Parasagitta euneritica

A species of arrow worm found in the Pacific Ocean. Arrow worms are a major component of plankton.

Get ready for a wriggly Wednesday, because we have wormsign! When people think of animals, we tend to think of things like dogs, cats, elephants, and so on. However, animals like that are actually on the large side, most animals are much smaller. There are whole ecosystems out there filled with creatures so small you wouldn't even notice them unless you were looking. And any ecosystem needs a predator. This is where arrow worms come in, the apex predators of the planktonic realm.

(Image: a microscope image of an arrow worm. It is an elongated, tube-shaped animal with fins on either side and a small tail fin. The animal is a translucent white and organs are visible through the skin. Next to it are a measuring instrument and a small crustacean. End ID)

Arrow worms are members of the phylum Chaetognatha, which means "bristle jaw". They used to be thought of as their own thing with no surviving relatives, but they have recently been grouped together with rotifers and other tiny animals in a clade called Gnathifera. You know how there's no such thing as a fish because all the different things we call fish are actually really distinct from each other? Yeah, it's even worse for worms. At least most things we call fish are all in the same phylum. Arrow worms are in a completely different phylum from roundworms, flatworms, proboscis worms, segmented worms, etc, but most people just lump them all together as worms. Arrow worms are really small, the largest species getting to about 10 cm long and most being considerably smaller than that. They have torpedo-shaped bodies with external fins that are the source of the name. People thought they looked like the fletching on arrows. Most species are transparent, but some deep-sea species are orange.

(Image: the front end of an arrow worm seen biting a larval fish. The fish is translucent and skeletal. The worm has sharp bristles at the bout that are gripping the fish. End ID)

Arrow worms have three body segments, the head, trunk, and tail, divided from each other by internal membranes. The outside of the body is protected by a tough but flexible cuticle. The head is elongated and at the tip is the mouth. On either side of the mouth 4 - 14 curved spines that are attached to flexible muscle. The spines are used to grab prey and move it into the mouth. In some species, the spines can inject neurotoxin into prey to help kill it. When not in use, the mouth and spines are covered by a membrane to help streamline the animal. The mouth leads to a muscular pharynx (throat) that uses mucus to help food pass into the intestine, where the food is digested. I have found sources that say the intestine leads to an anus and other sources that say that arrow worms have no anus and excrete their waste through the skin. Most sources go with the worms having no anus. Also on the head are a pair of compound eyes (which are reduced or absent in some deep-sea or cave species) and a ring of cilia that probably sense chemicals. All over the body are bristles that sense the movement of the water. The nervous system is very simple and is centered on a nerve ring that circles the pharynx and leads out to the rest of the body. Arrow worms have no respiratory system, they absorb dissolved oxygen through the skin. The circulatory system is very simple. Arrow worms have a pair of lateral fins on either side of the body and a tail fin post-anus. Arrow worms are also one of the few animals species that act as host for giant viruses.

(Image: an image of an arrow worm with the organs and body parts labeled. Sourced from Wikipedia. End ID))

Arrow worms are simultaneous hermaphrodites, meaning they possess male and female sex organs at the same time. The male gonads develop first, making them protandric. The testicles are located at the base of the neck. Sperm is placed in a capsule called the spermatophore and ejected from the body at an organ called the seminal vesicle. During mating, each partner puts a spermatophore on the other's back. The spermatophore then releases sperm, which swim down a groove on the back to reach the oviduct, where eggs are released from the ovaries, along the tail. Fertilization happens either as the eggs are released or just after. Most species release their eggs to the water, but some will attach them to algae or carry them in a pouch on the back. Most arrow worms are semelparous, meaning they mate only once then die. Unusually for marine invertebrates, arrow worms do not have a larval stage. The offspring are miniature adults. The maximum observed lifespan for an arrow worm was 15 months.

(Image: an electron microscope image of the head of an arrow worm. The mouth is wide and has the scythe-like bristles emerging from either side. The head is attached to a long neck that is narrower than the mouth. End ID)

Arrow worms live worldwide in every marine habitat, including the deep sea and caves. Of all the marine zooplankton, only copepods have a greater global biomass. Most species are swimmers, but about 20% of known species live on the seafloor. They are ambush predators, moving slowly or staying still until prey comes within range, then darting forward to catch it. Arrow worms have the fastest muscle contractions of any animal, which helps with their quick charges. To swim, they wriggle their bodies up and down. A common swimming pattern is to swim upwards then glide downwards, over and over again. Pelagic species are known to practice diel vertical migration, a mass migration of countless species of animal that migrate to shallow water at night, then back to deep water in the day. Juveniles tend to live in shallower water than adults and larger species are generally found in colder water. Their primarily prey is copepods and water fleas, but they will also eat amphipods, krill, and the eggs and larvae of fish and invertebrates. Some species are cannibalistic. Some reports indicate certain species may be omnivores who also feed on algae and organic detritus. Arrow worms are a crucial food source for many larger animals, including commercially important species. Not a lot is known about their natural behavior as it is hard to simulate their conditions in the lab and hard to observe them in the wild.

(Sources - 1, 2, 3, 4)

Chaetognatha is a phylum of predatory marine plankton commonly known as arrow worms. They are found in all marine waters, including deep sea and polar oceans, making up a large portion of plankton worldwide. Most chaetognaths are dart-shaped and transparent, though some deep-sea species are orange. They range in size from 2 to 120 millimetres (0.1 to 4.7 in).

Chaetognaths hunt using 4-14 hooked, grasping spines on either side of their mouth. When the animal is swimming, these spines are covered and protected by a flexible hood. Their spines and teeth are made of chitin, and the head is protected by chitinous armor. Most chaetognaths have two compound eyes, as well as sensory bristles along the sides of their body and over their head and neck which allow them to detect the swimming vibrations of potential prey. They also have an organ on the top of their neck that is likely involved in chemoreception. Some use a neurotoxin to subdue prey. They feed on smaller planktonic species such as copepods, cladocerans, amphipods, krill, smaller chaetognaths, and even fish larvae. Some species are omnivores, also feeding on algae and detritus.

All species are hermaphroditic and reproduce sexually, some even having elaborate courtship rituals. Unusual among marine invertebrates, chaetognaths do not have a larval stage or undergo metamorphosis, instead gradually growing after hatching from eggs. Their lifespans are relatively short, with the longest recorded being 15 months.

Propaganda under the cut:

Rotifer. This phylum is made up of microscopic aquatic animals with a cilliated corona sourrounding the mouth

Chaetognatha. This phylum is made up of tiny predatory marine zooplankton, known as arrow worms

Dicyemida (Rhombozoa): A small phylum of wormlike parasites specialized to live in the kidneys of cephalopods. While its evolutionary relationships are still being debated, it is believed that they are closely related to Orthonectida. They reproduce asexually in their host until they reach a certain population density, at which point new larvae will grow into an adult form that will reproduce sexually.

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glorpiness rating: at 9:30 we'll travel to that location, 78% approximately

photographs by Chris Isaacs (sacisaacs) on iNaturalist. link to original observation