Flower urchin

Appearance

Flower urchins are relatively large sea urchins. They can reach a maximum diameter of around 15 to 20 cm .

Like most echinoderms, the body of adult flower urchins is equally divided into identical segments around a central axis in multiples of five . The rigid "shell" has five interambulacral segments separated from each other by five ambulacral segments, each of them are composed of smaller regularly interlocking plates. It is overlaid by a thin layer of skin in living individuals. The test is variegated in coloration, usually deep red and grey, though there are rare instances of green and pale purple. Each ambulacral segment is ornamented by a large purple zigzag pattern running along its length.

Two rows of tube feet emerge from the grooves on either side of each of the ambulacral segments . The tube feet are individually composed of a thin muscular stalk tipped with a small suction cup . The mouth is centrally located in the bottom surface of the test. It is surrounded by a ring of small plates overlaid by softer tissue known as the peristome. Embedded in the peristome are five calcareous "teeth" collectively known as Aristotle's lantern. These are used for grinding the flower urchin's food. The anus is situated on the upper surface of the test, directly opposite the mouth. Like the mouth, it is surrounded by a ring of small plates known as the periproct. Surrounding the anal opening are five smaller holes which are directly connected to the gonads inside the body cavity.

The most conspicuous feature of flower urchins are their pedicellariae . Flower urchins possess four types of pedicellariae, distinguished by form and function, but only two are abundant. The first type are the ophicephalous pedicellariae. They resemble tube feet, except they end in three small claws rather than suction cups. These are used to keep the body surface clear of algae, encrusting organisms, and unwanted debris.

The second type are the globiferous pedicellariae which superficially resemble flowers . These are more specialized and are used for defense against predators and larger ectoparasites. Globiferous pedicellariae also end in a three-valved claw-like grasping appendage, like they do in ophicephalous pedicellariae, but they are much larger. The valves are connected to each other by a distinctive circular membrane around 4 to 5 mm in diameter. They are pinkish-white to yellowish-white in color with a central purple dot and a bright white rim. Each valve ends in a sharp fang-like tip which is capable of penetrating human skin. The base of the valves also house venom glands. Some authors further subdivide globiferous pedicellariae into two subtypes based on size - the trumpet pedicellariae and the giant pedicellariae. The other two types of pedicellariae - tridentate and triphyllous - are rare or restricted only to certain areas of the test.

The relatively blunt spines are quite short and are usually hidden below the flower-like pedicellariae. They can vary from white, pink, yellow, light green, to purple in coloration with lighter-colored tips.

Other members of the genus ''Toxopneustes'' are similar in appearance and can be mistaken for flower urchins. ''Toxopneustes roseus'' can be distinguished by the uniform coloration of their tests of pink, brown, or purple. It is also restricted to the East Pacific and thus aren't found together with flower urchins. ''Toxopneustes elegans'', which is only found around Japan, can be distinguished by the presence of a distinctive dark stripe just below the tips of their spines. ''Toxopneustes maculatus'' is a very rare species known only from specimens from Réunion, Christmas Island, and the Palmyra Atoll. It can be distinguished by the bright violet coloration on the bottom and in a band around the middle of their tests.

Naming

The commensal alpheid shrimp ''Athanas areteformis'', can sometimes be found living among the spines of flower urchins . The intestines of flower urchins can also serve as habitats for the commensal flatworm ''Syndesmis longicanalis''.

Flower urchins are also common hosts of the zebra crab, ''Zebrida adamsii''. These tiny crabs are obligate symbionts of sea urchins. They cling to the spines on the outer surface of the sea urchin test using their highly specialized walking legs. Because their ability to walk on substrates like sand is impaired, zebra crabs spend their entire benthic life stage attached to sea urchins, switching between hosts only during the mating season. Usually only one zebra crab is attached to an individual sea urchin outside of the mating season, but larger sea urchins can be hosts to two . The area of the test they inhabit is characteristically smooth; completely devoid of spines, pedicellariae, tube feet, and even epidermis. It is unknown if they physically destroy and/or consume these appendages or if they use other stimuli to induce the host sea urchins to autotomize. Although previously considered harmless commensals, authors have since reclassified them as parasites. In addition to the visible external damage, a 1974 study also observed abnormal behavior and coloration among infected sea urchins. They also appear to be immune to the flower urchin's venom.

Distribution

Flower urchins are widespread and common in the tropical Indo-West Pacific. They can be found north from Okinawa, Japan, to Tasmania, Australia in the south; and west from the Red Sea and the East African coast, to Raratonga in the Cook Islands in the east.

They are found among coral reefs, coral rubble, rocks, sand, and seagrass beds at depths of 0 to 90 m from the water's surface. They may sometimes partially bury themselves on the substrate.

Behavior

Flower urchins are among the numerous species of sea urchins known as "collector urchins", so named because they frequently cover the upper surfaces of their bodies with debris from their surroundings. This behavior is usually referred to as "covering" or "heaping". Flower urchins are usually found almost completely covered in objects like dead coral fragments, shells, seaweed, and rocks. These are held fast to their bodies using their tube feet and pedicellariae.

The function of this behavior is not well understood. Some authors believe that the debris serve as ballast, preventing them from being swept away by wave surges when feeding; while others believe that they may function as some sort of defense against predation. A 2007 study has hypothesized that the behavior may serve as protection from UV radiation during daytime.

Habitat

Flower urchins are widespread and common in the tropical Indo-West Pacific. They can be found north from Okinawa, Japan, to Tasmania, Australia in the south; and west from the Red Sea and the East African coast, to Raratonga in the Cook Islands in the east.

They are found among coral reefs, coral rubble, rocks, sand, and seagrass beds at depths of 0 to 90 m from the water's surface. They may sometimes partially bury themselves on the substrate.

Reproduction

Flower urchins are dioecious , but it is almost impossible to determine the sex of an individual by external characteristics alone. A possible method is by examining the external characteristics of the genital pores . In males, they are generally short, cone-shaped, and extrude above the body surface; while in females they are usually sunken. However this is not reliable, as 15% of the cases can exhibit the opposite characteristics. All other external features, like shape and size of the tests or color of the spines are indistinguishable between the two sexes.
Flower urchins have a chromosome number of 2n = 42.

Relatively little is known of the spawning behavior of flower urchins. Like other sea urchins, fertilization happens externally. Males and females release free-swimming gametes directly into the water currents in mass spawning events. In Okinawa, Japan, a 1994 study identified the spawning season of flower urchins as occurring in winter, at the same time as the closely related and sympatric ''Toxopneustes elegans''. It also recorded possible natural hybrids resulting from instances where the eggs of ''Toxopneustes pileolus'' are fertilized by the sperm of ''Toxopneustes elegans''.

In Taiwan, a 2010 study observed flower urchins spawning in May on the years 2007 and 2009. They occurred on highly similar conditions: in the afternoon low tide of the spring tide immediately after a new moon. During the event, spawning individuals discard the debris that usually cover their bodies before releasing their gametes into the water. Another study published in 2013 did not find any obvious correlation between lunar and tidal cycles to the mass spawning behavior of the flower urchin populations in southern Taiwan. It did note that the spawning patterns appeared to be non-random, with higher spawning rates on daytime on certain dates. However, the study was conducted within a span of only five months .

Food

Flower urchins feed on algae, bryozoans, and organic detritus.

Predators

Flower urchins have few predators. They are known to be toxic to fish. One of the few organisms capable of consuming flower urchins with no apparent adverse effects is the predatory corallimorph ''Paracorynactis hoplites''. However it is unknown if flower urchins are among its natural prey.

Defense

Unlike most other venomous sea urchins, flower urchins and related toxopneustids do not deliver their venom through spines. Instead, the venom is administered through the flower-like globiferous pedicellariae. If undisturbed, the tips of the globiferous pedicellariae are usually expanded into round cup-like shapes. They possess tiny sensors on their inside surfaces which can detect threats by touch and chemical stimuli. When agitated or brushed against by a potential threat, the pedicellariae will immediately snap shut and inject venom. The claws of the pedicellariae may also break off from their stalks and adhere to the point of contact, retaining the ability to continually inject venom for several hours.

The potency of the pedicellarial venom is believed to be directly related to the size of the pedicellariae. Thus individuals with larger globiferous pedicellariae are considered to be more dangerous than individuals with more numerous but smaller globiferous pedicellariae.

Uses

In Okinawa, fishermen observed numerous individuals of the predatory crown-of-thorns starfish gathering around the remains of the internal organs of flower urchins. A follow-up study by Japanese researchers in 2001 confirmed that the viscera of flower urchins could indeed attract crown-of-thorns starfish in both aquarium and open sea experiments. The attractant compounds were isolated and identified as arachidonic acid and α-linolenic acid. The authors believe that this discovery may be used to augment population control measures of the crown-of-thorns starfish, which are highly destructive to coral reefs.