Topic: Sea stars – Asteroidea

  • Sjøstjernen Pontaster tenuispinus

    Pontaster tenuispinus (dorsal)

    Photo: Arne Hassel / IMR
  • Pteraster pulvillus_dorsal_R1331-501.jpg

    Pteraster pulvillus (dorsal).

    Photo: Arne Hassel / IMR
  • Crossaster squamatus ventral 1 R1114-443.JPG

    Crossaster squamatus (ventral)

    Photo: Arne Hassel / IMR

Sea stars belong to the echinoderms, together with the brittle stars, sea urchins, sea cucumbers and sea lilies.

General appearance

Body as a rule pentagonal, with five arms symmetrically radiating from the central disk. However, there are several exceptions, Crossaster spp. have 8-13 arms. The shape of the arms is also variable, from long and slender in Urasterias lincki, to short and stout in species having an almost pentagonal shape, as in the flat and rigid Ceramaster granularis. Pteraster and Hymenaster spp. are also pentagonal. Poraniomorpha hispida is an example of a strongly convex form.

Crossaster – 8 to 13 arms

  • Crossaster papposus fra ryggsiden.jpg

    Crossaster papposus (dorsal side).

    Photo: Arne Hassel / IMR
  • Crossaster papposus fra undersiden.jpg

    Crossaster papposus (ventral).

    Photo: Arne Hassel / IMR
  • Crossaster papposus ryggskjelett og hudgjeller.jpg

    Dorsal side of Crossaster papposus, showing skeleton plates, some pointed spines, and the bulbous papulae used for respiration.

    Photo: Arne Hassel / IMR
  • Crossaster papposus arm med sidebørster.jpg

    Crossaster papposus (marginal paxillae).

    Photo: Arne Hassel / Havforskningsinstituttet

Pontaster tenuispinus 

  • Pontaster tenuispinus ryggsiden.jpg

    Pontaster tenuispinus (dorsal side).

    Photo: Arne Hassel / IMR
  • Pontaster tenuispinus undersiden.jpg

    Pontaster tenuispinus (ventral).

    Photo: Arne Hassel / IMR
  • Pontaster tenuispinus sugeføtter i armfuren.jpg

    Pontaster tenuispinus – tube feet in the ambulacral groove, surrounded by strong adambulacral spines.

    Photo: Arne Hassel / IMR

Pteraster obscurus

  • Pteraster obscurus ryggsiden.jpg

    Pteraster obscurus (dorsal side).

    Photo: Arne Hassel / IMR
  • Pteraster obscurus fra undersiden.jpg

    Pteraster obscurus (ventral).

    Photo: Arne Hassel / IMR
  • Pteraster obscurus fra siden.jpg

    Pteraster obscurus (side view).

    Photo: Arne Hassel / IMR
  • Pteraster obscurus munnparti og pigg-kammer.jpg

    Pteraster obscurus (webbed adambulacral spines and mouth area).

    Photo: Arne Hassel / IMR

Ctenodiscus crispatus

  • Ctenodiscus crispatus dorsal 1 R1150-459.jpg

    Ctenodiscus crispatus (dorsal)

    Photo: Arne Hassel / IMR
  • Ctenodiscus crispatus oral 1 R1150-459.jpg

    Ctenodiscus crispatus (ventral).

    Photo: Arne Hassel / IMR
  • Ctenodiscus crispatus madreporite

    Madreporite and numerous dorsal paxillae in Ctenodiscus-crispatus.

    Photo: Arne Hassel / IMR
  • Ctenodiscus crispatus 2 R42-62.JPG

    Ctenodiscus crispatus is common in the Barents Sea.

    Photo: Arne Hassel / IMR

Star or biscuit?

  • Ceramaster granularis ryggside.jpg

    Ceramaster granularis (dorsal side).

    Photo: Arne Hassel / IMR
  • Ceramaster granularis underside.jpg

    Ceramaster granularis (ventral side).

    Photo: Arne Hassel / IMR
  • Ceramaster granularis 4-armet ryggside.JPG

    A rare 4-armed Ceramaster granularis (dorsal side).

    Photo: Arne Hassel / IMR

Anatomy

Sea stars have an internal skeleton with numerous calcareous plates, held together by soft tissues and muscles. Groups of short spines, paxillae, often cover the dorsal side (Pontaster tenuispinus, Ctenodiscus crispatus), and stronger spines are numerous on the adambulacral plates lining the ambulacral groove on the underside of the arms. In Pteraster spp. these spines are united by a web, making a series of combs from the tip of the arm to the mouth.

The mouth, surrounded by jaws with spines, is placed centrally on the ventral side. The stomach has extensions into each arm. If anus is present, it opens on the dorsal side. 

Two or four rows of tube feet radiate along the underside of the arms in the ambulacral groove. 

  • Hymenaster pellucidus dorsalt st.830605.1.JPG

    The supradorsal membrane covering Hymenaster pellucidus (family Pterasteridae).

    Photo: Arne Hassel / IMR
  • Hymenaster pellucidus oralt 1 st.830605.1.JPG

    Hymenaster pellucidus (ventral side, with the fin-like actino-lateral membrane clearly visible).

    Photo: Arne Hassel / IMR

Sea stars have a hydraulic water vascular system, with internal water canals connected to the tube feet via muscular sacs, ampullae, to operate them for locomotion. The vascular system also distributes nutrients and waste products and has a major role in the respiration. It has a connection to the outside through the madreporite opening on the dorsal side.

The blood system is reduced. Respiration is accomplished by skin gills, papulae, on the surface of epidermis. Sea stars have a nervous system, but no brain. Light sensitive eye spots may occur at the tip of the arms.

Many sea stars have pedicellariae, small surface appendages with movable jaws (valves), for protection against attack, and for cleaning the skin. In crossed pedicellariae the two valves cross each other like the jaws in a wrench, another type is called straight pedicellariae. Both are numerous in the Arctic sea star Urasterias lincki.

  • Urasterias lincki dorsal 1 R1421-513.JPG

    Urasterias lincki 

    Photo: Arne Hassel / IMR
  • Urasterias lincki dorsal disk straight pedicellariae R1421-513.jpg

    Straight pedicellariae on dorsal side of Urasterias lincki

    Photo: Arne Hassel / IMR
  • Urasterias lincki straight pedicellariae cleaned 2 R1421-513.jpg

    Cleaned straight pedicellarie valves in Urasterias lincki.

    Photo: Arne Hassel / IMR
  • Urasterias lincki crossed pedicellariae R1421-513.jpg

    Two arm spines on Urasterias lincki, covered with crossed pedicellariae. 

    Photo: Arne Hassel / IMR
  • Urasterias lincki crossed pedicellariae cleaned composite R1421-513.jpg

    Two crossed pedicellariae and a single valve from Urasterias lincki. Treated with chlorine.

    Photo: Arne Hassel / IMR

Diet

Most sea stars are carnivorous and feed on bivalves, gastropods and other echinoderms. Examples are Asterias rubens and Crossaster papposus. If the prey is large, they may protrude the stomach to start digestion externally. Other species are detritus feeders, utilizing mud as the food source (Ctenodiscus crispatus).

Development

Larval development is usually planktonic. The planktonic larvae are bilaterally symmetrical. Pteraster and Hymenaster spp. have internal development in a brooding chamber under the supradorsal membrane.