The effect of algal turbidity on larval performance and the ontogeny of digestive enzymes in the grey mullet (Mugil cephalus)

Abstract

23A study comprised of two trials was carried out to determine and compare the effects of water 24 turbidity produced by live microalgae and inert clay particles on the larval rearing of grey mullet 25 (Mugil cephalus). ITrial 1 evaluated the effect water turbidity generated by microalgae on grey mullet 26 larval performance and digestive enzyme activities along ontogeny. In particular, two microalgae 27 (Nannochloropsis oculata and Isochrysis. galbana) were used, whereas water turbidity levels (0.76 28 and 1.20 NTU) and a non-microalgae control (0.26 NTU) were tested from 2 to 23 dph grey mullet 29 larvae (5 treatments). The higher turbidity (1.2 NTU) larvae (5 dph) consumed markedly (P < 0.05) 30 more rotifers than other treatment fish, independently of the microalgae type. Alkaline phosphatase 31 activity was ca. 8 times higher and α-amylase activity increased 5.3 times in 79 dph fish compared to 32 40 dph individuals. The ratio of alkaline phosphatase and leucine-alanine aminopeptidase indicated 33 gut maturation occurred around 61 dph, as well as a transition from carnivorous to omnivorous 34 feeding habits. Trial 2 compared the most effective N.occulata produced turbidity level (1.2 NTU) 35 with the identical water turbidity produced by inert clay on larval performance. M. cephalus larvae 36 exposed to high algal turbidity demonstrated superior performance (P < 0.05), in terms of rotifer 37 ingestion, dry weight gain and survival, in comparison to cohorts reared under the clay treatment as 38 well as a lower microalgae produced turbidity. These findings suggested that water algal turbidity is 39 not the dominant factor determining improved grey mullet larval performance. 40 41 (Papandroulakis et al., 2002; van der Meeren et al., 2007; Bentzon-Tilia et al., 2016). On the other 47 hand, it remains speculative how algal supplementation contributes to larval growth and survival or 48 whether this benefit is species-specific. The biochemical composition of algal species (e.g. fatty 49 acids) varies considerably and it is entirely possible that particular compounds secreted from the algal 50 cell (e.g. polysaccharides) and/or are released during digestion might stimulate the immune system 51 or enhance the digestive process in larvae (Hemaiswarya et al., 2011). In addition, water turbidity 52 from specific algal concentrations may modify the light milieu for larvae, providing optimal 53 backlighting for larvae to facilitate live prey identification (e.g. rotifers), foraging behavior and 54 thereby enhancing hunting success (Rocha et al., 2008). 55The grey mullet (Mugil cephalus) is an economically important euryhaline and eurythermal species 56 contributing to sizable fisheries of estuarine and coastal regions in a variety of countries. It has been 57 traditionally farmed extensively in ponds and enclosures in the Mediterranean region, South East 58 Asia, Korea, Taiwan, China, Japan and Hawaii (FAO, 2018) and has been recently recognized as a 59 highly valued candidate for more intensive aquaculture (Whitfield e...

Ontogeny of Expression and Activity of Digestive Enzymes and Establishment of gh/igf1 Axis in the Omnivorous Fish Chelon labrosus

“…Correspondingly, in our study an increasing trend of pga2 and atp4a expression from 21 dph onward coincided with proliferation of gastric glands in this species at 23 dph (Figure 1), although maximum expression levels and therefore, expected fully stomach functionality, was not achieved until 50 dph. The decreasing trend measured beyond this point is not in agreement with findings in species with carnivorous habits, in which the mRNA levels of these genes increase toward advanced developmental stages [5,9,17,34].…”

“…Interestingly, amylase activity both per larva and per mg dry body mass showed a similar pattern; both showed low values for most part of the development but exhibited a dramatic increase from 64 dph onward. This sharp escalation, not only in total secretion but also in synthesis rate of this enzyme over time, is in accordance with results obtained in another mullet species (M. cephalus), where amylase specific activity increased significantly at 79 dph [34]. Moreover, the increases in amylase gene expression and activity obtained here cannot be attributed to changes in dietary carbohydrates content, since the same commercial feed was supplied at the mentioned sampling times.…”

“…This comparatively increasing role of amylase to either trypsin or lipase within the digestive equipment could be a strong support for inclusion of starch or other low cost carbohydrate-based compounds in formulated diets of C. labrosus post-larvae/early juveniles, in a similar way to what has been suggested in M. cephalus [34], making this species a great candidate for an economically and environmentally sustainable aquaculture.…”

“…While pepsinogen expression and pepsin specific activity patterns are usually reported to be in line during the ontogeny of fish species [5,9,35], our attempts to detect pepsin activity by using a highly sensitive fluorometric substrate, failed. Similarly, no acid protease activity was detected in the larvae of another mullet species (Mugil cephalus) until 79 dph, and the authors suggested a crucial role of alkaline proteases in this family [34]. Thick-lipped grey mullet of larger size (45-329 g), has shown decreasing pepsin activity levels by age, pointing to a minor role of stomach digestion even in adult stages [30].…”

The ontogeny of greater amberjack digestive and antioxidant defence systems under different rearing conditions: A histological and enzymatic approach

“…During the first stages of development until its transformation into a juvenile, numerous changes take place in the digestive system of fish larvae in terms of its morphology and functionality (Gisbert, Nolasco, & Solovyev, 2018; Papadakis, Kentouri, Divanach, & Mylonas, 2013; Papadakis et al., 2009; Przybył, Ostaszeweska, Mazurkiewicz, & Wegner, 2006). Over this period, activity of the digestive enzymes is affected by a number of different factors, and their levels are closely related to the state of maturation of the secreting digestive tissues, which show important variations between species, water temperature and rearing conditions (Koven et al., 2019; Lazo, Darias, & Gisbert, 2011). Therefore, the knowledge of the digestive competence of a fish is essential in order to understand the digestive physiology of larvae and to adjust the feeding protocols to dietary qualitative and quantitative characteristics (Campoverde, Rodriguez, Perez, Gisbert, & Estevez, 2017; Gisbert et al., 2018) contributing to the optimization of diets (Campoverde et al., 2017; Zambonino‐Infante et al., 2008) and to the proper understanding of functions and limitations in the processing capacity of the digestive system.…”