Although regeneration in adult sea stars is known in many species, little work has been done on the phenomenon in the larvae.

Ochre star Pisaster ochraceus regenerating
a portion of arm tip perhaps from damage 1X

In the first study of its kind, researchers at Friday Harbor Laboratories surgically bisect Pisaster ochraceus bipinnaria¹ and brachiolaria lavae, and observe that both halves regenerate completely in 12-14d² (see photo on Right and photo series below). Despite removal of the mouth and esophagus, the posterior half of the larva at Day1 is able to capture and ingest phytoplankton particles.  However, feeding by the anterior larva does not commence until Day7, following regeneration of the digestive organs and coelom. Note that by Day12 both anterior and posterior larvae have completely regenerated and, by Day14, are essentially indistinguishable from control larvae.

Given that the surgical bisection may simulate natural damage to the larvae during its stay in the plankton, the authors suggest that population models incorporating past data on larval mortality may be using mortality estimates that are unrealistically high.  The authors note that their study is the first on regeneration of lost body parts requiring organogenesis in a metazoan larva³.  Vickery & McClintock 1998 Nature 394:140; for a detailed review of regeneration in echinoderm larvae see Vickery et al. 2001 Microsc Res Tech 55: 464.

NOTE¹  bipinnaria of the sea star Luidia foliata are also used with similar results sd those for the ochre star

NOTE²  culture temperature is not given

NOTE³  do the authors actually mean “invertebrate metazoan larva”? Much work on regeneration in amphibian larvae has been done

It has long been known that sea-star larvae can clone themselves but, until recently, the factors regulating this were not well understood.  A clone originates as a small bud on a mature bipinnaria larva and this soon detaches and develops into another bipinnaria larva (see photo on Left). The experiments described here involve culturing larvae of ochre stars Pisaster ochraceus at the Friday Harbor Laboratories, Washingtn under 9 combinations of 3 temperatures¹ and 3 food concentrations².

Results show that larvae at the medium temperature and on the highest ration grow fastest (see graph upper Right). These larvae produce the most clones. Most cloning occurs after the onset of the brachiolaria stage (see histogram lower Right).  All larvae die in the high-temperature treatments and those in the low-temperature treatment fail to attain the brachiolaria stage. In another experiment, larvae fed on a mixed diet of phytoplankton grow more and produce significantly more clones than ones fed on monospecific³ diets.

The clones develop normally and after about 2wk they are functionally and morphologically indistinguishable from larvae in the cultures from which they originate.  The authors conclude that production of most clones under conditions of optimal temperture and food may serve to increase larval populations when the environment is most conducive to larval growth.  Vickery & McClintock 2000 Biol Bull 199: 298.

NOTE¹ low: 7-10^oC, medium: 12-15 ^oC, and high: 17-20^oC; only results for the intermediate treatment are shown here

NOTE²  5x10², 5x10³, and 5x10⁴ cells ^. ml^-1, of an equal mix of diatoms Chaetocerous calcitrans, and 2 flagellates Dunaliella tertiolecta and Isochrysis galbana

NOTE³  growth of larvae on Isochrysis galbana seems the same as on a mixed diet, suggesting that this species may represent the major nutritional contribution of the 3 species in the mixed diet

A later study by the same research group at Friday Harbor Laboratories provides further details of regenerative capacity and process in sea-star larvae, most notably, those of Pisaster ochraceus.  Surgically bisected bipinnaria larvae (21d of age) show the progression of organogenesis in the anterior half as shown in the drawings on the Left. Note that no part of the coelomic pouches remain after bisection and these later form as lateral evaginations of the newly formed digestive system. 

If arm tips or whole arms are removed they survive for over 2wk and swim about, but never regenerate (see photos on Right). The larva itself will regenerate the missing arms but posterolateral arms never seem to attain the same length as before. Vickery et al 2002 Biol Bull 203: 121.

NOTE  the authors provide information on regeneration in west-coast sea stars Luidia foliata and sand dollars Dendraster excentricus, but only information on the ochre star is considered here