Onychophora

Diversity

Phylum Onychophora , or velvet worms, contains approximately 180 species of bilaterally symmetrical, coelomate organisms that somewhat resemble caterpillars. Onychophorans range from 5 mm to 15 cm in length, with homonomous bodies and small heads. The head carries a pair of annulated, fleshy antennae and a pair of small eyes at their bases, with large, chitinous lenses and a well-developed retinal layer, as well as a pair of jaws surrounded by circular lips, and a pair of fleshy oral papillae, also known as slime papillae. The sticky secretions produced by the latter structures are used to capture prey, which includes other small invertebrates. Living species are divided into two families, Peripatidae and Peripatopsidae , which live in mutually exclusive geographical regions; peripatids are circumtropical, while peripatopsids are circumaustral. Members of the two families differ in their number of legs (generally greater in peripatids), the position of their gonopore (on a more posterior body segment in peripatopsids), and their reproductive habits. All of the currently known living species are terrestrial, living mainly in dark, moist microhabitats. Research indicates that they are either modified arthropods or represent a link between annelids and arthropods, most recent molecular phylogenies favor the first hypothesis.

Geographic Range

The two onychophoran families live in mutually exclusive regions. Peripatids are circumtropical, commonly found in Chile, South Africa, and Australasia, while peripatopsids are circumaustral, found in Southeast Asia, west equatorial Africa, northern South America, Central America, Mexico, and the Antilles.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

All currently living onychophoran species are terrestrial, although the fossil record shows they were likely aquatic at some point, the shift to land probably took place during the Ordovician period. They are nocturnal and photonegative, living primarily in dark, moist microhabitats such as forest litter and soil, within bromeliads, or in rotten logs. A few species are cave dwelling or live in drier woodlands and grasslands.

• Habitat Regions
• temperate temperate
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• savanna or grassland savanna or grassland
• forest forest
• rainforest rainforest
• scrub forest scrub forest

Systematic and Taxonomic History

Early researchers considered onychophorans to be a link between annelids and arthropods , as they share characteristics with both phyla. Recent molecular research has suggested that onychophorans are actually modified arthropods, supporting the shared morphological characteristics between these phyla, including a chitinous cuticle, as well as features of their respiratory system and embryogenesis. Onychophorans and arthropods also share an evolutionary relationship with phylum Tardigrada , and together form the unranked taxon Panarthropoda . This clade belongs to superphylum Ecdysozoa , which contains other molting invertebrates, such as nematodes , kinorhynchs , and loriciferans , but the evolutionary relationships between these larger groups are still the subject of active debate. As relatively poorly studied organisms, research on the evolutionary relationships within the phylum is sparse, although the monophyly of the phylum and its two families has consistently been strongly supported.

Physical Description

Onychophorans range from 5 mm to 15 cm in length, with homonomous bodies and small heads. The head houses a pair of annulated, fleshy antennae and a pair of small eyes at their bases, which are direct, with large, chitinous lenses and a developed retinal layer; a pair of jaws surrounded by circular lips; and a pair of fleshy oral papillae, also known as slime papillae. These animals are covered in a thin, flexible, permeable, chitinous cuticle, which overlays a thin epidermis. The cuticle is characteristically covered in tubercles or papillae with sensilla, arranged in rings or bands around the trunk and appendages. These protrusions are covered in scales, giving the animals a velvety appearance that gives rise to their common name. Under the epidermis, onychophorans have a dermis of connective tissue and layers of circular, diagonal and longitudinal muscles. These animals have smooth and striated muscle tissue and their hemocoels are partitioned into sinuses; the coelom is restricted to the gonodal cavities. They have hemocoelic, hydrostatic skeletons. Onychophorans are generally blue, black, green or orange. Onychophorans have 13 to 43 pairs of lobopodal walking legs; peripatopsid species have a fixed number of legs, while peripatid species may have a variable number of legs. The conical legs are filled with hemocoelomic fluid and have extrinsic muscle extensions. Each leg has a multi-spined terminal claw and a distal transverse pad on which it rests while walking. Onychophorans are typically sexually dimorphic, females are larger than males. In species where the number of leg pairs are variable, females also have more legs.

The mouth opens into a chitin-lined pharynx and esophagus (foregut), which lead to a long, straight intestine where most digestion and absorption occurs. The hindgut typically loops over the intestine before it leads to the anus located on the last body segment (terminally or ventrally). Each leg-bearing body segment also has a pair of nephridia with associated nephridiopores near the base of each leg, with the exception of the fourth and fifth legs, where nephridiopores are located distally on transverse pads, and the segment where the gonopores are located, which has no nephridia. Nephridia are connected internally to a sacculus (coelomic end sac) which leads to a nephridioduct, together; the sacculus and nephridioduct are called the segmental gland. Each nephridioduct has a contractile bladder that opens via the nephridiopore. Some species have eversible sacs via hemocoelic pressure, or vesicles opening near the nephridiopores as well, which may help to take up moisture.

Velvet worms also have well-developed circulatory systems, similar to arthropods. The heart is tubular, opens at both ends, and lies in a pericardial sinus. The blood, which is colorless and has no oxygen binding capabilities, enters the heart through lateral ostia and leaves anteriorly, flowing throughout the hemocoel within the body’s sinuses. Hemal channels are also located under the cuticle, between the circular muscle and oblique muscle layers, and are likely important as part of the hydrostatic skeleton. Gas exchange occurs via diffusion across the body wall and through tracheae, which open through small spiracles located between the bands of body tubercles. Tracheal units are small and only provide gases to the tissues immediately adjacent to them.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes shaped differently

Development

Onychophorans may be oviparous, viviparous or ovoviviparous. In oviparous species, large, oval, yolky eggs (up to 2 mm in diameter), with chitinous shells are produced and laid through an ovipositor. Early, superficial, intralecithal cleavage occurs, due to the large size of the yolk, followed by the development of a germinal disc. Ovoviviparous species may produce large yolky eggs (up to 1.5 mm in diameter), or smaller eggs (0.5 mm in diameter) with little to no yolk and thin membranes. Early cleavage of ovoviviparous embryos is intralecithal in those eggs with yolks; development has not been described well for non-yolky ovoviviparous eggs. Viviparous species produce small, spherical, non-yolky eggs and may or may not have placentas, when no placenta is present, eggs are even smaller and cleavage is total and equal. Vivipary is the most commonly observed reproductive strategy for this phylum. Onychophorans develop directly and hatch, or are born as juveniles with developed body segments and organs.

Reproduction

Mating behavior has seldom been observed and seems to vary widely amongst species. Both dermal and vaginal inseminations are known. Males of some species ( Florelliceps stutchburyae , Planipapillus annae ) have specialized head structures with which they transfer a spermatophore directly to a female's genital opening; during the transfer, a male is held in place by the female's lodopod claws. Others deposit spermatophores on females’ bodies, triggering a breakdown of integument so that the sperm may pass into the hemocoelic fluid, through which it travels to the ovaries. Females of some species have seminal receptacles, which are enlargements of the uterus. Although the mating systems of most onychophorans have not been determined, polygyny is most likely, as vivipary is the most common reproductive strategy.

• Mating System
• polygynous polygynous

Onychophorans are dioecious, with the exception of one known parthenogenetic species ( Epiperipatus imthurni ), and sexually dimorphic, females are larger than males, and potentially have more legs. Males have a pair of elongated testes, their sperm ducts join into a single tube, where spermatophores up to 1 mm in length are formed. The single sperm tube opens in a posteroventral gonopore. Females have a pair of ovaries that are mostly fused and located in the posterior region of their bodies, connecting to gonoducts that fuse into a uterus, also opening through a posteroventral gonopore. Fertilization is internal.

Peripatopsids may be oviparous, oviviparous or viviparous and produce 6 to 23 offspring a year, while peripatids are viviparous and only produce 1 to 8 offspring per year. Peripatopsid eggs develop within 6 to 17 months, or are born after 11 to 13 months of gestation; the gestation period for peripatids is approximately 1 year. Some onychophorans breed once a year, while others may breed multiple times in a year. Gestation in viviparous species can be up to 15 months, a female may have embryos of varied ages developing in her uterus at any given time.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• parthenogenic parthenogenic
• sexual sexual
• asexual asexual
• fertilization fertilization
  • internal internal
• viviparous viviparous
• ovoviviparous ovoviviparous
• oviparous oviparous

Beyond gamete production, the parental investment exhibited by onychophorans is unknown. In captivity, some species have been observed in adult-newborn groups, in the wild, groupings of young individuals are often found.

• Parental Investment
• no parental involvement
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

Onychophorans typically live several years; females are not known to reproduce until reaching at least 1.4 years of age.

Behavior

Onychophorans are typically solitary; they are also photonegative and nocturnal. They are most active in humid and damp environments; activity greatly decreases during dry periods. Onychophorans move using their lopodal legs and extending and contracting their bodies using hydrostatic forces. Movement is from front to back, in a wave; when an anterior segment elongates, their legs lift and move forward and, when it contracts again, anterior legs and segments are pulled forward.

• Key Behaviors
• terricolous
• nocturnal nocturnal
• motile motile
• sedentary sedentary
• solitary solitary

Communication and Perception

Onychophorans have large, bilobed cerebral ganglia dorsal to the pharynx, which are attached to a pair of ventral nerve cords (via transverse segmental commissures) and supply nerves to the head elements, also giving rise to the paired nerves of the appendages and body wall. The tubercles and papillae on their body surfaces are covered in sensilla. They have a small dorsolateral eye at the base of each antenna, which are direct, with a chitinous lens and well-developed retinal layer. There is evidence that males of some onychophoran species secrete a pheromone from papillae on their legs, attracting both males and females in order to disperse and colonize new habitats such as rotting logs.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

• Other Communication Modes
• pheromones pheromones

• Perception Channels
• visual visual
• tactile tactile
• chemical chemical

Food Habits

Onychophorans are carnivorous, typically feeding on small invertebrates such as beetles, termites, and other insects, as well as snails and worms. They sometimes pursue their prey into small spaces. Onychophorans produce a powerful adhesive in their slime glands, which open at the end of their oral papillae; they can shoot this substance up to 30 cm to trap prey. Onychophorans use their jaws to grasp and cut prey and inject salivary secretions, which are produced by paired salivary glands and delivered along a median dorsal groove on the jaws. These secretions begin to digest the prey so the semi-liquidized tissues can be sucked into the mouth.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore
  • eats non-insect arthropods
  • molluscivore molluscivore

Predation

The adhesive substance used by onychophorans to ensnare prey may also be used to evade predators. Likewise, the lighter coloration of juveniles may be a type of camouflage. Their predators include birds, centipedes, and spiders, as well as Hemprichi's coral snakes, which are believed to feed almost exclusively on onychophorans.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

The regular molting cycles of onychophorans may be, in part, an anti-ectoparasite mechanism. Phoretic mites have been reported on some onychophorans, as have unspecified bacteria reproducing in their mid-line cuticular pits.

Economic Importance for Humans: Positive

Outside of scientific research, there are no known positive effects of onychophorans on humans.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known adverse effects of onychophorans on humans.

Conservation Status

Onychophorans are not considered threatened or in danger of extinction.

Other Comments

This phylum was part of the early Cambrian marine diversification and has an extensive fossil record throughout North America and Asia. Beyond their move from water to land, little has changed for these species over the last 530 million years.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

References

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