Habitat depth = 250- 1000 m
(I) Flaccisagitta: anterior and posterior lateral fins are well separated and very distinct.
1. Anterior lateral fin is very short (1/6 of total body length) and semi-circular; only 2-4 anterior teeth per side: F. hexaptera. Habitat depth = 0-500m.
2. Ova extend only several millimeters up body cavity in mature specimens, total body length up to 30 mm, anterior lateral fin elongate (1/4-1/5 of total body length): F. enflata. Habitat depth = 0-200m.
(II) Pseudosagitta: anterior and posterior lateral fins are connected by thin extension of posterior fin tissue.
1. Tail section of body is constricted and only 1/8 of total body length: P. lyra (=P. scrippsae). Habitat depth = 0-500 m
2. Tail section of body is large up to 25% of total body length: P. maxima. Habitat depth = 400-800 m
(B) Eukrohniidae: overall appearance tapered due to long, tapering lateral fins.
1. Orange-pigmented gut, up to 40mm body length: Eukrohnia fowleri.
2. Oil droplets in gut: Eukrohnia hamata (see B-3: most likely) or Eukrohnia bathypelagica (see B-4: rarely).
3. Ova straight, extending up to 1/3 of body length; lateral fin more or less connected with body posteriorly: Eukrohnia hamata. Habitat depth = 400- 1000m.
4. Ova curled up in body; lateral fin well separated from body posteriorly: Eukrohnia bathypelagica. Habitat depth = 1000-2500m.
5. Recently reported from off Mexico, Eukrohnia macroneura, mature at 14 mm, lacks eyes, rigid slender body and Eukrohnia bathyantarctica, similar to E. fowleri noted above, but with eyes that lack pigmentation and about half the number of teeth (up to 16).
(C) Heterokrohniidae: overall appearance "square", "blocky", one pair lateral fins are blunt, often missing. Most Heterokrohnia are bathy-benthopelagic. Presumably, all have orange-salmon pigmented guts when alive.
1. At first glance looks like Caecosagitta macrocephala, but larger up to 45mm; intricate pattern of orange pigment on head and neck: Heterokrohnia murina. Habitat depth = 2500-4500+m, usually within 100m of the bottom.
2. Slender, easily mistaken for E. bathypelagica at first glance, with little pigment in the gut, body length less than 30mm: H. mirabilis. Habitat depth = 2000-3000m. Most common species of Heterokrohnia. If it has short scale-like anterior teeth, it may be Heterokrohnia heterodonta.
3. Body less than 10 mm with conical seminal vessicles in contact with lateral fins but separated from the tail fin: Heterokrohnia wishernae
4. Body less than 10 mm with rounded seminal vessicles in contace with both the lateral fins and tail fin: Heterokrohnia alvinae.
Many new species of deep-sea Heterokrohnia have been described in the last several years. Save any suspected Heterokrohnia, note trawl number, depth and preserve in 5% formalin (NOT ETHANOL).
Species mentioned in this key: