Effects of supplemental wild zooplankton on prey preference, mouth gape, osteological development and survival in first feeding cultured larval yellow tang (Zebrasoma flavescens)

Highlights

• Determination of larval gut contents through a whole body proxy and metabarcoding reduces effort for identification of prey selection

• Maximum mouth gape heights and widths, documented herein for the first time for Yellow Tang larvae, further facilitates identification of suitable prey sizes for first feeding acanthurids and other marine ornamental larvae.

• Characterization of osteological development for Yellow Tang larvae through the first week of life aids in determination of appropriate prey types based on developmental stage and potential capture ability.

Abstract

Yellow Tang, Zebrasoma flavescens, are a popular species in the aquarium industry. Currently, all Yellow Tang are collected from wild stocks, with collection in Hawaii accounting for the bulk of the individuals found in the trade. Although culture of this species has recently been successful, survival remains low and mass mortality events, particularly associated with the transition from endogenous to exogenous feeding, continue to be observed. Improving survival and gaining a more thorough understanding of the drivers behind early prey selection, such as mouth gape and osteological development, are of primary importance in advancing the culture of Yellow Tang. For the present study, a supplemental size-sorted mixture of wild zooplankton was offered in conjunction with and compared to the cultured nauplii of Parvocalanus crassirostris to determine the effect on survival. Prey selection was determined via metabarcoding. Osteological development was documented and mouth gape measured from 1 to 6 days post hatch to better determine appropriate prey types and size ranges. Survival was significantly lower in larvae fed cultured nauplii supplemented with wild zooplankton, 6.9% v. 18.5%. Regardless of the presence of wild zooplankton, larvae preferentially selected for P. crassirostris based on molecular determination of prey consumption. However, it is likely that protozoans were mostly excluded from this molecular analysis possibly due to the type of primer selected. Mouth gape height increased by approximately 100 μm and mouth gape width by approximately 55 μm, during the study period. Although no significant difference in growth was calculated between treatments, larvae receiving the wild zooplankton supplement diet were observed to grow at a slightly slower rate than larvae being fed only cultured copepod nauplii. The rate at which the mouth gape increased suggests that the currently cultured and utilized copepod nauplii may not be an optimally sized prey item shortly after first feed. These findings indicate that first feeding Yellow Tang larvae are capable of consuming larger prey at an earlier age than they have been offered in previous rearing attempts, but that the larvae may continue to benefit from the presence of less elusive prey items, such as ciliates or rotifers, due to their delayed osteological development. Osteological development was slow with very little ossification occurring by 6 days post hatch and only in those larvae fed solely cultured nauplii. However, their selection of copepod nauplii suggests that some aspect of this prey item is preferential to the first feeding larvae and may elicit a hunting response essential to initiating exogenous feeding.

Introduction

Yellow Tang, Zebrasoma flavescens (Bennett 1828), are one of the most prevalent species in the global aquarium trade and are currently only available to consumers through wild collection (Wabnitz et al., 2003). Collection of the 300,000 to 400,000 Yellow Tang traded annually occurs almost solely in Hawaii and accounts for 84.3% of all marine ornamental fish collected in the state (DLNR, 2015). Although collection in Hawaii is accomplished without the destructive use of explosives or chemicals, exploitation of Yellow Tang has resulted in localized stock reduction (DLNR, 2010; 2015; Ogawa and Brown, 2001; Walsh et al., 2003). To address this issue, fish replenishment areas (FRAs) and similar sites that prohibit the collection of animals within their boundaries have been established in past years to alleviate fishing pressure and aid in increasing Yellow Tang densities (DLNR, 2015; Tissot and Hallacher, 2003). While FRAs appear to have helped stabilize the population of Yellow Tang in the state (DLNR, 2015), the continued removal of these ecologically important reef fish from the environment remains controversial resulting in recent legislation suspending the issuing of permits pending an environmental assessment (Umberger v Dept. of Land and Natural Resources, 2017).

Aquaculture has the potential to offer consumers a sustainable alternative to wild caught ornamentals. However, only about 22% of the 1471 marine ornamental species found annually in the aquarium trade have been bred in captivity and even fewer species, approximately 1.8%, are commercially available (Sweet, 2016; Tlusty, 2002; Wabnitz et al., 2003; Young, 1996). The challenges associated with the culture of marine ornamentals are often linked to the physical and physiological characteristics of the larvae (Olivotto et al., 2011; Yúfera and Darias, 2007). In particular, the transition from endogenous to exogenous feeding is a period of high mortality, as a variety of factors can affect first feeding incidence, and ultimately survival, including prey preference, mouth gape and osteological development, as it relates to hunting and prey capture ability (Olivotto et al., 2011; Yúfera and Darias, 2007).

Yellow Tang were first successfully cultured in late 2015 at the Oceanic Institute of Hawaii Pacific University (OI) through the implementation of important prior findings pertaining to live feeds and environmental parameters (Callan, 2016; Callan et al., 2017; Holt et al., 2017). Despite the achievement of culturing the first member of the family Acanthuridae, survival was low, approximately 1.3% from egg to juvenile (Holt et al., 2017). Continued low survival, 25–50%, through the first week in culture suggests that the transition to exogenous feeding continues to be a bottleneck in the culture of Yellow Tang (Callan et al., 2017).

Although it is widely believed that most marine larval fish consume copepod nauplii as prey, little is known about the natural diet of larval acanthurids (Llopiz and Cowen, 2009; Sampey et al., 2007). Guts excised from small wild Acanthuridae larvae (2–3 mm body length) mainly contained pteropods, but also, in smaller quantities, copepod nauplii (Llopiz and Cowen, 2009). Offering prey smaller and less elusive than copepod nauplii, such as dinoflagellates or ciliates, has resulted in some success in increasing survival in first feeding marine ornamental larvae when fed instead of, or in addition to, copepod nauplii or rotifers (Leu et al., 2009, Leu et al., 2015; Nagano et al., 2000). While offering a wider range of size options and taxa of prey could elucidate preferential prey items for target species, very few published studies have compared rearing success using wild zooplankton to cultured prey items in their methods. The few studies incorporating wild plankton, mainly used wild collected eggs with very few resulting juveniles or have used a mixture of feed items resulting in low survival through the first feeding stage (Baensch, 2014; Baensch and Tamaru, 2009; Olivotto et al., 2006). What, if any, prey items were consumed by larvae during these studies is either unknown or unreported. Direct comparisons between wild zooplankton and cultured prey could offer important insights into Yellow Tang feeding behavior and provide much needed early life history information for this species.

Determining potential larval diets for marine ornamentals is further exacerbated by the fact that mouth gape height (MGH) and width (MGW) measurements are absent for many species. It is generally agreed that to promote larval feeding prey width should be 25–50% smaller than the mouth gape of the species to be cultured, but an absence of gape measurements for first feeding ornamentals makes determining this range impossible (Shirota, 1970; Yúfera and Darias, 2007). The connection between gape and osteological development determines the size of possible prey that can be ingested and likely influences prey preference. Limitations in mechanical ability, resulting from the lack of a fully ossified jaw, means that first feeding larvae are often restricted to prey that are small and non-elusive, and therefore incapable of a successful escape (Wittenrich et al., 2009; Wittenrich and Turingan, 2011). Despite the fundamental importance of these relationships, the osteological development of most marine ornamental species and how it may affect prey preference remains understudied.

The main objective of the current study was to determine if the mass mortality observed surrounding first feeding could be mitigated in cultured first feeding Yellow Tang through the dietary supplementation of a diverse mixture of wild zooplankton. Molecular analysis was used to identify which prey items were consumed by larvae and whether these selections coincided with increased feeding incidence at 3 days post hatch (dph) and increased survival to 8 dph. Maximum MGH and MGW measurements, along with observations of osteological development, were used to further the knowledge on appropriate prey size range and potential capture ability for this species.