Million-legged carpet

Taxonomy and discovery

The first specimens retained for study were collected in October 1926 during a survey of sheltered reefs east of Isla Mocha. The zoologist Elena Aravena recorded them as mobile patches of encrusting tissue after an animal crossed a scratch in a collecting tray overnight. Preserved specimens contracted into thick, folded discs, concealing the mouth rows and substantially reducing their apparent width. Aravena and the comparative anatomist Henrik Sohl described Thalassopatina polyposa in 1928. Their dissections of relaxed specimens traced the oral branches into a connected gut and established that each disc was one animal.^[1]

The genus name combines the Greek thalassa, "sea", with the Latin patina, a shallow dish. The specific name refers to the numerous feet. Aravena and Sohl assigned the animal to the sea cucumbers but left its family uncertain. Its skin contains small perforated plates and branching rods of calcite, the material that forms the internal skeleton of other echinoderms. These structures, called ossicles, remained identifiable in specimens whose softer tissues had deteriorated. Ferrada's 1964 revision established Thalassopatinidae on the form of the ossicles, the repeated oral openings and the extensive branching of the ventral water-vascular canals.^[2]

Sequence comparisons by Viedma and Kehl placed Thalassopatina close to the stichopodid sea cucumbers. Subsequent classification within Synallactida retained the family because of its expanded sole and the loss of distinct external rows of tube feet. The five principal radial canals and associated nerve cords can still be traced internally, despite the animal's strong bilateral organization.^[3]

The common name arose from counts published by Rojas and Noll in 2019. Earlier accounts used “carpet holothurian” and estimated the number of feet from isolated pieces of preserved skin. Those estimates varied by nearly an order of magnitude because contraction changed the apparent density. The 2019 study combined photographs of living soles with whole-surface maps, separating the number of feet from the area occupied by a contracted animal.^[4]

Description

Body form

Large adults usually measure 65 to 90 cm across and 30 to 40 cm from front to rear. The long transverse axis is perpendicular to the usual direction of travel. The leading edge forms a shallow curve between rounded lateral lobes; the rear margin is more deeply scalloped. The outline changes as the animal follows cracks and projections in the underlying rock. Its central body is 12 to 18 mm thick, and the outer skirt tapers to less than 3 mm. When lifted, an animal folds under its own weight. Fluid pressure and tension in the muscular, collagenous body wall maintain its shape.^[2]

The upper surface is mottled olive-grey or brown, with pale granules over the embedded ossicles. Fine grooves outline low ridges above the larger gut branches. This pattern varies between individuals and persists through growth, allowing photographs to identify animals without attached tags. The underside is cream-coloured, shading to translucent white at the edge. Short, blunt papillae occur on the upper surface, but the walking feet are confined to the sole.

Mouths and digestive system

The mouths occupy four staggered rows in an 8–10 mm-deep band beneath the leading margin. Large adults usually have 200 to 500. The reference animal mapped by Rojas and Noll had 320 mouths, about 80 per row, across an 80 cm-wide front. Openings in the same row were about 10 mm apart; successive rows were offset sideways by about 2.5 mm. Each aperture is 0.5 to 0.8 mm across and has a small crown of short, lobed feeding tentacles. The crowns can extend separately while the body remains close to the rock.^[4]

Each mouth leads through a short foregut into a local digestive branch. These branches join collectors serving broad sectors of the body, with transverse connections between sectors. Waste passes through smaller hindgut branches to a row of outlet pores along the rear margin. A continuous body wall and coelom surround the entire gut, and the animal has one branched gonad. The repeated mouths are parts of one individual.^[2]

The mouth-by-mouth banquet is a feeding apparatus with 320 independently supplied positions, adjustable to the animal's four mouth rows. At one sitting, neighbouring mouths can receive diatoms, softened kelp and plain seawater from separate outlets. Coloured food particles reveal where their gut branches meet. A mouth can stop gathering while its neighbours continue; the table measures how the animal controls its mouths locally.

Tube feet

An extended tube foot is 0.6 to 1.0 mm long and ends in a pad 0.12 to 0.18 mm across. Each foot is a hollow muscular tube connected to a small internal ampulla. Compression of the ampulla moves fluid into the foot, extending it; contraction of the foot's longitudinal muscles shortens it. Glands in the terminal pad secrete a temporary adhesive. Release involves a change in the pad's secretions and peeling of its edge from the rock.^[6]

The feet cover most of the ventral surface, leaving the anterior feeding band, posterior outlet band and a narrow outer border bare. They form irregular local rows. At the reference density of 600 feet per square centimetre, a square millimetre contains about six feet. Their small terminal pads occupy only a fraction of this area, leaving space for each foot to extend, incline and retract without displacing all its neighbours.

Rojas and Noll's reference specimen measured 80 cm across and 36 cm from front to rear. Its mapped foot-bearing surface covered 2,000 cm² and carried an estimated 1.2 million tube feet. The estimate was obtained by counting feet in calibrated photographs from across the sole and weighting the counts by the area of each region. Density was lower behind the feeding band and higher near the lateral margins. The published total describes that specimen; adults of different sizes have different counts, and damaged or regenerating patches introduce further variation.^[4]

Physiology

The water-vascular system retains five main canals, joined by transverse connections and a network of progressively finer branches. Separate branches supply the walking feet and oral tentacles. Local changes in pressure operate groups of feet. Nerve cords run beside the main canals and connect to a dense peripheral network; small nerve loops around the mouths communicate along the feeding edge. Contact with a sharp projection causes nearby feet to withdraw while surrounding feet remain attached.^[6]

Oxygen passes across the thin body wall and tube feet. Two reduced respiratory trees open into an enlarged central hindgut chamber, which draws in water through the middle pore of the posterior row. In laboratory measurements at 11 °C, oxygen uptake fell when sediment coated the upper surface, even while that pore remained clear. Animals responded by arching the margin and moving toward exposed rock. Their tolerance of suspended sediment is greater than their tolerance of a settled layer.^[7]

Distribution and habitat

The confirmed range extends along the Chilean coast from the reefs near Quintero to the Gulf of Corcovado. Most records come from the mainland shore and nearby islands between 35° and 42° south. Within this range, the species is discontinuously distributed. Long stretches of apparently suitable coast separate reefs on which several animals can be found within a few metres of one another.^[5]

T. polyposa occurs mainly at depths of 7 to 42 m, below the zone of frequent wave scour. It occupies gently sloping bedrock and large, stable boulders among kelp holdfasts. Thin sediment films provide food, but mobile sand prevents firm attachment and buries the feeding surface. Animals are scarce on vertical faces and on rock densely covered by mussels or branching growth. Juveniles occupy small hollows and the undersides of ledges; their distribution is poorly represented by surveys that count only exposed adults.

Water temperatures at regularly occupied sites range seasonally from about 8 to 15 °C. At the northern end of the range, records are concentrated near persistent coastal upwelling. In sheltered southern channels, the animal is absent from the shallow, low-salinity surface layer after heavy rain. Lira found adults immediately below that layer, attached to rock between 12 and 18 m depth. Surveys made during different seasons can therefore produce different upper depth limits at the same locality.^[5]

Locomotion

Movement consists of overlapping cycles of attachment, shortening and release. A foot inclines forwards, presses its terminal pad to the rock and develops tension as it shortens. After releasing, it swings forward through the thin gap beneath the body. Adjacent feet tend to enter the same phase at slightly different times, producing bands of activity that travel across the sole. These are metachronal waves, comparable in timing to the successive strokes of cilia, although the feet move much more slowly.^[6]

The bands are easiest to see through transparent plates. On uneven rock they divide around obstacles, fade and re-form. A complete foot cycle commonly takes about three seconds, but individual feet can remain attached through several neighbouring cycles. Kehl and Sáez recorded ordinary forward speeds of 0.05 to 0.15 mm per second on clean, roughened glass. A continuously moving animal at 0.1 mm per second would travel 36 cm in an hour; field tracks are shorter because feeding and stationary periods interrupt movement.

The million-foot treadmill keeps a walking carpet in view above a transparent underwater belt. The floor retreats at about 0.1 mm per second, matching the animal's advance while a camera scans successive patches of the sole. Freshly coated sections return beneath the feeding edge, allowing hundreds of mouths to graze while more than a million feet repeatedly walk over the same loop of ground. The recordings distinguish local attachment waves from movement of the body as a whole.

Direction is set by the orientation and timing of local strokes. During a turn, the outer side advances while feet on the inner side hold or make shorter strokes. A tight turn produces folds in the inner margin. The animal can reverse over short distances, particularly when the leading edge meets a crevice too narrow for the thick central body. Sustained feeding runs proceed with the long mouth-bearing edge foremost, across the animal's shorter front-to-rear axis.

In stronger currents, the margin settles against the rock and a greater proportion of feet remain attached. Small patches can lose contact without detaching the whole animal. This distributed attachment also permits movement across shallow cracks: feet behind a gap continue to hold while those beyond it establish contact. Deep fissures halt progress. The million-legged carpet has not been observed swimming, and individuals overturned by divers take several hours to regain their normal position.^[6]

Feeding and ecology

The species is a surface deposit feeder. Tentacles around each mouth press against the rock, collect loose material and bend inward to deliver it. Several tentacles may gather food while another is being cleared at the same opening. Adjacent crowns work at different rates, so a patch of loose detritus can be collected without stopping the whole feeding edge. In undisturbed animals, the front margin rises by a few millimetres to expose the crowns. After a diver approaches or casts a shadow across the animal, the tentacles withdraw and the margin closes.^[8]

Gut contents include benthic diatoms, microscopic fragments of softened kelp and fine mineral particles coated with bacteria. Arce and colleagues found that the animals collected fine debris from recently shed kelp after it had softened and acquired a microbial film. Fresh blades placed in the path of feeding animals were usually crossed without being eaten. The rear pores discharge short, loosely bound faecal threads, which break apart in weak water movement. A feeding track can be recognized for several hours by its reduced cover of loose detritus.

The staggered mouth rows gather food across most of the strip that the body will occupy, before the feet pass over it. Successive rows work adjacent narrow tracks. The rock behind a feeding animal retains attached growth but carries less loose detritus, leaving a broad, irregularly paler trail. Food intake is concentrated in the first hours after dusk at exposed sites, although animals beneath dense kelp or overhanging rock also feed by day. Individuals repeatedly photographed at one reef moved between a small number of feeding patches and a sheltered resting position. Their routes changed after storms redistributed sediment. They sometimes crossed earlier tracks but seldom stopped to feed on them until a new film had accumulated.^[8]

Small grazers occupy the space around the margin. Amphipods shelter in its folds, and juvenile gastropods have been found among the dorsal granules. These associations appear temporary: animals kept on clean plates lost most of their accompanying fauna within a week. Larger adults can cover young barnacles and mussels while feeding, but their small mouths do not remove firmly attached shellfish.

Recorded injuries are concentrated at the edge of the body. Crabs clip the thin margin, leaving crescent-shaped scars that gradually close as the surrounding tissue expands. The thick central body is rarely missing from living specimens. Detached marginal pieces seal their wounds but have not developed into complete animals in culture. Small wounds acquire functional feet within several weeks. Where the feeding band has been damaged, new oral buds appear after the wound closes. The pattern of dorsal granules can remain altered after the outline has recovered.^[11]

Reproduction and development

The sexes are separate and externally similar. Eggs and sperm are released into the water through a small opening on the upper surface near the front of the body. At Isla Mocha, spawning has been recorded in late winter, usually during the first hours of darkness. Several animals at a site may spawn within the same evening, although many nearby adults remain inactive. Fertilized eggs are approximately 170 μm in diameter.^[9]

The embryo develops into a transparent, feeding auricularia larva, with a continuous band of cilia used for swimming and collecting small plankton. It later contracts into a non-feeding doliolaria stage. Settlement follows 18 to 24 days after fertilization in cultures maintained at 11 °C. The newly settled juvenile is about 0.7 mm long and bears one mouth and five primary tube feet. It remains close to the settlement crevice while its first oral tentacles begin collecting material from the surface.

The broad adult form develops gradually. Young animals first resemble short, flattened sea cucumbers with a narrow sole. The lateral body wall then expands faster than the front-to-rear axis. New mouths bud along and behind the growing anterior margin. The first additions flank the original opening; further buds form between and behind established mouths, producing staggered rows. Their gut branches join the existing collectors. New tube feet form across the expanding sole, and additional outlet pores open along the rear margin. The first pale dorsal granules become visible before the juvenile reaches a centimetre in length.^[9]

Growth varies with temperature and food supply. In a marked cohort followed by Yáñez and Curín, representative front-to-rear lengths were 5 cm after one year, 14 cm after two years, 28 cm after four years and 36 cm after six years. Width increased more rapidly, reaching about 4, 17, 50 and 80 cm at the same ages. Corresponding mouth counts were 24, 88, 224 and 320. These measurements describe surviving animals under local conditions. Sexual maturity generally occurs between four and six years. Following two warm winters, juvenile counts fell at three northern monitoring sites while adult counts changed little.^[10]

Study and conservation

Field surveys use overlapping photographs and the persistent pattern of dorsal granules to recognize individuals. Measuring a living animal requires a scale in the same plane as the body; a ruler placed above the substrate produces appreciable errors in oblique photographs. Counts of tube feet and mouths require an unobstructed view of the underside. For this purpose, animals are allowed to crawl from rock onto a transparent plate, which can be photographed from below without lifting the body.^[4]

The species is not commercially harvested. Local disturbances include sediment deposited during coastal construction and displacement of boulders used for attachment. At monitored reefs, adults survived brief periods of turbid water but disappeared from surfaces that retained a sediment layer. Recruitment recovered more slowly than adult occupancy after the affected rock was cleared. Escalona and Viedma recommended retaining permanent photographic plots because a census of large animals alone can miss several years of failed recruitment.^[11]

Escalona and Viedma classified the species as Data Deficient in their 2023 assessment. No range-wide population estimate has been published. The southern channels remain sparsely surveyed, and most long-term observations come from a few reefs accessible to scientific divers. These records document local losses; the population trend across the species' range remains unknown.^[11]

Cuteness-optimized form

A cuteness-optimized form of Thalassopatina polyposa was first developed in 2032 as a companion animal kept in marine aquaria. Its designers used plush aesthetics to make its slow grazing and many-mouthed feeding appealing to observe.

The plush form has a softly rounded, scalloped body, a velvety surface, pastel flower-like dorsal tufts and a small smiling face. It is a live animal with plush aesthetics. Its soft covering is living tissue, and it grows, feeds and moves under its own power.

The form retains the carpet's tube-foot locomotion and multiple feeding mouths beneath the broad leading edge. The visible smile is a curved facial marking; food enters through the mouths on the underside. It requires a seawater habitat and a supply of food suitable for a living deposit feeder.

See also

• Cuteness-optimized animals — animals whose anatomy is designed for cuteness.
• Polystomy — the presence of multiple functional mouths.
• Mouth-by-mouth banquet — an apparatus for feeding individual mouths of a carpet.
• Million-foot treadmill — an underwater belt for observing tube-foot locomotion.

References

1. ↑ a b c Aravena, E.; Sohl, H. (1928). “Thalassopatina polyposa, un holotúrido laminar de la costa de Mocha”. Anales de Zoología Litoral. 3 (2): 41-67.
2. ↑ a b c Ferrada, L. (1964). “The calcareous deposits and systematic position of Thalassopatina”. Studies in Pacific Echinoderms. 8: 103-138.
3. ↑ a b Viedma, R.; Kehl, M. (2020). “Molecular relationships of Thalassopatinidae within Synallactida”. Southern Marine Systematics. 26 (1): 19-36.
4. ↑ a b c d e Rojas, I.; Noll, B. (2019). “Surface area, podial density and total podial number in the carpet holothurian”. Journal of Benthic Morphometry. 14 (3): 201-224.
5. ↑ a b Lira, A. (1977). “Distribución batimétrica de Thalassopatina polyposa en Chile central y austral”. Cuadernos del Bentos Austral. 5: 7-39.
6. ↑ a b c d e Kehl, M.; Sáez, P. (2012). “Local coordination and temporary adhesion in the locomotion of Thalassopatina”. Marine Invertebrate Mechanics. 17 (2): 88-116.
7. ↑ Ibáñez, C.; Mora, D. (1998). “Cutaneous gas exchange in a flattened subtidal holothurian”. Comparative Littoral Physiology. 9 (4): 271-289.
8. ↑ a b c Arce, V.; Millán, F.; Kessler, J. (2015). “Detrital films and feeding paths of a kelp-forest holothurian”. Ecology of Temperate Reefs. 22 (1): 55-79.
9. ↑ a b Mancilla, S. (1986). “Desarrollo larvario y metamorfosis de Thalassopatina polyposa”. Anales de Zoología Litoral. 31 (2): 145-173.
10. ↑ Yáñez, O.; Curín, L. (2021). “Growth and recruitment of the million-legged carpet across a coastal temperature gradient”. Ecology of Temperate Reefs. 28 (3): 233-258.
11. ↑ a b c d Escalona, N.; Viedma, R. (2023). “Photographic monitoring, marginal injury and recruitment in Thalassopatina”. Cuadernos del Bentos Austral. 42: 61-94.