Nucleic acid derived indices or instantaneous growth rate as tools to determine different nutritional condition in cuttlefish hatchlings

Journal of Shellfisheries Research, August, 2004 by Antonio V. Sykes, Pedro M. Domingues, Jose P. Andrade

ABSTRACT Two groups of 100 cuttlefish hatchlings each were used to determine the duration of the yolk reserves, during which growth can be obtained with no food supply. One group was fed live grass shrimp ad libitum from the 3rd day of life onwards, while the other was starved during the whole experiment. The experiment lasted 7 days, because this was the 1st day where a negative growth rate was obtained. Nucleic acid derived indices and instantaneous growth rates (IGR) were used as a way to describe their condition. when all the yolk reserves would be exhausted and to determine the most accurate tool to express growth and condition. RNA/DNA ratios described clearly the differences (P < 0.05) between fed and starved hatchlings cultured at 23 [ or -] 2[degrees]C, especially from day 4 onwards. Correlation between IGR and RNA/DNA ratios was only attained for the starved population (R = 0.90; P = 0.0374). DNA concentration was identical (P > 0.05) between groups and did not displayed a clear pattern with increasing age. Correlation between IGR and DNA was only attained for the fed population (R = -0.90; P = 0.0374). RNA concentration values were different from day 5 onwards. Any of the nucleic acid derived indexes were not accurate enough to express growth and condition, so IGR seems to be the most accurate and inexpensive way to describe hatchlings growth and condition in controlled conditions.

KEY WORDS: condition indices, cuttlefish, DNA/RNA ratios, growth rates, hatchlings, accuracy methodologies, Sepia

INTRODUCTION

When culturing any species in intensive aquaculture, time of first feeding is of extreme importance. Food should be available when larvae or hatchlings are finishing the absorption of the inner yolk reserves and starting to feed externally. Cuttlefish are known to be voracious feeders throughout their life cycle and accept a wide range of prey (Guerra 1985, Castro & 1989, Pinczon du Sel & Daguzan 1992, Domingues et al. 2001a. Domingues et al. 2003). Some authors (Wells 1958, Richard 1971, Richard 1975, Boletzky, 1983, Boletzky 1987, and Nixon 1985) have studied the embryonic development and the early life history of cuttlefish and set the end of the inner yolk reserves around the 3rd day of life. However, according to Domingues et al. (2001b), cuttlefish in unfed or poor nutritional condition (as result of prey with poor nutritional profile) can last as much as 20 days. The only way to determine optimal timetables for first feeding, thus optimizing cuttlefish production, is the use of condition analysis. Condition is a measure of the physical status or well being of an animal and may be used to evaluate growth or survival rates (Bolger & Connoly 1989, Ferron & Leggett 1994).

Nutritional condition of larval stages plays an important role in the knowledge of the recruitment of marine species. Nutritional condition of fish larvae can be evaluated using several methods: morphometric, histologic, and biochemical (Chicharo 1993). However, costs and results associated with each methodology are not similar between them. So, the main questions are which methodology to choose when trying to evaluate the condition of the animal, and the compromise between costs and accuracy of results to achieve. The use of morphometric methodologies are standards in most aquaculture and fishery laboratories around the world. Nowadays, biochemical methodologies related to nucleic acid derived indices are becoming standards also.

One of the most commonly used methods for determining the nutritional condition and growth is the RNA/DNA ratio (Buckley 1979, Buckley 1980, Buckley & Lough 1987, Clemmesen 1988, Clemmesen 1990, Robinson & Ware 1988). The use of this methodology is based on the assumption that DNA is present in constant concentrations, under changing environmental conditions or during starvation (Richard et al. 1991), whereas RNA varies, decreasing in starving animals (Buckley 1984, Buckley & Lough 1987). According to Bulow (1987), RNA concentrations are higher in tissues with faster growth rates or with a higher rate of protein synthesis. The RNA/DNA ratio indicates quantities/concentrations of RNA per cell and is the most accurate when estimating tecidular proteossynthetic activity (Bulow 1987, Buckley 1981, Buckley 1984). The use of RNA concentration in a tissue as an indicator of growth is based on the assumption that RNA is related to the potential for protein synthesis (Houlihan 1991). However, large variation in the RNA/DNA ratio may occur in fed larvae (Clemmesen 1988, Raae et al. 1988). Recent studies suggest that the RNA/DNA ratio is one of the best indicators of the nutritional condition of several marine organisms (Clemmesen 1994, Bailey et al. 1995, Chicharo 1997, Chicharo et al. 1998, Chicharo et al. 2001).

Biochemical methodologies have been proposed to evaluate condition in post-hatch and juvenile cuttlefish (Clarke et al. 1989, Pierce et al. 1999, Koueta et al. 2000). One of those is the RNA/ DNA nutritional condition ratio. However, until now, only RNA concentrations in the muscle were shown to be directly correlated with growth in Octopus vulgaris (Houlihan et al. 1990) and Sepia officinalis (Castro & Lee 1994).

The objective of this research is to determine: (1) how long cuttlefish hatchlings could survive on inner yolk reserves after hatching, using nucleic acid derived indices (RNA/DNA ratio, [DNA]/g and [RNA]/g) and instantaneous growth rates (IGR) as ways to describe their growth and condition and (2) the best describer for condition, based on a compromise of money spent, results achieved, and when to apply it.