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CHAETOGNATHS Arrow Worms The Chaetognatha constitutes a distinctive phylum of about 100 marine species ubiquitous in the world’s oceans. These flattened, transparent-to-opaque creatures are easily distinguished from all other taxa. They may reach 1–2 cm, but most inshore arrow worms are smaller. Arrow worms are common along the coast and in the mouths of major estuaries during warm seasons. Most arrow worm species are so sensitive to minor changes in salinity that oceanographers can identify water masses as oceanic or coastal in origin by noting which specific arrow worms are present. Although their relationship to other invertebrate phyla is uncertain, new information suggests that the closest arrow worm relatives are roundworms (Phylum Nematoda). Arrow worms are hermaphroditic, with the male gonads developing first. Although self-fertilization is possible in the lab, cross-fertilization may be more common in nature. Fertilized eggs are often encased in a gelatinous matrix that may float at the surface or be attached to submerged vegetation. The eggs develop directly (without metamorphosis) into juvenile arrow worms. Several generations a year may occur. Most of the arrow worms likely to be encountered in nearshore waters were all conveniently in the genus Sagitta until Tokioka (1965) revised the phylum and divided Sagitta into nine genera. More recently, workers in the Gulf of Mexico have expanded on Tokioka ’s revisions, resulting in name changes for most of the local “Sagitta” species. Some, but not all, workers have accepted the newer classification. Both old and new names are listed in the identification section. Arrow worms often remain quiescent while waiting to ambush prey. When passive, they sink for a period and then swim upward in rapid short bursts as undulating waves pass from head to tail. Stiffness in arrow worms is caused by pressure in their internal “hydroskeleton .” They swim by alternating contractions of dorsal and then ventral longitudinal muscles. Many species undertake diel vertical migrations. Arrow worms are nonvisual predators. Arrays of sensory cilia detect hydrodynamic vibrations of prey, triggering sudden attack when victims are within range. A flick of the tail and flex of the body propel the arrow worm forward as it unsheathes its chitinous grasping hooks (or spines) around the mouth to capture the prey, primarily copepods. A venom (tetrodotoxin ) associated with the smaller teeth subdues the prey, which are then swallowed whole. Newly hatched arrow worms feed in the same fashion as the adults but on smaller CHAETOGNATHS 291 prey, especially copepod nauplii. Other common foods include tintinnids, barnacle larvae, and, on occasion, fish larvae and other arrow worms. When they are abundant in coastal waters, arrow worms can have a significant effect on copepod populations. Predaceous copepods, such as Candacia and Oncaea, and larger decapod larvae, fishes, squid, ctenophores , and jellyfishes eat arrow worms. IDENTIFICATION HINTS Arrow worms and some elongate larval fishes appear similar on first inspection, especially if the fish’s eyes have been lost (not uncommon). Arrow worms are distinguished from fishes by their anterior hooks and the presence of identical lateral fins on both sides (Fig. 27). Because the teeth and spines are often enclosed within the hood, they are often not visible through a dissecting microscope. Instead, use the following features to identify arrow worms to species. The location of the ovaries relative to the fins is usually an excellent characteristic in mature specimens. Variations in the lengths of the strings of eggs are common among individuals. Inspect multiple specimens before making a firm identification. The position of the ventral ganglion is often diagnostic. It is most easily seen from the side view, where it appears as a swelling on the ventral surface. Note the presence or absence of a fork at the anterior end of the intestine. This feature may not be visible in opaque specimens. The presence or absence of a collarette, a thin membrane on either side of the anterior trunk. Note its length and width. Collarettes are often rubbed off during collection, but remnants may still be found. Presence of a collarette is thus a more reliable characteristic than its absence. You may need to alter lighting to see the collarettes. Whether the rays on the lateral fins are complete (the rays extend from the outside of the fin to the body) as opposed to being restricted to the outer fin margin (incomplete ) may be diagnostic. Sometimes the shape of the tail section, possession of seminal vesicles, or presence of hairs along the trunk can be useful identification features. If these...


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