Pinpointing biomarkers of sea urchin health for aquaculture - a step toward sustainable harvesting

February 10, 2019

Sea_urchin_2

Details on photo and microscope images at bottom of article.

Sea urchins are members of the phylum Echinodermata, a taxonomic group that includes sea stars. Their shape is unlike that of sea stars however, as many species appear as hard, colourful balls covered in spines, bearing a resemblance to pin cushions. Amongst the spines and poking through their calcified outer shell (called a “test”), are countless spaghetti-like tube feet that urchins use to slowly crawl across the sea floor. Their mouths, located on the underside of their bodies, consist of five teeth that allow the urchin to feed on one of their preferred foods, algae.

Interestingly, sea urchins have what is termed a biphasic life history. Following fertilization, embryos develop into tiny larvae that swim through the water feeding on plankton. Similar to humans and most other animals, a larvae’s body has dual symmetry. Once larvae have sufficiently developed (termed “competent larvae”), it settles onto the ocean floor and begins the second phase of its life, first as a juvenile and then eventually becoming an adult sea urchin with the recognizable appearance described above (and shown in the image). Although not immediately obvious to the onlooker, the sea urchin’s body has, similar to sea stars, five-part symmetry in the second phase of its life.

Some species of sea urchins are consumed by humans and their gonads, commonly called roe, are considered a delicacy in parts of the world. There is concern that market demands will outpace sustainable harvesting of natural populations, alternatively sea urchin aquaculture prevents a viable option. Due to their biphasic life history, there are challenges in aquaculture. For instance, several weeks are required to complete the transition from larval to settled juvenile urchin and the development of a mouth and digestive system alone can take several days. During this period, newly settled juvenile urchins rely on their larval reserves for energy. 

Fadl et al. (2017) set out to investigate aspects of sea urchin biology that are important to aquaculture but are poorly understood. Specifically, they looked at molecular and genetic markers related to the metamorphosis of urchins from the planktonic larval stage into juvenile settlement onto benthic habitat (i.e., the sea floor). Depletion of larval energy reserves could correlate with changes in certain molecular and genetic markers.

For their study, they used the purple sea urchin, Strongylocentrotus purpuratus, a species found along the North American coast from Alaska to Mexico that has been extensively used in research. Urchins were spawned and reared in the lab and divided into groups that were exposed to different conditions including: artificially grown bacterial biofilm, natural bacterial biofilm, larval food (a microalgae), filtered artificial sea water, and/or natural sea water. Bacterial biofilms were included in the study since they have been shown to influence settlement of larvae. Another experiment involved starving larvae for five days for comparison against larvae that were consistently fed. 

Fadl et al. (2017) assessed urchin growth by measuring spine length and test diameter of settled juveniles before they had developed a mouth and digestive system and started feeding. They also measured the expression of a molecular signaling pathway (abbreviated name IIS/TOR) in larvae and juveniles. This pathway occurs in cells and is important in regulating life history traits such as body size, growth, reproduction, and aging. The IIS/TOR signaling pathway could be a useful biomarker from an aquaculture perspective because it is influenced by an urchin’s nutritional status and hence relates to health.

They hypothesized that changes in the IIS/TOR pathway would reflect the changes in feeding mode from larval to settled juvenile. In sum, their results show a strong correlation between expression of certain IIS/TOR genes and purple sea urchin growth. Starving larvae for a brief period affected growth of settled juveniles and was also reflected in IIS/TOR expression. Given the similarities in the biology of purple sea urchin and other edible species, the information provided by Fadl et al. (2017) could be used to develop new biomarkers of health for wild populations of sea urchin as well as for those grown in aquaculture.

 

Details on photo and images:

Photo of adult purple sea urchin Wikimedia license.

Top microscope image - juvenile sea urchin after settlement. js: juvenile spines, t: test, m: mouth, ptf: primary tube feet, as: adult spine. Scale bar 50 microns.

Bottom microscope image - 6-day-old juvenile. as: adult spine, sb: stereomic bud, bs: basal part of spine, s: stem. Scale bar 50 microns. 

 

Read the full article (open access):

www.heliyon.com/article/e00412  

References:

Fadl AEA, Mahfouz ME, El-Gamal MMT, and Heyland A. 2018. Onset of feeding in juvenile sea urchins and its relation to nutrient signalling. Invertebrate Reproduction & Development. doi: 10.1080/07924259.2018.1513873. 

Fadl AEA, Mahfouz ME, El-Gamal MMT, and Heyland A. 2017. New biomarkers of post-settlement growth in the sea urchin Strongylocentrotus purpuratus. Heliyon 3 (2017) e00412. doi: 10.1016/j.heliyon.2017.e00412.