Velvet worms, also known as Onychophora (from Ancient Greek: ονυχής, onyches, "claws"; and φέρειν, pherein, "to carry"), are a unique phylum of soft-bodied, segmented invertebrates. Their velvety texture, due to the scaly papillae on their skin, and worm-like appearance give them their common name. Often ambiguously called peripatus, after the first described genus, Peripatus, these creatures are endemic to terrestrial environments. This article delves into the dietary preferences and feeding strategies that define these fascinating animals, as well as their evolutionary adaptations and ecological roles.
Overview of Velvet Worms
Velvet worms are generally considered close relatives of the Arthropoda and Tardigrada, with which they form the proposed taxon Panarthropoda. They are segmented animals with a flattened cylindrical body cross-section and rows of unstructured body appendages known as oncopods or lobopods ("stub feet"). They reach lengths between 0.1 and 22 cm (0.04-8.66 in) depending on species, with the smallest known being Ooperipatellus nanus and the largest known Mongeperipatus solorzanoi. The number of leg pairs ranges from as few as 13 (in Ooperipatellus nanus) to as many as 43 (in Plicatoperipatus jamaicensis). Their skin consists of numerous, fine transverse rings and is often inconspicuously coloured orange, red or brown, but sometimes also bright green, blue, gold or white, and occasionally patterned with other colours.
Predatory Nature and Diet
All 230 known velvet worm species are predatory. Velvet worms share much of their ecology with predatory centipedes, including their feeding patterns. They are carnivores feeding on small invertebrates like woodlice, termites, barklice, millipedes and centipedes, worms, spiders, and cockroaches.
Hunting Strategy
The velvet worm hunting strategy goes something like this: seek, stalk, sense, incapacitate, bite and inject, wait to liquefy, and consume. Once locked on its potential target, the velvet worm will sneak up on it with remarkable steadiness. When it gets close enough, it will swiftly brush its antennae against the still-unsuspecting insect, probably to check its size.
Slime Production and Use
The papillae are equipped with large and complex glands that produce protein-based milky slime, sticky at first and hard and water-insoluble when dry - like glue. Since the papillae have muscles like all other legs, the velvet worm can point and squirt his “hunting glue” in the direction it wants and at quite some lengths - up to two feet.
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The Onychophora forcefully squirt glue-like slime from their oral papillae; they do so either in defense against predators or to capture prey. The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction. High speed films show the animal expelling two streams of adhesive liquid through a small opening (50-200 microns) at a speed of 3 to 5 m/s (10 to 20 ft/s).
The velvet worm squirt is not straight - the stream oscillates in milliseconds, governed by the “spray nozzle’s” musculature. That is why velvet worm sticky spray can cover an entire surface - and the whole prey. However, they are pretty economical because the slime can take weeks to regenerate. The slime glands themselves are deep inside the body cavity, each at the end of a tube more than half the length of the body. The tube both conducts the fluid and stores it until it is required. The distance that the animal can propel the slime varies; usually it squirts it about a centimetre, but the maximal range has variously been reported to be ten centimetres, or even nearly a foot, although accuracy drops with range. It is not clear to what extent the range varies with the species and other factors. The slime can account for up to 11% of the organism's dry weight and is 90% water; its dry residue consists mainly of proteins-primarily a collagen-type protein. 1.3% of the slime's dry weight consists of sugars, mainly galactosamine. The slime also contains lipids and the surfactant nonylphenol.
Biting and Digestion
The prey is now completely covered in sticky goo, which quickly turns into stiff fibers - its protein shackles. After waiting a bit, the velvet worm moves in, finds a weak, soft spot on the prey’s body, and bites in with a pair of jaws. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", or mandibles, which are strongly hardened and resemble the claws of the feet, with which they are serially homologous; early in development, the jaw appendages have a position and shape similar to the subsequent legs. The jaws are divided into internal and external mandibles and their concave surface bears fine denticles. They move backward and forward in a longitudinal direction, tearing apart the prey, apparently moved in one direction by musculature and the other by hydrostatic pressure. The surface of the mandibles is smooth, with no ornamentation. The cuticle in the mandibles (and claws) is distinct from the rest of the body. Two salivary glands discharge via a common conductor into the subsequent "throat", which makes up the first part of the front intestine.
Because of its sensitivity to drier daytime conditions, velvet worms have only a few daily hours to hunt and feed. It is thought that this evolutionary pressure made them such voracious predators and led to sociality - group living and foraging - in some velvet worm species. Velvet worms spend about 90% of their feeding time ingesting the prey.
Novel Feeding Behaviors in Costa Rican Velvet Worms
Recent studies have revealed novel feeding behaviors in Costa Rican velvet worms, including food hiding, parental feeding investment, and ontogenetic food shift.
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From December 2016 to January 2018, individuals from eight morph-species of onychophorans from the sub-family Neopatida were housed in terraria. They were fed once a week with live or freshly killed domestic crickets introduced from 19:00 to 20:00h. Prey not consumed after 12h was removed to prevent contamination.
Food Hiding
The onychophorans hid food in burrows, and under moss or bark: small prey was dragged, while pieces of larger prey were carried in the mouth. Hiding unfinished food protects it from predators and scavengers, and has evolved independently in many species, from invertebrates to mammals. Food hiding may be particularly important in onychophorans because they hunt only a few times per month and are slow to process food. Unlike E. rowelli, which dispute food, these species shared the prey.
Prey Consumption
Specimens from Gandoca started the feeding process on the prey’s thorax, while those from Batán, Quesada, Sarapiquí and Fortuna, as well as Epiperipatus biolleyi and Peripatus solorzanoi, started feeding by the prey`s head; the Agujas and San Vito species started by the abdominal region.
Parental Feeding Investment and Ontogenetic Food Shift
During their first two weeks of life, the young only fed on the adhesive threads used to capture prey by the adults, rather than on the prey itself, apparently a form of parental feeding investment. After those two weeks, there is an ontological diet shift and adult females of the Gandocan morph-species and P. solorzanoi, shared the prey with their offspring (or the young found prey by themselves). An ontological diet shift is present in a variety of organisms, and results from strong selection to optimize the use of limited resources.
Two hypotheses have been proposed: (1) the young feed on adhesive because it is a better food than the prey; or (2) that initially they cannot process prey because their digestive system is immature.
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Digestive System
The digestive tract begins slightly behind the head, the mouth lying on the underside a little way from the frontmost point of the body. Here, prey can be mechanically dismembered by the mandibles with their covering of fine toothlets. On entering the central intestine, food particles are coated with a mucus-based peritrophic membrane, which serves to protect the lining of the intestine from damage by sharp-edged particles. The intestinal epithelium secretes further digestive enzymes and absorbs the released nutrients, although the majority of digestion has already taken place externally or in the mouth. The rear intestine opens into the rectum, where undigested food material arrives.
Social Behavior and Feeding
Members of the genus Euperipatoides, one of the most widely studied genera of velvet worms, form social groups of up to fifteen individuals. Dominance hierarchy is quickly established and maintained within these groups, with the superior individuals often chasing and biting the subordinate ones.
Habitat and Ecological Role
Velvet worms are found in both tropical and temperate zones of the Southern Hemisphere. While peripatids live in equatorial and tropical regions, peripatopsids are restricted to the south of the Equator. They are abundant in Central and South America, the Caribbean islands, West and South Africa, New Guinea, Australia, New Zealand, and Asia. In Asia, they are mostly found in northeastern India, Thailand, Indonesia, and Malaysia.
Being photophobic (light-hating), they resort to dark and humid habitats, particularly in the rainforests and a few in grasslands, where they hide in crevices in the soil. Some prefer living among moss cushions and leaf litter, in termite tunnels and rotting wood, and under tree logs and stones.