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  Reproduction & development
 

Sex ratios are usually 1:1 in temperate coast sea stars, and in echinoderms in general.  Gonads mature through late autumn and winter, and spawning is in the spring coincidental with seasonal appearance of warmer seawater temperatures and phytoplankton food for the larvae, if they are of the feeding type.

Topics relating to reproduction & development include selected genera, considered here, and EGG SIZE & ENERGY CONTENT, LARVAL CLONING & REGENERATION, and TO BROADCAST SPAWN OR BROOD?, presented in other sections.

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  Selected genera
  After some general introductory studies, reproduction is considered in detail for several west-coast sea stars, beginning with Crossaster, Henricia and Leptasterias in this section, and MEDIASTER, ORTHASTERIAS, PATIRIA, PISASTER, PTERASTER, PYCNOPODIA, and SOLASTER in their own sections.
 
Research study 1
 

There are 4 types of developmental patterns in asteroids, named according to the type of larva development involved.  The first is planktotrophic, as in Pisaster ochraceus, where free-swimming, feeding photograph of a partially dissected ochre star Pisaster ochraceusbipinnaria larvae are produced from relatively small eggs.  The larvae are drawings showing life cycle of the sea star Pisaster ochraceus for several weeks and they feed on phytoplankton.  They later mature into the settling stage, the non-feeding brachiolaria. Because of the low parental investment in each egg, a female may have high fecundity – perhaps numbering in the hundreds of millions of eggs.  These larvae often have complex structures for swimming, feeding, and settlement. 

Ochre star Pisaster ochraceus dissected for
study. The predominant organs visible are the
pyloric stomach and pyloric ceca for digestion,
and the gonads , not ripe in this individual.
There are 5 pairs of gonads in total, only 2 of
which (in adjacent arms) are featured here. A
gonopore is located between each arm pair 0.5X


drawing of larva of Solaster endecaphotograph of sea star Solaster endeca courtesy Ron Lon, Simon Fraser University, Burnaby, British ColumbiaThe second developmental type is pelagic lecithotrophic, such as in the rose star Crossaster papposus, blood star Henricia leviuscula, Mediaster aequalis, sun stars Solaster dawsoni, S. endeca, and S. stimpsoni, and, uniquely, in the cushion star Pteraster tesselatus (other Pteraster species, such as P. militaris, brood their young).  Here, eggs are large and yolky, and develop into swimming, or floating, non-feeding larvae.  Parental investment is high in this type of development and fewer eggs are produced.  These larvae may have special attachment structures for use during settlement.


Sea star Solaster endeca 0.4X Photo
courtesy Ron Long, Simon Fraser
University, British Columbia



drawing of a demersal larva of an asteroidThe third developmental type is demersal, involving large, yolky eggs, and larvae that are non-feeding and drift near the sea bottom (see drawing lower Left).  For this reason, even though developmental time may be relatively long, dispersal is limited.  There are no examples of this type of development in west-coast asteroids.

drawing of Leptasterias hexactis brooding eggsThe fourth type is brooding, such as in Leptasterias spp. (see drawing on Right), where the eggs are held within the arms of the hunched-up female, and in most slime stars Pteraster spp., where the eggs are retained in a special nidamental or brood chamber.  The young are released at the crawling juvenile stage.  In Leptasterias the juveniles simply crawl away from the brood mass, while in Pteraster militaris, they crawl out of the large ventilatory opening, the osculum.  McEdward & Janies 1993 Biol Bull 184: 255; McEdward & Janies 1993 Biol Bull 184: 255; for a review of larvae and life-cycle patterns in asteroids and other echinoderms see McEdward & Miner 2001 Can J Zool 79: 1125.

NOTE  lit. “plankton feed” G.  Most or all planktotrophic asteroid larvae feed on phytoplankton

NOTE  lit. “yolk” “food” G., referring to the fact that the developmental stages derive their nourishment from yolk stores rather than from food obtained from the plankton

 
Research study 2
 

sea star Evasterias troschelliIn comparison with species that brood their young, asteroids with planktotrophic larvae would be expected to have greater opportunity for gene flow and, thus, much broader geographic distribution.  To test this idea scientists in Alaska analyse the genetic structure of 5 species of sea stars from 6 locations to determine levels of intra- and inter-population variation.  Surprisingly, the results show higher heterozygosity in Leptasterias epichlora, a brooder, than in either of the free-spawners Pisaster ochraceus and Evasterias troschelli, or in L. polaris, a brooder with circumpolar distribution.  Additionally, the authors demonstrate: 1) that hybridisation between L. epichlora and L. hexactis can occur in some situations, 2) that there is no subspecies of P. ochraceus, as has been proposed in earlier literature, and 3) that E. troschelli is a single panmictic species.  The authors conclude that genetic variation in sea stars is unrelated to mode of reproduction.  Stickle et al. 1992 Can J Zool 70: 1723.

NOTE  allozyme variation at 16-25 loci (overall, 33 different enzymes are examined)

NOTE  unstructured population with random mating

Evasterias troschelli exposed at low tide 0.6

 
Research study 3
  In a similar larger-scale follow-up investigation the same research groups investigates the genetic structure of Leptasterias at 37 intertidal locations between Alaska and California.  By use of both morphological and genetic criteria, 4 species are confirmed: L. epichlora, L. hexactis, L. aequalis, and L. aspera.  No more than 2 species are found at any given location, and hybridisation is rare (only 0.9% of 1085 specimens).  All species are morphologically and genetically similar, but could usually be separated by at least one diagnostic allozyme locus.  A 5th species, tentatively identified as L. pusilla, may be present at some sites in central California where it overlaps with the similar L. hexactis at the latter's southern limits of distribution.  Foltz & Stickle 1994 p291 In, Echinoderms through time (David et al, eds) Balkema, Rotterdam; see also Foltz 1998 p. 235 In, Echinoderms (Proc 9th Int. Echinoderm Conf., San Francisco) (Mooi & Telford, eds.) Balkema, Rotterdam.
 
Research study 4
 

During their free-living existence in the plankton, larvae of asteroids may feed (planktotrophic) or, alternatively, may subsist on yolk stores provided by their maternal parent (lecicthotrophic).  All swim as embryos, beginning as early as the blastula stage.  A question asked by researchers at Friday Harbor Laboratories, Washington relates to how early in development the embryos of  species with different nutritional modes begin to swim.  Prior to the onset of swimming behaviour, planktonic embryos (stages prior to the larval one) can only drift passively.  Swimming allows an embryo to adjust its position in the water column, thus removing it from exposure to benthic predators and perhaps allowing it to avoid capture by at least some pelagic predators. Of 8 asteroids examined, swimming commences earlier in those species whose larvae are planktotrophic (4 species, mean time from first cleavage = 36h) than in species whose larvae are lecithotrophic (4 species, 65h).  Swimming in the first group, moreover, begins in the blastula (3 spp.) or early gastrula stage (1 sp.), while swimming in the second group does not commence until the gastrula.  The study really focuses on the adaptive value of rapid embryonic development, and uses time to swimming as an indicator. By presenting just this short section on asteroids, the ODYSSEY does not do justice to the wealth of comparative data presented in the entire study.  Staver & Strathmann 2002 Biol Bull 203: 58.

NOTE  the question is much broader than this, involving a comparison of age and stage of first swimming in planktonic embryos/larvae in 34 invertebrate species (from 10 phyla) from an evolutionary standpoint.  The study is presented in this sea-star section because of its (the study’s) relatively large representation of asteroids with different modes of larval dispersal

NOTE  asteroid and other echinoderm embryos employ uniciliated cells to swim

 
 

Juvenile sea stars are notoriously difficult to spot, whether one is searching among intertidal rocks or SCUBA-diving. Whether this owes to cryptic behaviour of the young stages or to low probability of settlement and recruitment succes is not known. Here are a few images of young stages: Photo of Solaster dawsoni courtesy Kirt Onthank and Dave Cowles, Walla Walla University, Washington wallawalla.edu.
photograph of a juvenile rose star Crossaster papposus
Rose star Crossaster papposus 0.3X
photograph of juvenile sun star Solaster dawsoni
Sun star Solaster dawsoni 1.3X
 
photograph of juvenile sun star Solaster dawsoni
Sun star Solaster dawsoni 4X
photograph of juvenile sea star Solaster stimpsoni
Solaster stimpsoni 1X
photograph of sea star Solaster endeca
Solaster endeca 3X (or S. dawsoni?)
 
photograph of juvenile sunflower star Pycnopodia helianthoides
Sunflower star Pycnopodia helianthoides 8X
photograph of juvenile sunflower star Pycnopodia helianthoides
Sunflower star Pycnopodia helianthoides 3X
photograph of juvenile sunflower star Pycnopodia helianthoides
Sunflower star Pycnopodia helianthoides 0.5X
 
 

Crossaster

 

drawing of juvenile Crossaster papposusA study on early development of Crossaster papposus at the Millport Marine Station, Scotland reveals the following features.  Spawning in springtime leads to fertilisation of eggs that float at the surface.  Gastrulation at about 5d proceeds to a lecithotrophic larva that spends about 20d in the plankton.  The larva then settles and attaches to the bottom by a single photograph of juvenile Crossaster papposussucker.  By 4wk post-fertilisation, the first tube feet develop and become functional, although the young sea star is still anchored for a few days by its original attachment sucker.  Meanwhile, spines have been forming from about 22d.  After about 7wk a mouth appears, with the anus appearing a few days later.  Gemmill 1920 Quart J Microsc Sci 64 (N.S.): 155.

NOTE  the species has a circumboreal distribution ranging from the north Atlantic to the north Pacific.  On the west coast it lives from Alaska to Puget Sound

 

Aboral view of juvenile sun
star Crossaster papposus 2X

 
 

Henricia

 
Research study 1
 

drawings comparing ambulacral spines formation and shape in two species of Henricia sea stars, H. leviuscula and H. pumila
photographs of sea-star species Henricia leviuscula and H. pumila courtesy Eernisse et al. Zootaxa 2329: 22Two known species of Henricia inhabit west-coast shores.  The larger, more common, and more familiar species is H. leviuscula.  It occurs from southern Alaska to Puget Sound, Washington and is free-spawning with pelagic larvae.  A second, newly described species H. pumila occurs from Sitka, Alaska to Ensenada, Baja California and is smaller, with mottled colour, and broods its young. The 2 species can additionally be differentiated on the basis of spine patterns along the ambulacral grooves (see drawings), and on shape and configuration of aboral spines.  A brood chamber in H. pumila is formed by arching the disc and spiraling the arms to create a concavity around the mouth.  Eggs of 1.1mm diameter are released from gonopores located between the arms.  Eggs of H. leviuscula are reddish-brown in colour and are relatively large, about 1.5 x 1.2mm. In the San Juan Islands spawning is in springtime.  The authors note that there are no published reports of the larvae of H. leviuscula feeding, and the large size of the eggs suggests that the larvae may be lecithotrophic. Eernisse et al. 2010 Zootaxa 2329: 22.

NOTE  for many years this small, mottled version has been lumped together with H. leviuscula as Henricia spp.  The authors suggest that there may be other brooding species yet to be described in the Henricia spp. complex

NOTE  mottled hues of gray, brown, red, orange, yellow, and lavender

 
 

Leptasterias

 
Research study 1
 

diagram of gonads and brooded egg mass in the 6-armed sea star Leptasterias hexactisSome west-coast sea stars brood their eggs and a free-living larval stage is absent.  In the 6-armed sea star Leptasterias hexactis the female parent hunches over its egg mass during several months in winter.  The mass may comprise as few as 50, or as many as 1500, eggs.  At spawning, the female catches the eggs as they are released from the gonopores with her tubefeet and transfers them to the brooding area.  The eggs are fertilised on the way to, or within, the brooding area.  The fertilisation membrane is adhesive and the embryos stick together in a cluster.  The eggs hatch in early spring to crawling juveniles.  Brooding provides protection, allows cleaning, and maintains the eggs in a uniform environment. Chia 1966 Biol Bull 130: 304.

NOTE  the taxonomy of 6-armed species in the genus Leptasterias is in a state of uncertainty on the west coast.  Only 3 species names, hexactis, aequalis, and epichlora are used in the ODYSSEY, but there may be more (including some in Alaska).  All brood their eggs. Chia 1966 Syst Zool 15: 300; Lambert 1981 The Sea Stars of British Columbia B.C. Prov Museum Handbook # 39; Kwast et al 1990 Mar Biol 105: 477; Flowers & Foltz 2001 Mar Biol 139: 475; Hrincevich et al. 2000 Am Zool 40: 365; Lamb & Hanby 2005 Marine Life of the Pacific Northwest Harbour Publishing, B.C.

NOTE  in San Juan Island, Washington the range is from 100-2000 eggs .  Menge 1974 Ecology 55: 84.

 
Research study 2
  Studies on Leptasterias hexactis at the Friday Harbor Laboratories, Washington provide the following details of early development (10-13oC). Chia 1968 Acta Zoologica 49: 321.
 

newly hatched juveniles of sea star Leptasterias hexactisgastrulation: 6-7d after fertilisation
early brachiolaria larva: 15d
late brachiolaria: 20d, the larva is bilaterally symmetrical with one dorsal brachiolar arm and two ventral ones
hatching: 21d
metamorphosis: 29d, note the appearance of the 5 adult arms
mouth opens: 49d
metamorphosis complete
: 60d
young adult: 90d, 7mm diameter

Stages shown in bold type are illustrated below (photo above Right: newly hatched juveniles 0.1X):

 
drawing of late brachiolaria larva of sea star Leptasterias hexactis drawing of early metamorphosis of sea star Leptasterias hexactis drawing of post-metamorphic sea star Leptasterias hexactis showing cross-sectional view of gut system photograph of early juvenile of sea star Leptasterias hexactis
 
Research study 3
 

photograph of sea star Leptasterias hexactis oral viewDuring the 2- period of brooding, female Leptasterias hexactis do not feed.  Since maintenance must go on including continual production of oocytes, each of which takes 2yr to mature, then the female must mobilise nutrient reserves and/or resorb tissues for nutrients and energy.  Studies at Friday Harbor Laboratories, Washington on brooding females show that food reserves in the pyloric caeca are depleted after only 4wk of brooding, and nutrients must be utilised from other sources for the remaining 4wk, perhaps from tissue resorption. Why winter brooding?  One reason relates to the idea of not feeding while brooding.  Cooler temperatures will reduce metabolism and, hence, the need to feed.  Also, it is advantageous for juveniles to appear in springtime when newly settled barnacle and mussel spat are becoming abundant as food.  Chia 1968 J Zool, Lond 154: 453; Chia 1969 Biol Bull 136: 185.

 



Oral view of sea star Leptasterias hexactis 1.4X

 
Research study 4
 

drawing of extent of arm attachment by brooding Leptasterias hexactis in different conditions of wave exposuregraph showing extents of arm attachment to the substratum in female Leptasterias hexactis in different wave-exposure conditionsWave-exposure has a negative effect on fecundity in Leptasterias hexactis.  The explanation for this is straightforward: in wave-exposed areas the brooding female uses more of its arm length to attach to the substatum (see drawings, and graph on Right); hence, less is graph showing number of eggs brooded by Leptasterias hexactis in different wave-exposure conditionsleft to form the brood chamber and fewer eggs are brooded (see graph lower Left). The approximate 45% greater length of arms required to form the brood chamber in females in protected areas translates to about 90% more eggs being brooded.  Menge 1974 Ecology 55: 84.

 

 
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