THE EFFECT OF SETTLEMENT SUBSTRATES AND FEED ON EARLY STAGE GROWTH OF THE SEA URCHIN Tripneustes gratilla: IMPLICATIONS FOR HATCHERY PROTOCOLS AND ECHINOCULTURE DEVELOPMENT
Introduction
Successful larval settlement and metamorphosis has been linked to the recognition of specific substrates or substratum-specific biochemical signals. In many intensive aquaculture systems, the required morphogenesis- inducing substances are often absent. S ea urchin settlement and metamorphosis cues are highly species-specific and may include microalgae, natural surfaces (e.g. porous rocks), adult conspecifics, coralline and other macroalgae, specific chemicals, and bacterial biofilms, particularly those found on the surface of macroalgae. C ommercial abalone and urchin farms often use biofilms that develo p in the systems naturally for larval settlement (Affan et al., 2015); but this method is often highly inconsistent and seasonal and largely considered ineffective for Tripneustes gratilla (<2% settlement, with poor post-settlement survival) (Mos et al., 2011). Certain cues may well induce settlement but not necessarily sustain growth and ensure survival of larvae during the post-settlement phase (Mos et al., 2011), because the settlement substrate is either indigestible or nutritionally poor . T. gratilla requires a diet capable of promoting high survival and growth during these fundamental stages of development. This study investigated settlement success, post-settlement survival and growth of T. gratilla during different periods of the juvenile phase fed a variety of natural and artificially created substrates/diets.
Materials and Methods
N ine settlement substrates were tested, including natural substrates of Ulvella lens , fresh Ulva and Nitzschia, as well as artificially created substrates using alginate with the addition of either dried Ulva , dried Isochrysis galbana , a probiotic Vibrio midae SY9, a combination of V. midae SY9 & an Ulva exctract , ethanol solvent (control) or no additive (control). Competent larvae were settled directly into small troughs (L:200×W:105×D:45mm) contained within a flow- through experimental system. Each treatment had 4 replicates within randomly assigned tanks. Competent larvae (n=35) were placed in each trough and the number of larvae to successfully completed metamorphosis within a 60h period was recorded. Successfully settled larvae were maintained in their respective treatments for a period of 4 weeks to assess post-settlement growth (by measuring test diameter of 10 random individuals) and survival, which was recorded weekly for each treatment using a Nikon SMZ1500 stereomicroscope. Substrates were replaced once a week when the individual troughs were cleaned. After 4 weeks, urchins from the best performing treatment (U. lens ) were used to assess the effects of weaning urchins from post-larval feeds on to the macroalga Ulva . Urchins from the latter treatment were randomly divided into 2 treatments of 4 replicates each. One treatment continued to receive U. lens for three weeks before being fed fresh Ulva for an additional 3 weeks, whereas the second treatment group was offered fresh Ulva for the entire 6 week period .
Results and Discussion
Settlement success varied greatly between substrates. The 3 best performing treatments, not significantly different from each other, were Ulva , Ulvella lens and Nitzschia; which induced 67, 62 and 41% larval settlement, respectively . Settlement success was less than 30% for the remaining treatments, with no significant differences between them. Survival of larvae settled and maintained on the U. lens substrate by the end of the 4 week growth trial was significantly higher (61%) compared with all other treatments , which had an average survival < 20%.
Juvenile urchin test diameter (TD) at week 4 was significantly greater in the U. lens treatment (3507µm), compared with all other treatments that had an average TD of 508µm, with no significant differences between the latter treatments . Thus, diets other than U. lens do not appear to be suitable post-settlement diets for T. gratilla, even though both Ulva and Nitzschia induce high settlement . Following the dietary change at week 4, urchins maintained on Ulvella were significantly larger at week 7 (6874µm; Fig 1) compared to juveniles fed fresh Ulva (5260µm). After 7 weeks both treatments were offered only Ulva , but the difference in TD did not change between the 2 treatments and urchins maintained for 7 weeks on Ulvella remained significantly larger at the end of week 10 (14278µm TD), compared with juveniles transferred to a diet of fresh Ulva three weeks earlier (11638µ m TD).
Our study suggests Ulvella i s a superior substrate for T. gratilla larval production, capable of producing juvenile sea urchins of ca. 1. 5cm in just 10 weeks post- settlement. These findings have important implications for the commercial production of this high value species, highlighting the importance of suitable substrates for both settlement and post-settlement feeding. The study further shows that the timing of weaning juveniles from post-settlement diets to adult feeds can have significant effects on growth and potential effects on overall production times, which could have significant effects on timelines for the production on u rchins for harvest.
References
Mos, B., Cowden, K., Nielsen, S. & Dworjanyn, S., 2011. Do cues matter? Highly inductive settlement cues don’t ensure high post-settlement survival in sea urchin aquaculture. PLoS One, p. 6.
Affan , M.A., Khomayis, H.S., Lee, J.B., Al-Harbi , S.M., Touliabah , H.E.S. and Abdulwassi , N.I., 2015. Settlement and growth of larval and juvenile abalone on single and mixed strains of benthic diatoms. Thalassas : An international journal of marine sciences, 31(2), pp.59-65.Akakabe, Y. & Kajiwara , T., 2008. Bioactive volatile compounds from marine algae: feedging attractants. Journal of Applied Phycology, 20(5), pp. 211-214.
