Despite the numerical abundance and richness of species of asteroids on the west coast, there are surprisingly few studies on their growth and regeneration.

Sea star Orthasterias koehleri in the
process of regenerating a missing arm 1X

Do sea stars have an anterior-posterior axis?  They superficially appear to be radially symmetrical but, in fact, their single, offset madreporite and stone canal (and associated elements of the hemal system) signify 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 the 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 posterior” 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  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

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

The need to mark sea stars over long term growth studies in the field is hampered by the propensity for plastic and metal disc or filament tags to rip free from tissues over time.  Vital stains, such as Nile Blue Sulfate and Neutral Red, can be used to mark sea stars over periods of up to 10mo with no ill effects.  Even the purple-coloured morph of Pisaster ochraceus can be visibly stained, not on its aboral (upper) surface, but on its ossicles and on the lighter-coloured soft tissues of the oral surface.  When the dye fades in several months, it can be re-applied.  Feder 1955 Calif Fish Game 41: 245.

NOTE  a vital stain is one that has no deleterious effect on the health of a test organism.  The author remarks that stained individuals of Pisaster ochraceus have grown 30% in a 4-mo period, suggesting that the dyes are not harmful, although controlled tests would be necessary for complete confidence.  Nile Blue Sulfate requires a seawater solution of 1g ^. liter^-1 (dissolve the powder first in 1ml tap water) and Neutral Red, 3-4g ^. liter^-1.  Immerse the sea star entirely and apply the stain with a syringe, or stain selective arms by dipping them in the stain for 5-10min, taking care to aerate the solution during the process

Pink star Pisaster brevispinus, resting after
a meal, with simulated markings 0.4X

A study in Monterey Bay, California shows that laboratory growth of Pisaster ochraceus on an ad libitum diet of sea mussels Mytilus californianus is much greater than field growth in 3 different populations (see graph on Left). Other results from the study show that arm length is too variable a parameter to use to express growth in Pisaster, and perhaps in other asteroid species (see graph on Right). Not only are arms not usually straight in ochre stars, but inter-ossicle distances may vary depending on habitat, time of collection, whether the tide is in or out, and activity level (whether sedentary, feeding, locomoting).  Note also in the graph on the Right the apparent difference in live mass in individuals from the laboratory population versus those from the field populations for a given arm length.  This could be because the laboratory animals are actually fatter, or perhaps because, as noted by the author, the laboratory animals, being continually submerged, take on a plumper appearance than field animals.  In any case, the author cautions against solely using arm length in growth studies. Feder 1970 Ophelia 8: 161.  

NOTE the author uses vital stains to mark individuals for successive growth measurements in the field populations

Based on data from a study of Crossaster papposus in Alaska using 207 marked animals and successive measurements on the same animals over several years, growth is found to be slow, even at a young age, leading to essentially no growth at all when they are large. An individual 16cm in diameter may be 20yr old and, at the largest diameter of 34cm, an individual may be 30yr old or more. Data presented by the authors are highly variable, but significant. The authors use a coding system to number their field specimens, done as shown in the drawing on the Left by application of neutral red dye to the arms using different numbers and positions of stripes (see Research Study 2 above). The dye sets in about 3min and is readable for up to 5yr. In experiments where arms are half or completely removed from test individuals in the field, 4yr are required to regenerate the former to their original full lengths and the latter to half their original lengths. A large individual of 24cm diameter with 2 arms severed at their bases takes nearly 9yr to regrow both arms to 75% of their original sizes. Carlson & Pfister 1994 Mar Biol 133: 223.

NOTE the marking system uses the madreporite as a reference point, with the first arm clockwise to it (viewed from above) being designated as Arm #1. The example shown in the drawing is "Rose Star No. 425" (the double stripes on arms 10 & 11 designate the "400s", while the other stripes in clockwise order designate the "20s" and the number "5", respectively). Crossaster papposus in this study commonly have 11 arms