Reproduction & development
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  Egg size & energy content
  Topics relating to reproduction & development include egg size & energy content, considered here, and SELECTED GENERA, LARVAL CLONING & REGENERATION, and TO BROADCAST SPAWN OR BROOD?, presented in other sections.
 
Research study 1
 
photograph of Pteraster tesselatus or militaris

Large yolk-rich eggs, such as those of Pteraster tesselatus, are generally associated with lecithotrophic, pelagic development and with brood protection. It is almost axiomatic in marine-invertebrate developmental biology that such large-sized eggs are correspondingly energy-rich.  However, studies at Friday Harbor Laboratories, Washington on eggs of P. tesselatus reveal no significant correlation between volume and energy content (see graph).  Thus, at least in P. tesselatus, egg size is not a reliable predictor of parental investment, nor can it be used as a correlate of such important larval features as size, developmental rate, settlement-delay capability, and size at metamorphosis, nor of potential for juvenile growth and survival.  McEdward & Coulter 1987 Evolution 41: 914.

NOTE  a point of interest here is that most studies on energy contents of invertebrates eggs, including this one, use dichromate oxidation against a glucose standard as the favoured method of assessment (Research Studies 1-3 here and several others throughout the ODYSSEY).  Although handy to use, this technique is now thought to underestimate significantly the energy contents of small eggs of marine invertebrates because of its tendency to oxidise proteins incompletely.  Incomplete oxidation of carbohydrates and fats is apparently not an issue, but smaller eggs of planktotrophic developers, especially echinoderms, contain proportionately more protein than the larger eggs of lecithotrophic-developing species.  The magnitude of the potential error can be large.  For example, in critiquing the dichromate oxidation method the authors cited below note that eggs of green sea-urchins Strongylocentrotus droebachiensis contain 12.2mJ . mg-1 dry mass as measured from summing the energetic value of their measured biochemical components, a value that is about 1.5 times greater than that reported from dichromate oxidation.  The former value is equivalent to 23mJ . mg ash-free dry mass, close to the value of 22.5mJ . mg-1 as measured by direct calorimetry (combustion in a bomb calorimeter).  The authors estimate that use of dichromate oxidation in studies of small invertebrate eggs may underestimate the true energy value by about 35%.  Pernet & Jaeckle 2004 Biol Bull 207: 67.

NOTE the 44 eggs used in the study are siblings, taken from a single female, and are estimated to represent about 1% of the female’s total spawn. The authors acknowledge that data from a single female may not represent the true situation within the species; however, see the similar conclusion arrived at in Research Study 2 below that employs many females

 

graph showing the relationship between egg volume and energy content for sea stars Pteraster tesselatus
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Research study 2
 

photograph of sea star Solaster stimpsonigraph showing organic content vs. volume in eggs of the sea star Solaster stimpsoniAnother investigation by the same research group as in Research Study 1 above takes a slightly different approach to the question of parental investment and egg size in sea stars. The sea star used in the study, Solaster stimpsoni, is similar to Pteraster tesselatus in that it also has lecithotrophic larval development.  In this study organic content versus volume is measured in 658 eggs from 25 females at 4 locations around the Bamfield Marine Sciences Centre, British Columbia.  Overall means for the 25 females are 114 µg carbon at an egg volume of 0.4mm3 (see accompanying graph).  As in the previous Research Study, the authors conclude that egg size is not a reliable indicator of parental investment per offspring.  McEdward & Carson 1987 Mar Ecol Progr Ser 37: 159.

 

Sea starSolaster stimpsoni tangled in the
feeding tentacles of a terrebellid worm 0.6X

 
Research study 3
 

In a later comparison involving 5 asteroid and 2 holothuroid species in the Bamfield area of British Columbia, the authors find good correlation of energy content with egg volume, but only when mean values for different species are compared (i.e., interspecific; see graph on Left). These interspecific results are more like what the researches expected, that is, graph showing energy content vs. volume for eggs of the sea star Mediaster aequalislarge eggs contain more energy than small eggs and represent a greater level of parental investment. However, when intraspecific data are examined, for example, for the sea star Mediaster aequalis, graph showing the relationship between energy content and volume in eggs of several sea star and sea cucumber speciesthen the correlation is much less good (see graph on Right). Species selected in the study are ones with pelagic lecithotrophic development.

The authors suggest that the variation in egg size may result from imprecision or asynchrony in vitellogenesis within the large populations of oocytes, but its adaptive value is unclear.  However, given that there is sufficient energy in the eggs to allow completion of metamorphosis, is it possible that the variation is a strategy to spread out the larvae both temporally and geographically to maximise recruitment?  McEdward & Chia 1991 J Exp Mar Biol Ecol 147: 95.

NOTE in their published graph the authors include mean values from an unequal number of females (1-3) to plot the regression.  It is replotted here using a single mean value for each species

 
Research study 4
 

graph showing diameter of eggs produced by sea stars Leptasterias epichlora at 2 sites in San Juan Islands, Washingtongraph showing number of eggs produced by sea stars Leptasterias epichlora at 2 sites in San Juan Islands, WashingtonA study at Friday Harbor Laboratories, Washington addresses potential site-differences in female Leptasterias epichlora in relation to egg size, number of eggs produced, and protein content of the eggs.  Two sites are compared: one at Deadman Bay where females are larger (8-12g live mass), more wave-exposed, and have access to a greater number and diversity of prey; and another site at Point Caution where females are smaller (2-5g), more sheltered from waves, and have access to fewer prey numbers and species.  The author predicts that females will produce many small eggs in the favourable site (Deadman Bay) versus fewer, larger, eggs in the unfavourable site (Point Caution). 

graph showing protein content of eggs of sea stars Lepttasterias epichlora at 2 sites in San Juan Island, WashingtonThe data, however, show the reverse. Larger females at the favourable site (Deadman Bay) produce significantly larger eggs (see graph at Left) and more of them (see graph upper Right).  Additionally, protein content is significantly higher in eggs produced at the favourable site (see graph lower Right) and this leads to hatching of larger-sized juveniles (1.7mm vs. 1.5mm diameter at Deadman Bay and Point Caution, respectively).  The author concludes generally that these results accord with life-history models that incorporate similar aspects of female size, nutritional status, and extent of care devoted to the young.  George 1994 J Exp Mar Biol Ecol 175: 121.

NOTE the author notes the taxonomic uncertainty of the Leptasterias-complex of species.  Three species are thought to occur in the San Juan Islands where this study is done, with L. epichlora being the most common at the 2 study sites.  Refer to Chia 1966 Syst Zool 15: 300 and Kwast et al. 1990 Mar Biol 105: 477 for more discussion of these species

NOTE the author never really explains the justification for this prediction, as common sense suggests the opposite. Perhaps this hypothesis, defined in the INTRODUCTION part of the paper, is meant to be the NULL hypothesis, with the RESEARCH hypothesis being the opposite (and the one that is supported by the results)

 
Research study 5
 

photograph of an ochre star Pisaster ochraceus in seaweedA later study by the same author at Friday Harbor Laboratories, Washington reveals remarkable phenotypic plasticity in reproduction in ochre stars Pisaster ochraceus.  First, sea stars collected from a wave-exposed habitat, Mar Vista, are larger and have larger pyloric ceca than ones collected from a wave-protected site, Point Caution.  Interestingly, the bigger females actually produce smaller eggs than the smaller females.  In turn, the bipinnaria larvae from the larger eggs of the latter group are initially larger and develop faster than ones developing from the smaller eggs of the former group.  Moreover, if experimentally graphs showing the relationship between larval size and size of female and whether the larvae are fed or starvedstarved, the larger bipinnariae survive better than the smaller ones.  However, if fed, survival is actually higher for bipinnariae from small eggs.  Thus, both egg types may have equivalent fitness depending on environmental conditions. 

Under starvation conditions, both sets of bipinnaria adopt a wider and longer form, whereas under fed conditions, they are narrower and shorter (see graphs). These larger bipinnariae have greater ciliary-band lengths and thus greater feeding capacity.  They also have larger mouths and stomachs, thus maximising food intake and processing, and presumably leading to greater survival.  The author suggests that the additional energy for the extra growth in outer body wall and ciliary bands may come at the expense of gut-wall thickness, noted to be greater in fed individuals than in starved ones. The study is an interesting one and deserving of further research. George 1999 J Exp Mar Biol Ecol 237: 203.

NOTE  food used in culture of the larvae is the alga Rhodomonas sp. at densities up to 10,000 cells . ml-1

 
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