Comparative developmental biology of Meridiastra mortenseni and Patiriella regularis: The role of egg size

Abstract

The common New Zealand cushion stars Meridiastra mortenseni and Patiriella regularis are both members of the Family Asterinidae. In many coastal areas around New Zealand they are sympatric. Each of these species exhibits planktotrophic development, the ancestral form of larval development in the Asterinidae. The most striking difference between M. mortenseni and P. regularis is the difference in egg size, with P. regularis having been previously reported to have significantly smaller eggs than M. mortenseni. No study to date has described embryogenesis and larval development in M. mortenseni as has been previously done for P. regularis. Nor has any study looked at lipid profiles or utilization in either of these species beyond the third day of development. This was the first study to do each of the aforementioned things. M. mortenseni had larger eggs and larvae throughout development. The early stages of larval development for M. mortenseni happened across a similar time frame to that of P. regularis with the development of a feeding bipinnaria on day 3. The biggest difference in development was through the brachiolaria larval stages when M. mortenseni developed at a much faster rate resulting in larvae reaching the advanced brachiolaria stage in 4-5 weeks compared to P. regularis for which development to the advanced brachiolaria stage took 7-9 weeks. M. mortenseni developed a left posterior enterocoel off the left side of the archenteron which is a unique characteristic of P. regularis. Using thin-layer chromatography and flame ionisation detection M. mortenseni was found to contain the same lipid classes as P. regularis with a greater quantity of maternally invested lipid. Both species showed similar increases in each lipid class throughout development. Phospholipid and triglyceride showed the largest increase in quantity. The biggest difference between species was in phospholipid of which M. mortenseni contained significantly more throughout larval development. The results of this research agrees with previous studies showing that species with larger eggs have higher maternal investment, produce larger larvae and have a shorter larval duration.