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  Symmetry
 
Research study 1
 

In an early publication on symmetry in asteroids, researchers on the Harriman Alaska Expedition 1899 study arm multiplication and origin of bilaterally symmetrical elements in adult sunflower stars Pycnopodia helianthoides.  Although they do not study early development, the authors are able to collect specimens ranging in size from a few millimeters diameter, with 6-8 arms, to adults of nearly 1m in diameter.  In a 6-arm individual the arms are labeled as follows.  The one clockwise to the madreporite is Arm I, then skip the next one, then come Arms II, III, IV, and V (see 2 drawings on Left below)).  Arms after the 6-arm stage are added in pairs.  The skipped arm is Arm A, which is an important one because the next 2 arms are budded between I and A, and II and A.  Subsequent pairs are always added immediately clockwise to I and counterclockwise to II, photograph of juvenile sunflower star Pycnopodia helianthoidesas viewed from an aboral aspect (see Right drawing below), and note that they are added bilaterally with the plane A-IV as the axis of symmetry.  This pattern of addition of arms explains why most individuals have an even number of arms.  Only when growth of one of the arms is suppressed, which apparently happens from time to time, will an individual have an odd number of arms.  The authors assume that the original 5 arms, and perhaps all 6, originate in the larva prior to settlement.  In fact, they see the bilateral pattern of arm addition in the adult as just a continuation of the pattern seen in the bilateral brachiolaria larva.  The authors close their discussion by noting that some or perhaps many sea stars have during development a “larval organ”, that is used for temporary attachment to the substratum when settling, and then later discarded.  The position occupied by this organ in a larval Pycnopodia is the same as that of Arm A in the adult, leading the authors to suggest that the two may in some way be intimately related.  Ritter & Crocker 1900 Proc Wash Acad Sci 2: 247.

 

A 10-armed Pycnopodia helianthoides. The tell-tale (for
orientation) madreporite cannot be distinguished 4X

 
drawing of 8-arm juvenile of Pycnopodia helianthoides aboral view drawing of 8-arm juvenile of Pycnopodia helianthoides oral view drawing of 10-arm juvenile of Pycnopodia helianthoides showing positions of most recent arms
Aboral view of 8-arm juvenile Pycnopodia helianthoides showing location of new arms, added in pairs Oral view of same individual showing relative positions of new arms Aboral view of 10-arm individual showing that the newest arms are always clockwise to 1 & counterclockwise to 2
 
Research study 1.1
 

histogram showing righting times for a single sunflower star Pycnopodia helianthoides tested 50 times in successionIs there a functional anterior part of a sea star as it locomotes along or attempts to right itself?  Early observations on sunflower stars Pycnopodia helianthoides crawling in Bremerton Bay, Washington suggests that there is a functional anterior region, defined by the author as the axis of the oldest arms as the sea star crawls along. Without a description of early development of the post-larval arms in P. helianthoides, however, it is not possible from the author’s description to figure out which of the arms these “oldest” ones might be (but see Research Study 2 below).  The author states that “the first pair of…post-larval rays…is at right angles to the anterior-posterior rays” (p. 243), which suggests that these first, or “oldest arms”, actually extend out to one side or other, or perhaps both, of the rays that are designated as the anterior ones.  Whether this anterior end has a fixed orientation to the madreporite is not mentioned. Despite this confusion the author is clear that photograph of sunflower star crawling over a piece of kelpthe same set of arms is in the lead during different locomotory bouts and, after turning from an obstacle, the sea star resumes locomoting with this functional “anterior end” in the lead.  Other experiments show that 92% of righting responses are toward this “anterior end”.  Kjerschow-Agersborg 1918 Biol Bull 35: 232.

NOTE  most contemporary authors take a contrary view, that while certain arms do take on temporary dominance in locomotion, these change as an individual changes direction or activity. In this more modern view, localised control of movement temporarily resides in a node at the junction of radial and ring nerves on the arm that happens to be leading, and this changes as the leading arm changes

NOTE  46 out of a total of 50 trials; all trials, however, are done on the same specimen over the course of a single day (10cm diameter with 13 arms, at 10oC).  As readers, we are impressed with the apparent indefatigability of this individual, whose righting speed remains essentially constant over the course of the 50 trials (see histogram above)



Sunflower star Pycnopodia helianthoides crawling at high speed
along a piece of kelp in the direction shown by the white line. The
madrepoite can't be seen in this view, so is likely not at the front 0.4X

 
Research study 2
 

photograph of sunflower star Pycnopodia helianthoides with 23 armsDo sea stars really have an anterior-posterior axis?  They superficially appear to be radially symmetrical but, as noted in Research Study 1 above, their single, offset madreporite and stone canal (and associated elements of the hemal system) are testiment to a primary bilateral symmetry.  Also, the larvae of sea stars are almost universally bilateral.  The question is investigated in Pycnopodia helianthoides by monitoring the position of the madreporite relative to that of the 6 primary arms during early development.  According to this author’s interpretation, the first 5 arms become the functional anterior end of the animal (see drawing). Note the position of these primary arms to the madreporite. The next arm to appear, the sixth, becomes the “primary drawing showing one author's view of which is the functional anterior end of a sunflower star Pycnopodia helianthoidesposterior” arm, and all additional arms are added symmetrically to right and left sides as shown. The youngest arms, then, are always represented by the pair located adjacent to the 2 posterior primary arms. In an earlier paper this author states that an adult Pycnopodia always crawls with its functional anterior end in the lead, a point of view not necessarily shared by other biologists (see TUBE FEET & LOCOMOTION). Kjerschow-Agersborg 1922 Biol Bull 42: 202. 

NOTE  these are the first to appear in development of Pycnopodia and 5 of these, shown in the drawing, correspond to the arm-complement of 5-armed adult sea-star species.  Note that this author, having the benefit of seeing Pycnopodia in early development and having observed its orientation during locomotion and righting (see SEASTAR: LOCOMOTION), has fipped its orientation 180o from that shown in Research Study 1 above, and has indicated that Arm A (6th primary arm in the drawing) is added after the 5 primary arms (1st-5th primary arms in the drawing)

NOTE  for some reason the author conclude that P. helianthoides has a maximum of 20 arms as an adult, but the species commonly has more than this – 25 or more being not uncommon

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Let's see if you have been paying attention. CLICK HERE for a quiz on arm designation in the sunflower star Pycnopodia helianthoides.

 
Research study 3
 

A study at Hopkins Marine Station on early development in bat stars Patiria miniata emphasises a change from the bilateral symmetry of the early bipinnaria larva to a left-handed asymmetry as the coelomic pouches and hydropore (madreporite) appear.  At 15oC in culture, cleavage commences 2h after fertilisation, leading to a swimming blastula at 18h and to hatching soon after.  Gastrulation begins a few hours after hatching (see drawings below) and is completed about 30h after fertilisation.  At 72h the anterior coelomic pouches have pinched off and the posterior coelom begins to outpocket from the archenteron (this will form the gut).  Note that until the posterior coelom forms on the left side of the archenteron the larva is perfectly bilaterally symmetrica (middle view below)l.  The author notes that of 916 embryos examined, 88% exhibitthis normal left-handed asymmetry, while 11% have a right-handed asymmetry, with the posterior coelom forming on the right-hand side.  On Day 5 the hydropore (madreporite)  buds off typically from the posterior end of the left anterior coelom and is mostly (90%) on left side of the larva.  Apparently, the presence of about 10% of larvae with a reversed right-handed asymmetry is, up to the time of publication of this paper, thought to be unusual in echinoderms.  These right-handed larvae appear to be vigorous and develop normally in the culture flasks.  At 2wk of age the anterior coeloms fuse with one another, but not yet with the posterior coelom, and the mouth is well developed.  By 4wk the primordium of the adult sea star with its 5 budding arms is apparent (right view below, side view of early brachiolaria stage).  At this point in the study the description of development terminates owing to death of the cultures.  However, the author observes that prior to this many right-handed asymmetrical larvae are swimming amongst the normal left-handed ones, and notes that should this occur naturally, there would be no way to know from the radially symmetrical adult whether it derives from a larva with normal or reversed asymmetry.  Newman 1925 Biol Bull 49 (2): 111.

NOTE  the author concludes the work with a series of experiments attempting to induce right-handed asymmetry in laboratory culture.  Cooling the culture for a couple of hours to near 0oC seems to have some effect if it is done during the blastula stage (about 25% become right-handed), but not if it is done during the gastrula stage (only about 10% become right-handed)

  drawings of development of a bat star Patiria miniata
 
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