Onychophora

Distribution and habitat

Distribution

Velvet worms are found in the tropics and in the temperate zone of the Southern Hemisphere. Members of the family Peripatidae are found in the tropical regions of South America, Central America, the Caribbean islands, Gabon, Northeast India, and Southeast Asia. Meanwhile, members of Peripatopsidae are found in Chile, Australia, Southern Africa, New Guinea, and New Zealand. One deviation is the South African Peripatopsis capensis. This animal was inadvertently introduced to Santa Cruz Island in the Galapagos and co-occurs with native species. When looking at velvet worms as a whole, the majority of are found in Australia and South America.

Extinct onychophorans have been discovered in parts of the Northern Hemisphere which are currently uninhabited by the group. While the onychophoran affinities of Succinipatopsis and Helenodora are questioned, others like the Antennipatus (found in France) are at a minimum close relatives of crown-group Onychophora (Peripatidae and Peripatopsidae). This indicates that velvet worms were far more widespread in the past, but subsequently died off for unknown reasons.

Habitat

Velvet worms always sparsely occupy the habitats where they are found: they are rare among the fauna of their habitats.

All extant velvet worms are terrestrial, preferring dark, humid environments. They are often found in rain-forests, in micro-habitats such as moss cushions, leaf litter, rotting wood, termite tunnels, soil crevices, and under stones and tree trunks. They also occur in unforested grassland.

Some species live in caves, with at least one species living both on the surface and in caves. Velvet worms are already adapted to life in the dark and in crevices on the surface, so this has been described as an exaptation.

Some species of velvet worms are able to occupy human-modified land-uses, such as cocoa and banana plantations in South America and the Caribbean, but for others, conversion of rain-forests is likely one of the most important threats to their survival. (see Conservation).

Velvet worms are photophobic: They are repelled by bright light sources. Because the danger of desiccation is greatest during the day and in dry weather, velvet worms are usually most active at night and during rainy weather. Under cold or dry conditions, they actively seek out crevices in which they shift their body into a resting state.

Behavior

Feeding

Velvet worms are ambush predators, hunting only by night, and are able to capture animals at least their own size, although capturing a large prey item may take almost all of their mucus-secreting capacity. They feed on almost any small invertebrates, including woodlice (Isopoda), termites (Isoptera), crickets (Gryllidae), book/bark lice (Psocoptera), cockroaches (Blattidae), millipedes and centipedes (Myriapoda), spiders (Araneae), various worms, and even large snails (Gastropoda). Depending on their size, they eat on average every one to four weeks. They are considered to be ecologically equivalent to centipedes (Chilopoda).

The most energetically favorable prey are two-fifths the size of the hunting onychophoran. Ninety percent of the time involved in eating prey is spent ingesting it; re-ingestion of the slime used to trap the insect is performed while the onychophoran locates a suitable place to puncture the prey, and this phase accounts for around 8% of the feeding time, with the remaining time evenly split between examining, squirting, and injecting the prey. In some cases, chunks of the prey item are bitten off and swallowed; indigestible components take around 18 hours to pass through the digestive tract.

Onychophora probably do not primarily use vision to detect their prey; although their tiny eyes do have a good image-forming capacity, their forward vision is obscured by their antennae; their nocturnal habit also limits the utility of eyesight. Air currents, formed by prey motion, are thought to be the primary mode of locating prey; the role of scent, if any, is unclear. Because it takes so long to ingest a prey item, hunting mainly happens around dusk; the onychophorans will abandon their prey at sunrise.

Velvet worms literally creep up on their prey, with their smooth, gradual and fluid movement escaping detection. Once they reach their prey, they touch it very softly with their antennae to assess its size and nutritional value. After each poke, the antenna is hastily retracted to avoid alerting the prey. This investigation may last anywhere upwards of ten seconds, until the velvet worm makes a decision as to whether to attack it, or until it disturbs the prey and the prey flees. Hungry Onychophora spend less time investigating their prey and are quicker to apply their slime. Once slime has been squirted, Onychophora are determined to pursue and devour their prey, in order to recoup the energy investment. They have been observed to spend up to ten minutes searching for removed prey, after which they return to their slime to eat it. In the case of smaller prey, they may opt not to use slime at all. Subsequently, a soft part of the prey item (usually a joint membrane in arthropod prey) is identified, punctured with a bite from the jaws, and injected with saliva. This kills the prey very quickly and begins a slower process of digestion. While the onychophoran waits for the prey to digest, it salivates on its slime and begins to eat it (and anything attached to it). It subsequently tugs and slices at the earlier perforation to allow access to the now-liquefied interior of its prey. The jaws operate by moving backwards and forwards along the axis of the body (not in a side-to-side clipping motion as in arthropods), conceivably using a pairing of musculature and hydrostatic pressure. The pharynx is specially adapted for sucking, to extract the liquefied tissue; the arrangement of the jaws about the tongue and lip papillae ensures a tight seal and the establishment of suction. In social groups, the dominant female is the first to feed, not permitting competitors access to the prey item for the first hour of feeding. Subsequently, subordinate individuals begin to feed. The number of males reaches a peak after females start to leave the prey item. After feeding, individuals clean their antennae and mouth parts before re-joining the rest of their group.

Locomotion

Velvet worms move in a slow and gradual motion that makes them difficult for prey to notice. Their trunk is raised relatively high above the ground, and they walk with non-overlapping steps. To move from place to place, the velvet worm crawls forward using its legs; unlike in arthropods, both legs of a pair are moved simultaneously. The claws of the feet are used only on hard, rough terrain where a firm grip is needed; on soft substrates, such as moss, the velvet worm walks on the foot cushions at the base of the claws.

Actual locomotion is achieved less by the exertion of the leg muscles than by local changes of body length. This can be controlled using the annular and longitudinal muscles. If the annular muscles are contracted, the body cross-section is reduced, and the corresponding segment lengthens; this is the usual mode of operation of the hydrostatic skeleton as also employed by other worms. Due to the stretching, the legs of the segment concerned are lifted and swung forward. Local contraction of the longitudinal muscles then shortens the appropriate segment, and the legs, which are now in contact with the ground, are moved to the rear. This part of the locomotive cycle is the actual leg stroke that is responsible for forward movement. The individual stretches and contractions of the segments are coordinated by the nervous system such that contraction waves run the length of the body, each pair of legs swinging forward and then down and rearward in succession. Macroperipatus can reach speeds of up to four centimeters per second, although speeds of around 6 body-lengths per minute are more typical. The body gets longer and narrower as the animal picks up speed; the length of each leg also varies during each stride.

Sociality

The brains of Onychophora, though small, are very complex; consequently, the organisms are capable of rather sophisticated social interactions. Behavior may vary from genus to genus, so this article reflects the most-studied genus, Euperipatoides.

The Euperipatoides form social groups of up to fifteen individuals, usually closely related, which will typically live and hunt together. Groups usually live together; in drier regions an example of a shared home would be the moist interior of a rotting log. Group members are extremely aggressive towards individuals from other logs. Dominance is achieved through aggression and maintained through submissive behavior. After a kill, the dominant female always feeds first, followed in turn by the other females, then males, then the young.

When assessing other individuals, individuals often measure one another up by running their antennae down the length of the other individual. Once hierarchy has been established, pairs of individuals will often cluster together to form an "aggregate"; this is fastest in male-female pairings, followed by pairs of females, then pairs of males.

Social hierarchy is established by a number of interactions: Higher-ranking individuals will chase and bite their subordinates while the latter are trying to crawl on top of them. Juveniles never engage in aggressive behavior, but climb on top of adults, which tolerate their presence on their backs. Hierarchy is quickly established among individuals from a single group, but not among organisms from different groups; these are substantially more aggressive and very rarely climb one another or form aggregates. Individuals within an individual log are usually closely related; especially so with males. This may be related to the intense aggression between unrelated females.

Growth, development, and reproduction

Almost all velvet worms reproduce sexually. The sole exception is Epiperipatus imthurni, which have no males and reproduce by parthenogenesis. In most cases, velvet worms are sexually dimorphic. Females are usually larger than males and can often have more legs. All velvet worms have internal fertilization, though the way this is done varies widely. For most of them, a package of sperm cells called the spermatophore is placed into the female's vagina. In many species, fertilization happens only once. Because of this, copulation can happen before reproductive organs are even fully developed. In cases like this, sperm cells are kept in a special reservoir where they can survive for longer.

The detailed process by which this is achieved is in most cases still unknown, a true penis having been observed only in species of the genus Paraperipatus. In many Australian species, there exist dimples or special dagger- or axe-shaped structures on the head; the male of Florelliceps stutchburyae presses a long spine against the female's genital opening and probably positions its spermatophore there in this way. During the process, the female supports the male by keeping him clasped with the claws of her last pair of legs. The mating behavior of two species of the genus Peripatopsis is particularly curious. Here, the male places two-millimeter spermatophores on the back or sides of the female. Amoebocytes from the female's blood collect on the inside of the deposition site, and both the spermatophore's casing and the body wall on which it rests are decomposed via the secretion of enzymes. This releases the sperm cells, which then move freely through the haemocoel, penetrate the external wall of the ovaries and finally fertilize the ova. Why this self-inflicted skin injury does not lead to bacterial infections is not yet understood (though likely related to the enzymes used to deteriorate the skin or facilitate the transfer of viable genetic material from male to female). Velvet worms are found in egg-laying (oviparous), egg-live-bearing (ovoviviparous) and live-bearing (viviparous) forms.

In a recent peer-reviewed paper published in the "Journal of Zoology", researchers discovered that certain species of Peripatus exhibit a unique form of parental care. Unlike most invertebrates, where parental involvement is minimal, female Peripatus were observed actively guarding their eggs and even providing protection to their offspring after hatching. This finding challenges the conventional understanding of reproductive behavior in invertebrates and highlights the diversity of parenting strategies in the animal kingdom.

• Ovipary occurs solely in the Peripatopsidae, often in regions with erratic food supply or unsettled climate. In these cases, the yolk-rich eggs measure 1.3 to 2.0 mm and are coated in a protective chitinous shell. Maternal care is unknown.
• The majority of species are ovoviviparous: the medium-sized eggs, encased only by a double membrane, remain in the uterus. The embryos do not receive food directly from the mother, but are supplied instead by the moderate quantity of yolk contained in the eggs—they are therefore described as lecithotrophic. The young emerge from the eggs only a short time before birth. This probably represents the velvet worm's original mode of reproduction, i.e., both oviparous and viviparous species developed from ovoviviparous species.
• True live-bearing species are found in both families, particularly in tropical regions with a stable climate and regular food supply throughout the year. The embryos develop from eggs only micrometers in size and are nourished in the uterus by their mother, hence the description "matrotrophic". The supply of food takes place either via a secretion from the mother directly into the uterus or via a genuine tissue connection between the epithelium of the uterus and the developing embryo, known as a placenta. The former is found only outside the American continents, while the latter occurs primarily in America and the Caribbean and more rarely in the Old World. The gestation period can amount to up to 15 months, at the end of which the offspring emerge in an advanced stage of development. The embryos found in the uterus of a single female do not necessarily have to be of the same age; it is quite possible for there to be offspring at different stages of development and descended from different males. In some species, young tend to be released only at certain points in the year.

A female can have between 1 and 23 offspring per year; development from fertilized ovum to adult takes between 6 and 17 months and does not have a larval stage. Velvet worms have been known to live for up to six years.

Ecology

The velvet worm's important predators are primarily arthropods such as various spiders and centipedes, along with tetrapods such as rodents and birds. In South America, Hemprichi's coral snake (Micrurus hemprichii) feeds almost exclusively on velvet worms.

For defense, some species roll themselves reflexively into a spiral, while they can also fight off smaller opponents by ejecting slime. Various mites (Acari) are known to be ectoparasites infesting the skin of the velvet worm. Skin injuries are usually accompanied by bacterial infections, which are almost always fatal.

Evolution

External phylogeny

The closest living relatives of the Onychophora are the arthropods and tardigrades, with the three of forming the clade Panarthropoda. Each of these evolved from a broader group of legged, worm-like animals known as lobopodians. Below is a simplified phylogeny showing the clade's major groups (both extant and extinct) and where velvet worms fit within it.

Internal phylogeny

Below is a combined genus-level cladogram of velvet worms containing both extant and extinct species confirmed to be crown or stem-group velvet worms. This phylogeny does not analyze every species and lumps most peripatids into Neopatida, as many Peripatidae genera are paraphyletic. As of 2023, there are around 232 total living species, meaning this phylogeny should eventually be updated.

Genomics

As of February 2025, velvet worms have had only two nuclear genomes sequenced. These are of Euperipatoides rowelli (a peripatopsid) and Epiperipatus broadwayi (a peripatid). The first one is highly fragmented, while the second is less so, but still needs improvement. Velvet worms seem to display genome gigantism, with the more complete assembly (E. broadwayi) having an size of 5.60 giga-base pairs. Around 70.92% of its genome are repeat sequences, something that contributes to the bulk of its size. While less substantial, it also has very large introns, or parts of a gene that do not become proteins.

Taxonomy

Compared to other phyla, Onychophora has an odd taxonomic organization. This is partially a consequence of obsolete classification schemes. To illustrate the matter, the two main clades of velvet worms diverged around 274 million years ago during the Late Devonian, have since diversified, but are grouped as families rather than a class or order.

Velvet worms are divided into two main families (Peripatidae and Peripatopsidae). The group also contains three extinct genera (Antennipatus, Antennacanthopodia, and Tritonychus).

Within Peripatidae, the genera Eoperipatus (found in Southeast Asia) and Mesoperipatus (found in Gabon) were the most basal, while the rest of the group is found in tropical regions of the Americas. On the other hand, Peritpatopsidae can be divided into two main clades. One has members in Southern Africa and Chile, while the others live in Australasia.

Fossil history

Insights on decay and fossilization

Due to being soft-bodied, onychophorans need excellent conditions to fossilize. However, even when this happens, their fossils can be subject to taphonomic bias. Experiments were done with modern velvet worms to analyze their decomposition in various saline solutions. The study also investigated whether they experienced something called stemward slippage. In this phenomenon, animals are falsely categorized as more primitive due to the decay of certain features.

The researchers found that different features decayed at significantly different rates. Salinity and time of moult had little effect on decay, and the way things decomposed remained the same for different species (though it could happen at different speeds). Before any degradation, velvet worms flex into a S, U, or circular shape. Most flexing happens in the first 24 hours, but the process can continue for around two more days. In the early stages of decay, the epidermis and outer cuticle separate, causing a bloated appearance. The trunk elongates while increasing in width by around 10–30%. The limbs do the same, increasing in length and width by around 10–25%. Around the same time, the internal organs begin to degrade. This eventually culminates in the gut rupturing, destroying the other organs. In later stages of decay, the body cuticle shrinks close to its original size. A similar trend was found with the limbs, but it was just short of being statistically significant. It's around this time that many external features begin to deteriorate. These include the dermal papillae, leg rings, anus, gonopore, antenna, slime papillae, and eventually eyes. Interestingly, the dermal papillae on the trunk disappear faster than those on the limbs. Even as decay progresses, the body is still recognizable. This stops once the outer cuticle finally ruptures. After that, the animal's anatomy is extremely difficult to interpret. At this stage, the only identifiable features would be the chitinous jaws and claws.

Onychophorans are unlikely to experience stemward slippage since their defining features (jaws, feet, and slime papillae) are generally decay-resistant. However, decomposition has a significant impact on fossil anatomy. For starters, the preserved body outline is probably somewhat inaccurate, as this gets bloated in the decay process. A similar thing happens with the limbs, so this needs to be accounted for when analyzing locomotion and/or leg length. While fossilized onychophorans can appear to display patterning, these are not true to life — pigment granules are one of the first things to degrade and can easily move around in the body. Another finding is that characters such as internal organs or the body cavity are highly unlikely to fossilize. When it comes to placement of the mouth, even moderate decay makes it hard to tell if it is at the front or underside of the head. Additionally, fossils that lack decay-resistant features probably lacked them in life. For example, a fossilized onychophoran (or related animal) with eyes but no claws likely never had claws.

Emergence from lobopodians

Certain fossils from the Early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as lobopodians, were an evolutionary grade that gave rise to arthropods, tardigrades, onychophorans, and the extinct radiodonts.

How different lobopodians are related varies from study to study. However, with the exception of a single paper, Antennacanthopodia is the only animal to be confidently viewed as a close onychophoran relative. Antennacanthopodia lived during the Cambrian Stage 3 and possessed a variety of onychophoran traits: stubby lobopods, spiny foot pads, an annulated body, and small eyes behind the main antennae. An obvious difference from living onychophorans was that the animal had not one, but two pairs of antennae. These secondary antennae were shorter than the first pair and thought to be homologous with either the slime papillae or jaws of velvet worms. When Onychophora first arose or moved onto land is currently unknown. However, it could have plausibly happened between the Ordovician and Silurian – approximately 490 to 430 million years ago – via the intertidal zone.

Velvet worms and their diversification

The earliest potential onychophoran, Helenodora, originates from the Mid Pennsylvanian Mazon Creek lagerstätte in the United States. Originally described in 1980, Helenodora was long regarded as the earliest known onychophoran. This changed in 2016 when a paper by Murdock et al. doubted its affinity. Based on the absence of many decay-resistant features (claws, jaws, slime papillae) and unclear ecology (Carbotubulus, a late-surviving aquatic lobopodian, was found in the same formation), the authors of this study placed Helenodora as a basal lobopodian most closely related to Paucipodia.

The earliest definite onychophoran is the French Antennipatus from a Stephanian (Late Pennsylvanian) Lagerstätte in Montceau-les-Mines. This animal had a ventral (bottom facing) mouth, rings of dermal papillae on its trunk and limbs, and was at least somewhat terrestrial because of its slime papillae. However, due to the way it was preserved, it is unknown if Antennipatus belonged to the stem or crown group of the two living families of velvet worm (Peripatidae and Peripatopsidae).

Based on molecular dating, crown-group onychophorans diverged around 376 million years ago in the Late Devonian. This estimation happens regardless if Antennipatus is used to constrain the divergence date, or if no calibration is done at all. The earliest known crown-group onychophoran, Cretoperipatus, is known from multiple specimens in Burmese amber. All are from the same general location and date to a maximum age of around 98.79 million years ago (the earliest Cenomanian of the Late Cretaceous). Based on its morphology, Cretoperipatus was early-diverging member of Peripatidae, most closely related to the Asian genera Eoperipatus and Typhloperipatus.

Despite being preserved in amber, the affinities of the Cenozoic Tertiapatus and Succinipatopsis are surrounded in controversy. Some sources consider them to be onychophorans, while others dismiss this. Ultimately, these animals should be reanalyzed to better grasp their position on the tree of life.

Conservation status

The global conservation status of velvet worm species is difficult to estimate; many species are known only from their type locality where they were first described. The collection of reliable data is hindered by their low population densities, their typically nocturnal behavior, and possibly as-yet undocumented seasonal influences and sexual dimorphism. To date, the only onychophorans evaluated by the IUCN are:

• DD Mesoperipatus tholloni
• NT Plicatoperipatus jamaicensis
• VU Peripatoides indigo
• VU Peripatoides suteri
• VU Peripatopsis alba
• VU Peripatopsis clavigera
• EN Leucopatus anophthalmus
• EN Macroperipatus insularis
• CR Opisthopatus roseus
• CR Peripatopsis leonina
• CR Speleoperipatus spelaeus

The primary threat to velvet worms is destruction and fragmentation of their habitat due to industrialization, draining of wetlands, and slash-and-burn agriculture. The low population densities and restricted geographic ranges of many species means that localized disturbances of ecosystems could lead to the extinction of entire populations or species. Collection of specimens for universities or research institutes may plays a role on a local scale.

There is a pronounced difference in the protection afforded to velvet worms between regions. In South Africa, there are restrictions on both collecting and exporting, while in Australia, there are only export restrictions, and many countries offer no specific safeguards. Tasmania has a protection program that is unique worldwide: one region of forest has its own velvet worm conservation plan tailored to the species (Tasmanipatus barretti).