Asteroidea

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Expert

Brisingida morphotypes

Paxillosida morphotypes

Velatida morphotypes

Introduction

Asteroids, also known as starfish or sea stars, are among the most readily recognizable of marine invertebrates. They are one of five living classes of echinoderms and possess a fossil record extending to the Paleozoic (Mah & Blake 2012). Among the living Asteroidea, five distinct groups display support as distinct taxa, these include the Forcipulatacea, the Valvatacea, the Spinulosida, the Velatida, and the Concentricycloidea. Some of these groups, such as the Velatida and the Concentricycloidea occur almost entirely in deep-sea and/or cold-water settings, but nearly every family in every major group, includes representative taxa in deep-water habitats (Mah & Blake 2012).

Classification

Approximately 1900 species of asteroids in 370 genera have been described with a breakdown of diversity within living asteroids outlined by Mah and Blake (2012). Nineteen families of asteroids occur in cold-water settings, mainly in the deep sea and high-latitudes. Further outline of asteroid diversity in the CCFZ is provided below.

Basic Anatomy

Asteroids are dorsoventrally flattened animals, stellate to pentagonal in outline with most taxa possessing five rays, although this number may vary from six up to 50. Arms may be distinct from a smaller disk in more stellate forms to triangular and completely confluent with the disk in more pentagonal forms. Adult size varies widely. Some individuals are only a few millimeters across whereas others are more enlarged, massive forms that approach nearly a meter across (2-3 feet). Deep-sea forms are in a size range of approximately six to 30 cm in diameter (as adults).

Body forms of deep-sea asteroids are diverse and range from flattened, pentagonally-shaped valvatidans, such as in the Goniasteridae to moderately calcified, stellate forms with sharp spines, such as in the Astropectinidae. On the opposite end of the spectrum are taxa such as Hymenaster which include swollen, cushion-shaped species that possess a highly modified and reduced skeleton with a gelatinous body wall. Many species, however, possess a very flexible body wall which can be swollen with water, extending it well above the plane of the disk and arms.

Surface details on asteroids are the primary source of characteristics used in identification. Granules, spines, thickened skin or other features may be present on the top (=abactinal or aboral) and/or lateral surfaces. The presence or absence of a distinct border of plates around the edge is also an important character used in diagnosing different species. Some velatidans, such as Pteraster and Hymenaster have a large prominent opening present centrally on the disk, called an osculum which often opens and closes. More commonly among asteroids, is a tiny circular to polygonal plate present offset from the center of the disk called a madreporite or sieve plate. This is more often seen in shallow-water species. Large numbers of feathery, finger-like structures are observed along the radial regions of some in situ observations of living asteroids. These are the papulae, or gills, which are used for gas exchange with the water. Papulae can be withdrawn but are observed fully extended when the animal has, presumably, been undisturbed for long periods of time giving the animal a “fuzzy” look in situ.

Several species possess more than 5 rays. In the CCFZ, the number of multiple rays ranges from six to at most, 20. Mutliple arm number is most likely going to be observed in brisingids, such as Freyastera benthophila (with six rays) or various Freyella spp. (up to 15).

Tube feet for locomotion and the mouth are located on the underside, also called the oral surface, of asteroids. Tube foot grooves radiate from the mouth located on the central disk. Most asteroids possess only two rows of tube feet, but some forcipulataceans and velatidans possess four. Water is absorbed via the madreporite on the top surface and via the tube foot grooves. Many sea stars feed with an extrusible stomach which can be observed extended and digesting prey items, which range from encrusting invertebrates, such as sponges to detritus or active prey.

Many asteroids possess small (often 5.0 mm or less) wrench, or clam-shaped structures known as pedicellariae. The presence, absence or morphology of pedicellariae are often used in determining the group with which it is classified. Velatidans lack pedicellariae. Forcipulates possess a distinctive 3-piece pedicellariae with a wrench-like structure. These are variably used as feeding structures and defensive mechanisms but their function in most species remains poorly understood.

Ecology

Asteroids occur across a broad depth range, from the intertidal to 6000 m depths (Mah & Blake 2012) with many species occurring at bathyal to abyssal depths (e.g., Clark & Downey 1992). Cold-water asteroids, including those in the deep-sea are found at high-latitudes (Arctic and Antarctic) as well as in the Atlantic, Indian, and Pacific oceans (Mah & Blake 2012). Substrate occurrence across the Asteroidea is highly variable.

Although complete ecological data is unavailable for most deep-sea asteroids, many accounts of shallow-water species (e.g, Mauzey et al. 1969; Birkeland 1974) indicate that asteroids occupy important and diverse ecological roles. This is also likely the case in deep-sea faunas as well. Some species are active predators (e.g. the multi-armed Solaster spp. in the Solasteridae) whereas others are deposit feeders (e.g., the Paxillosida below) and still others display suspension feeding (e.g., the Brisingida), removing food from the water column. Some taxa, such as the Zoroasteridae are present in high abundance (see Mah 2007) which also implies that they occupy a significant role in benthic systems. Herein, we review ecological information for different taxa observed in the CCRZ.

Ecologically, deep-sea forcipulataceans are most easily perceived as “brisingid” vs. “non-brisingid.” The ecology of “non-brisingid” forcipulataceans, such as pedicellasterids, stichasterids and zoroasterids is poorly understood. When confidently identified, they are often sitting on the benthos with no other clear ecological/behavioral observations evident. Brisingids have been observed from submersibles holding their arms into the water current (Emson & Young 1994). High abundance of brisingids, as well as crinoids and other filter feeding echinoderms, is observed on topological features associated with ideal water current flow. Presumably, ecological structure is related to flow regimes, substratum and other factors related to current flow.

Feeding and life modes in Paxillosida are consistently attuned to soft-bottom habitats. Most are either detritivores, feeding on organic material from sediment or predators of infaunal invertebrates, such as gastropods and/or mollusks (Jangoux 1982). Many paxillosidans feed via sediment ingestion, i.e., swallowing large amounts of sediment, mud, etc. and digesting the organic materials. As a consequence, it is not unsual to observe a swollen disk filled with mud in astropectinids, porcellanasterids, goniopectinids or ctenodiscids.

Little is known of velatidan ecology. Observations of pterasterids, myxasterids, and korethrasterids suggest a broad range of substrates. Most observed animals are solitary. Taxa Occurring in the Clipperton Fracture Zone Region

Forcipulataceans are among the most familiar of sea stars. These include the familiar Asterias spp. , Pisaster ochraceus and Coscinasterias, the widely occurring multi-armed sea star, all occuring in shallow waters. In general, the taxa within this group are characterized by one distinctive character: distinctive three piece pedicellariae, which are claw or wrench-shaped. Not all forcipulataceans possess the distinctive “forcipulate” pedicellariae, although similar pedicellariae, and accompanying skeletal morphology, is mostly present. These pedicellariae are often associated with spines and are present in many taxa as large tufts. Although their full range of use is not well-understood, some forcipulates such as brisingids use their pedicellariae to feed on prey (Emson & Young 1994). Forcipulates have two to four rows of tube feet and often possess a small disk and long, tapering arms. Several groups, such as brisingids have more than 5 arms.

Deep-sea forcipulates are best represented by three groups: The Brisingida (Brisingidae & Freyellidae), the Zoroasteridae and the Pedicellasteridae. Brisingids are discussed below. Pedicellasterids were not observed in the photoset and are not frequently encountered.

The Paxillosida comprise a diverse clade of asteroids which have developed adaptations and life modes which adapt them for living in and/or among unconsolidated sediments in both deep and shallow habitats. Paxillosidans are diverse and occur in a wide range of ecological settings. Historically the group has included such diverse families as the Astropectindiae, the Luidiidae, the Porcellanaasteridae, the Goniopectinidae, the Ctenodiscidae and the Radiasteridae but recent molecular systematics (Mah & Foltz 2011) has since broadened the Paxillosida to include two previously separate cold-water/deep-sea groups, the Benthopectinidae and the Pseudarchasteridae.

Velatidans include some of the most rarely encountered of sea star groups, including the Pterasteridae, the Myxasteridae and the Korethrasteridae. Most occur in cold-water settings, primarily the deep sea with some in high-latitude settings. Although not observed among the photographs provided, myxasterids and korethrasterids are distinctive deep-sea asteroid taxa and as indicated earlier, are among the most rarely encountered of species. Korethrasterids are small, five-rayed stellate velatidans showing fine spination and delicate skeletons and are not likely to be observed from ROV or photosleds. Of the three known genera of myxasterids, Asthenactis and Myxaster are multi-armed (six to ten rays) with an osculum and often covered by fine, needle-like spines. Pythonaster possesses an osculum and five elongate arms. Although rarely encountered, known myxasterids are widely occurring throughout the Atlantic and Pacific.

Among the most prominent of unsual deep-sea asteroids are the Pterasteridae (Order Velatida). The Pterasteridae occurs widely in both shallow and deep-sea cold-water habitats and are among the most distinctive of asteroid groups. The most distinctive feature observed in pterasterids is that of the supradorsal membrane; essentially a second surface layer upheld by pole-like paxillar plates. The supradorsal membrane completely ranges from translucent to more fleshy and tunic-like in appearance, completely covering the proper abactinal surface of the animal. In nearly all instances, a distinctive opening on the disk’s center, called the osculum is present and opens/closes in order to permit water exchange with the papulae under the supradorsal membrane.

List of potential species

Species listed below were those collected by C.G. Welling aboard the R/V Governor Ray in 1980 from deep-sea settings adjacent to the CCFZ. These do not reflect species indicated by Tilot (2006). It is unknown if images are available which correlate with collected specimens. Identifications of only two species, Dytaster exilis and Hymenaster violaceus presented an annotated photographic atlas of echinoderm biodiversity from the Clarion-Clipperton Fracture Zone, including a checklist of possible species (Tilot, 2006). Maluf (1988) provided a checklist of echinoderms from the Eastern Pacific region, including those from the deep sea. Hendrickx et al. (2011) presented a checklist and sampling of recent deep-sea Asteroidea from slope cruises in the Gulf of California, Mexico. Important historical monographs of deep-sea Asteroidea from this region include those of Ludwig (1905) and H.L. Clark (1920). This treatment does not attempt to cover comprehensively all known or possibly known asteroid taxa. It is an overview of species from the general region based on NMNH (Smithsonian) records and from images provided by the workshop.

Paxillosida
Astropectinidae

  • Dytaster exilis Sladen 1889
  • Plutonaster sp.

Porcellanasteridae

  • Eremicaster crassus (Sladen 1883)
  • Hyphalaster inermis Sladen 1883
  • Styracaster caroli Ludwig 1907
  • Styrascaster elongatus Koehler 1907

Velatida
Pterasteridae

  • Hymenaster cremnodes H.L. Clark 1920
  • Hymenaster echinulatus Sladen 1882

Limitations of photographs

In most cases it is not possible to classify asteroids beyond family level without specimens. In some instances generic level determinations can be made as indicated in the guide below. However species level classifications are generally not possible.

References

  • Birkeland, C.1974. Interactions between a sea pen and seven of its predators. Ecological Monographs 44: 211-232.
  • Clark, A. M. and Downey, M.E. 1992. Starfishes of the Atlantic. Chapman and Hall, London.
  • Clark, H.L. 1920. Reports on the scientific results of the expedition to the Eastern Tropical Pacific, in charge of Alexander Agassiz, by the US Fish commission Steamer Albatross, from October 1905 to March 1905, Lt. Cmdr. L.M. Garrett, USN, Commanding. XXXII. Asteroidea. Memoirs of the Museum of Comparative Zoology 39(3): 70-154.
  • Emson, R.H. and C.M. Young. 1994. Feeding mechanism of the brisingid starfish Novodinia antillensis. Marine Biology 118: 433-442.
  • Gale KSP, Hamel J-F, Mercier A. 2013. Trophic ecology of deep-sea Asteroidea (Echinodermata) from eastern Canada. Deep-Sea Research I 80: 25-36.
  • Howell KL, Pond DW, Billett DSM, Tyler PA. 2003. Feeding ecology of deep-sea seastars (Echinodermata: Asteroidea): a fatty-acid biomarker approach. Marine Ecology Progress Series 255: 193-206.
  • Ludwig, H. 1905. Asteroidea. Memoirs of the Museum of Comparative Zoology at Harvard. 32: 1-292.
  • Mah, C. and D.B. Blake. 2012 Global Diversity and Phylogeny of the Asteroidea (Echinodermata). PLoS ONE 7(4): e35644. doi:10.1371/journal.pone.0035644
  • Mah, C.L. 2007. Phylogeny of the Zoroasteridae (Zorocallina; Forcipulatida): Evolutionary Events in Deep-Sea Asteroidea displaying Paleozoic Features. Zoological Journal of the Linnean Society 150: 177-210.
  • Maluf, Y. 1988. Composition and distribution of the central eastern Pacific echinoderms. Natural History Museum of Los Angeles County, Technical Reports 2. 242 pp.
  • Mauzey, K. P., C. Birkeland, P.K. Dayton. 1968. Feeding behavior of asteroids and escape responses of their prey in the Puget Sound region. Ecology 149: 603-619.
  • Tilot V. 2006. Biodiversity and distribution of the megafauna. Vol. 2. Annotated photographic atlas of echinoderms of the Clarion-Clipperton Fracture Zone. IOC Technical Series 69(2): 1-62.