Effects of salinity on the growth and mucous cells of the abalone Haliotis diversicolor Reeve, 1846
Effects of salinity on the growth and mucous cells of the abalone Haliotis diversicolor Reeve, 1846
Lota Alcantara Creencia . Tadahide Noro 0 1 2
0 T. Noro Kagoshima Prefectural College , 1-42-1 Shimo-ishiki, Kagoshima City 890-0005 , Japan
1 L. A. Creencia T. Noro Faculty of Fisheries, Kagoshima University , 4-50-20 Shimoarata, Kagoshima 890-0056 , Japan
2 L. A. Creencia (&) College of Fisheries and Aquatic Sciences, Western Philippines University , Sta. Monica, 5300 Puerto Princesa City , Philippines
This study was conducted to determine the influence of salinity on the growth of abalone Haliotis diversicolor Reeve, including the density and size of mucous cells. Abalone individuals were reared in the laboratory at salinities of 20, 25, 31, 35 and 40 ppt. The mucous cells of the lips, gills and digestive gut of H. diversicolor, which react to some forms of stress such as suboptimal salinity, were characterized following staining with Alcian Blue-Periodic Acid-Schiff's Reagent (AB-PAS). The specific growth rate in wet weight and shell length of H. diversicolor were highest at 31 ppt and lowest at 20 ppt (0.52 vs 0.15% d-1, and 0.058 vs 0.021 mm d-1, respectively). The abalone H. diversicolor tolerated salinity fluctuations within the range of 20-40 ppt, but growth was optimum at 25-35 ppt. Mucous cells of the lips and gills showed significant differences (ANOVA, df = 4, P = \0.001) in cell density and cell size, being less dense and larger at 31 ppt than at 40 ppt, which could be an effect of osmotic and ionic regulation. Consistent with reports in literature, salinity ranges of 25-35 ppt are suitable for growth of H. diversicolor. Results of this study indicated that areas with such salinity are favorable for stock enhancement and mariculture of the abalone H. diversicolor.
Acclimation; Fishery resource; Haliotis diversicolor; Japan; Mariculture; Stock enhancement
The abalone Haliotis diversicolor Reeve, 1846, commonly called ‘tokobushi’ in Japan, inhabit the rocky
littoral to sublittoral zone (Geiger and Poppe 2000). In Tanegashima, Kagoshima Prefecture, Japan, H.
diversicolor is an important fishery commodity with declining yield. Hence, the local government and fisheries
cooperative associations are managing this fishery resource. Interventions include regulating the minimum
size of harvest and releasing cultured juveniles in designated fishing grounds for stock enhancement
(Alcantara and Noro 2005, 2006)
. In the Philippines, stock enhancement of abalone H. asinina resulted in
spillovers of abalone outside the release site (Salayo et al. 2016). On the other hand, the mariculture of abalone H.
diversicolor involves hatchery, nursery and on-growing in inland facilities and grow-out in nearshore ponds
and cages (Chen and Lee 1999; Creencia et al. 2016).
Fishers tend to gather larger individuals of H. diversicolor near estuaries (pers. obs.) which may be related
to salinity. Salinity has some adverse effect on various aspects of the physiology of abalone. H. diversicolor at
salinities lower or higher than optimum have reduced immunity and resistance to infection (Cheng et al.
2004), survival (Chen and Chen 2000) and growth (Chen et al. 2000). According to Chen et al. (2000), growth
rate of H. diversicolor is optimum at 30–35 ppt with variable rates for different sizes of juveniles. We
hypothesized that the seasonal inflow of freshwater in estuaries and the resulting decrease in marine salinity
from 34–38 ppt to 31–33 ppt might have promoted faster growth of H. diversicolor (Chen et al. 2000). In
general, faster growth of marine animals in intermediate salinities has been correlated with reduced metabolic
rate and elevated assimilation efficiency (Boeuf and Payan 2001).
The lips, gills and digestive gut may react to changes in salinity level by adjusting the density and size of
mucous cells and other epithelial cells. In varying salinities, epithelial mucous cells may adjust to local
(Allen 1961; Gilles 1972; Yiyan et al. 2004; Di et al. 2012)
. Since mucous cells mainly
contain water (Davies and Hawkins 1998), we hypothesized that the size and density of mucous cells in the
lips, gills and digestive gut might reflect acclimation and adaptation to the prevalent water salinity.
Acclimation to salinity in marine mollusks occurs via cellular mechanisms such as reversible changes of protein and
RNA synthesis, alteration of molecular forms of enzymes, and regulation of ionic content and cell volume
(Berger and Kharazova 1997). Various mucous cells are partly responsible for ionic regulation as well as
lubrication of different organs (Crofts 1929; Davies and Hawkins 1998; Luchtel et al. 1997). However, there is
limited information on the effect of acclimation to changes in salinity on mucous cell number and size.
Because of the regulatory activity of mucous cells, alteration of salinity may have effects on the overall
physiology of abalone, as reflected in growth rate. Therefore, in this study we investigated the growth (wet
weight and shell length) of abalone and their mucous cells (characterized by density and size) when reared at
different salinities. The results will be useful for selecting sites for stock enhancement or mariculture and in
designing inland culture systems for H. diversicolor. Abalone raised from the hatchery are released to fishing
grounds to replenish the overfished wild population (Body 1987; Braje et al. 2007) and cultured to increase
abalone products in the market
(Troell et al. 2006, Wu and Zhang 2012)
One-year-old individuals of the abalone H. diversicolor produced from the hatchery facilities of Kagoshima
Mariculture Society in Tarumizu City, Kagoshima, Japan, with total wet weight of 1.67 ± 0.49 g (mean ±
SD) and shell length of 23.26 ± 1.90 mm, were used in this study. In the laboratory, the animals were
maintained in 10-L plastic tanks with static seawater. Each tank was provided with a 7 9 7 9 6 cm3
portable filter-aerator and a shelter made from an inverted length of plastic guttering 15 9 5 cm , with a
central hole 6 cm in diameter. A shelter was necessary to provide a substrate for attachment and to cover the
nocturnal abalone during daytime. Except for the control, salinity in each tank was gradually adjusted to the
desired level within 1 week to acclimatize the test individuals. Dechlorinated tap water was added to decrease
seawater salinity. Dissolved sea salt (Hagata, Japan sea salt) was added to increase salinity. The experiment
began after an acclimation period of 2 weeks. Two replicate tanks for each of five treatments [salinity levels of
20, 25, 31(control), 35 and 40 ppt] were arranged in a completely randomized design. These salinities were
chosen because they span the possible salinity ranges in coastal areas where H. diversicolor lives. The control
was the salinity of the nearby sea where the seawater in the laboratory was pumped.
Ten individuals of H. diversicolor (2 tanks 9 5 salinities) were maintained in each tank for 2 months.
Rehydrated blades of kelp (Laminaria sp., from Kagoshima City local supermarket) were provided ad libitum
and replaced every 3 or 4 days, simultaneous with change of the tank water. Water temperature, dissolved
oxygen (DO) (YSI 85, YSI, USA) and pH (Yokogawa pH82, Japan) of the tank water in each tank were
measured weekly. Values of percent DO saturation were computed using the DOTABLES on-line program
(available at: water.usgs.gov). There was no management of bicarbonate levels during the experiment.
Growth was measured weekly by recording wet weight (WW) (electronic balance, Shimadzu EL-120 W/
AC, Japan) and shell length (SL) (Vernier caliper, Mitotuyo Corp., Japan). The shells were blotted with paper
towels before measuring the wet weight. Repeat weighing after re-immersion and blotting indicated an
accuracy of ± 0.02 g. Specific growth rate was calculated as percentage increase in wet weight per day
relative to original weight. This measure was used rather than absolute growth rate to reduce variability with
size and age. On the other hand, to measure growth rate in shell length we used the absolute increase per day,
because shell length is not affected as much by size or age.
The formulae used were:
Specific growth rate (% d-1) = (ln wetW2 - ln wetW1)/(d2 - d1) 9 100
Growth rate in shell length (mm d-1) = (SL2 - SL1)/(d2 - d1)
where ln is the natural logarithm, wetW1 (g) is the wet weight of H. diversicolor at the start of the
experiment, wetW2 (g) is the wet weight of H. diversicolor at the end of the experiment, SL1 (mm) is the
initial shell length of H. diversicolor, SL2 (mm) is the final shell length of H. diversicolor, d1 is the initial day
of the experimental culture period, and d2 is the final day of the experimental culture period.
At the end of the tank period, two individuals were randomly chosen from each tank for histological
analyses. These individuals were relaxed in 5% ethanol then fixed in 10% seawater–formalin solution for
48–72 h and stored in 70% ethanol. About 1–2 cm of the lips, gills (lamellae) and digestive gut (stomach and
intestine) from each individual were taken and dehydrated in a graded ethanol series (70, 80, 85, 90, 95, and
100%) with two changes at 10–12 h each. This was followed by gradual infiltration and embedding with the
cold resin Glycol Methacrylate (GMA) (Technovit 7100 , HeraeusKulzer, Germany) at ethanol:GMA ratios
of 4:1, 2:1, 1:1, 1:3, 0:1,0:1 for 10–12 h each. Sections of tissues that were 2 lm thin were stained with Alcian
Blue–Periodic Acid–Schiff‘s Reagent (AB–PAS) to test for the presence of mucopolysaccharides and
glycoconjugates. The density (number per 0.01 mm2) and length (in lm) of mucous cells positive to AB–PAS
(blue or purple to pink or magenta) in each organ were compared among the treatments. Using
photomicrographs of ten different areas per individual sample, the number of teardrop-shaped mucous cells was
counted and their corresponding maximum dimension of length was measured. The density and size (average
of 20 cells) of mucous cells were measured using ImageJ (Image Processing and Analysis in Java). The ten
areas per individual were blocked in the statistical analysis. An average density was calculated from 10
observations 9 2 individuals 9 5 salinities = 200 observations (40 per salinity treatment). For size, an
average was calculated from 20 cells 9 2 individuals 9 2 tanks 9 5 salinities = 400 observations (80 per
To determine if the differences in the growth rates of H. diversicolor, and the density and size of mucous
cells positive to AB–PAS among the salinity treatments were significant, single-factor analysis of variance
(ANOVA) followed by pairwise multiple comparisons (Tukey Test) was performed using SIGMASTAT
(Systat Software Inc., California, USA).
The average measured salinity was usually a little higher than the nominal salinity (Table 1). Average DO in
tanks ranged from 6.13 to 6.76 mg L-1, the average DO saturation ranged from 84.96 to 86.48%, and the
average pH values ranged from 7.65 to 7.79. There were no significant differences (ANOVA, P [ 0.05)
between the two tanks in each treatment for salinity, DO or pH. Indoor water temperature during the
experimental period decreased from 24.2 C (October) to 12.4 C (December) as the ambient temperature
decreased with the progress of winter, which was similar to what would be experienced by abalone in their
natural marine habitat. The temperature was the same in all treatments and did not affect the readings of
salinity, DO or pH for the duration of the experiment. Survival rate of abalone was 100% in all treatments for
the duration of the experiment.
There were significant differences in specific growth rate of abalone H. diversicolor among the salinity
treatments (ANOVA, df = 4, P = \0.001). The specific growth rate was highest at 31 and lowest at 4 and
20 ppt (Table 2). Moreover, there were significant differences in SL growth rate of abalone among the salinity
treatments (ANOVA, df = 4, P = \0.001). The highest SL growth rate was obtained at 31 ppt and lowest at
40 and 20 ppt (Table 2).
Density of mucous cells
There were significant differences in the density of AB–PAS-positive mucous cells (Fig. 1) of the lips of
abalone among treatments (ANOVA, df = 4, P = \0.001). The density of mucous cells from the lips of
abalone reared at 40 ppt was significantly higher (P = 0.03) than in those raised at 31 ppt (Table 3). In the
gills, there were significant differences in the density of AB–PAS-positive mucous cells (Fig. 2) of abalone
among treatments (ANOVA, df = 4, P = \0.001). The density of mucous cells from gills of abalone reared at
40 and 35 ppt was significantly higher (P = \0.001) than cell densities of abalone raised at 20, 25 and 31 ppt
(Table 3). In the digestive gut, there were significant differences in density of AB–PAS-positive mucous cells
(Fig. 3) of abalone among treatments (ANOVA, df = 4, P = \0.001). Density of mucous cells from digestive
gut of abalone reared at 25 ppt (19.0 ± 1.9 cells per 0.1 mm2) were significantly higher (P = 0.04) than those
reared at 20 ppt (Table 3).
Sizes of mucous cells
There were significant differences in sizes of AB–PAS-positive mucous cells from the lips of abalone among
treatments (ANOVA, df = 4, P = 0.021). The lengths of the mucous cells of lips of abalone raised at 31 ppt
were significantly greater than those from abalone reared at 40 ppt (Table 4). In the gills, there were
Different letters superscript indicate significant differences (ANOVA, df = 4, P = \0.001, Tukey test), common letters indicate
no significant differences (P = 0.06)
significant differences in sizes of AB–PAS-positive mucous cells among treatments (ANOVA, df = 4,
P = \0.001). The mucous cells of gills of individuals reared at 31 ppt were significantly longer than those
reared at 40 ppt (Table 4). In the digestive gut, there were significant differences in sizes of AB–PAS-positive
mucous cells among treatments (ANOVA, df = 4, P = \0.001). The AB–PAS-positive mucous cells from
abalone raised at 20 ppt were significantly longer than those from abalone raised at 35, 31 and 25 ppt
Salinity in coastal areas where the abalone H. diversicolor lives may drop sharply during heavy rains and large
discharge events from rivers and streams. Alternatively, salinity may rise in tide pools during low tides with
high rates of evaporation. The normal range of salinity in H. diversicolor habitat is 32 ± 3 ppt. The 100%
survival in five different salinity levels tested indicates that the range of 20–40 ppt is tolerated by H.
diversicolor in culture. This is an important finding and supports the occurrence of H. diversicolor in some
estuarine areas where our results show a wider range than the 25–35 ppt marine water range suitable for the
growth of H. diversicolor reported by Chen and Chen (2000). In the earlier farming studies of H. diversicolor
in Taiwan, the reported salinity range is 30–35 ppt (Chen 1984). There is a wide range of salinity tolerance
across the genus Haliotis in general. The salinity tolerance of Haliotis laevigata Leach and Haliotis rubra
Leach is from 25 to 40 ppt and mortality occurs at 2 ppt outside this range (Edwards 2003). Individuals of
Haliotis asinina Linnaeus survive at lower salinity levels, 20.5 ppt without acclimation and down to 12.5 ppt
with gradual acclimation, but die at 10 ppt
(Singhagraiwan et al. 1992)
During the conduct of this experiment, water temperature was decreasing due to the onset of winter
(October–December) in the study site. Temperature was not considered to affect the growth of the abalone H.
diversicolor reared at five different salinities because they would experience such water temperatures in their
Different letters superscript indicate significant differences (ANOVA, df = 4, P = \0.001, Tukey test), common letters indicate
no significant differences (P = 0.06)
natural habitat. Generally, the effect of temperature on growth of various abalone species would depend on
what temperature range they are exposed to in their natural environment. For instance, in California the
abalone Haliotis rufescens Swainson that inhabits deep cool water has been reported to decrease growth at
increasing temperature, whereas the abalone Haliotis fulgens Philippi that inhabits warm water has increased
growth at higher temperature
(Vilchis et al. 2005)
. On the other hand, in Chile the abalones Haliotis discus
hannai Ino and H. rufescens cultured in tank systems have low growth during austral winter and high growth
during the austral summer Mardones et al. (2013).
As indicated by both the growth rates in wet weight and shell length, 31 ppt is the most favorable salinity
for H. diversicolor reared in culture, closely followed by 35 and 25 ppt. This implies that a salinity fluctuation
of ± 4–6 ppt from 31 ppt is still favorable for growth of H. diversicolor in the wild or in a culture facility. Our
results indicate a wider optimum salinity range than that of Chen (1984) and Chen et al. (2000) who reported
that the optimum salinity for the growth of H. diversicolor is from 30 to 35 ppt. In our study, we used
Laminaria as food, whereas Chen et al. (2000) gave formulated diet. Moreover, we have related the effect of
salinity to mucous cells which might have influenced the growth performance of abalone. Since mucous cells
principally contain water (Davies and Hawkins 1998), these cells are responsible for volume regulation of
mucous in the body to better adapt to salinity stress (Drew et al. 2001); hence, the changes in density and size
of mucous cells directly affect the growth of abalone. In another study, individuals of H. diversicolor of a
similar age raised in similar tanks, densities, and temperatures but at slightly higher salinity (32–34 ppt) have
a specific growth rate of 0.44% d-1,
(Alcantara and Noro 2006)
, similar to that of abalone in this study raised
at 35 ppt but lower than those raised at 31 ppt. The optimal range of salinity across marine mollusks having
similar natural habitats generally appears to be similar. For example, the scallop Argopecten purpuratus
Lamarck, the pearl-oyster Pinctada maxima Jameson and the clam Laternula marilina Reeve show the highest
growth between 27 and 30 ppt
(Navarro and Gonzalez 1998; Taylor et al. 2004; Zhuang 2005)
there are often differences among local habitats of other mollusks in general and among abalone in particular.
The higher growth rate exhibited at 40 ppt compared to 20 ppt (for weight) and at 35 ppt over 25 ppt (for shell
length) may suggest that individuals of H. diversicolor can better adapt to higher salinity. On the contrary,
individuals of H. laevigata and H. rubra can adapt better at lower rather than higher salinity within the range
of 25–40 ppt (Edwards 2003).
Although abalone are exclusively marine species, results of previous studies
(i.e. Singhagraiwan et al. 1992;
and this study reveal that their growth performance is enhanced at intermediate salinity. The
control treatment seawater (31 ppt) was sampled near (* 800 m) a river mouth and thus was slightly diluted
by river influx. Several studies have speculated on the physiological advantages of lower salinity on growth.
Greater growth of some mollusks at lower salinities is due to the lower energetic cost of ionic and osmotic
regulation, high food intake, efficient food conversion ratio, high retention of nutrients, low excretion of
metabolites and low oxygen consumption (Ghiretti 1966). On the other hand, at higher salinities abalone
spends more energy in counteracting stress rather than investing in growth, thus resulting in reduced growth
rate (Morash and Alter 2016). It has been implicated that during seasonal salinity fluctuations the chemical
balance between the environment and the body fluids is disrupted and efforts to regain homeostatic balance is
an energetically costly process for abalone (Martello et al. 1998; Morash and Alter 2016). Further, suboptimal
low salinity causes added stress to the abalone Haliotis varia Linnaeus when exposed to toxic substances
(Lasut 1999). As in other marine mollusks, cell volume is regulated during variation in external salinity using
intracellular free amino acids as osmotic solutes (Baginski and Pierce 1975; Amende and Pierce 1980) causing
changes in water content and weight of abalone (Morash and Alter 2016). In our study, the lowest salinity (20
ppt) is the least favorable to the growth of H. diversicolor. This can be explained by their adaptive capacity at
this particular salinity (Javanshir 2013) which affects their feeding behavior, metabolism and growth (Riisgard
et al. 2012). Favorable feeding behavior, metabolism and growth of H. diversicolor aqualitis, a subspecies of
H. diversicolor, all benefit at 25–37 ppt
(Yan et al. 2009)
The lips, gills and digestive gut of abalone are some of the organs that may show pronounced reactions to
different salinity levels. The lip is directly exposed to the environment and contains many mucous and sensory
cells. Changes in the cells of the lip may affect the abalone’s ability to graze and ingest. Salinity directly
influences feeding of mollusks (Broom 1985; Riisgard et al. 2012). According to Chen et al. (2000), H.
diversicolor feeds on artificial diets at salinity range of 20–38 ppt which means that they feed on a wide
salinity range. In another study, the gastropod Lithopoma tectum has increased ingestion of algae after
exposure to reduced-salinity water (Irlandi et al. 1997). In the case of abalone, highest growth of those reared
at 31 ppt could be due to high intake of algal food compared to those at 40 and 20 ppt.
The bipectinate gills are positioned under the shell openings and are thus directly exposed to the salinity of
the environment because the water passes through them. This organ is very sensitive because each filament is
innervated and contains several mucous and ciliated epithelial cells
(Wanichanon et al. 2004)
. In the gills,
mucous cells are important in detecting salinity stress. According to Drew et al. (2001), the abalone Haliotis
rubra Leach are able to regulate the swelling of tissues due to exposure to hyposaline stress. Thus, salinity
stress are able to affect the growth of abalone by diverting energy usage to addressing the stressful condition.
In this study, salinity stress caused by prolonged exposure to hypersaline (40 ppt) culture environment results
in higher density and smaller sizes of mucous cells in externally located lips and gills but not in internally
located digestive gut. The digestive gut is located internally, hence its function is not influenced by salinity.
However, mucous cells in the digestive system may produce hormones such as hydrolases that are active in
osmoregulation, control of food intake and growth regulation (Gilles 1972; Harris et al. 1998; Di et al. 2012).
Implications on stock enhancement and mariculture
Results of this study has application in choosing areas with average salinity of 31 ± 4 ppt for stock
enhancement and open sea culture system of abalone H. diversicolor. To ensure favorable survival and growth
of abalone released or reared in the site, one of the important considerations is the potential seasonal
fluctuation of salinity due to rainy or dry season and other factors that may cause drastic change in water salinity.
For land-based mariculture system of abalone, the findings will be helpful in designing and developing
mariculture facilities and protocols that will minimize salinity stress and other forms of stress to optimize
production and produce high-quality abalone product.
Generally, the growth (wet weight and shell length) of H. diversicolor was greatest at 31 ppt and lowest at 40
and 20 ppt with those at 35 and 25 ppt having average growth performance. Mucous cells positive to AB–PAS
of lips and gills had their highest density and smallest cell size at 40 ppt and lowest density and largest cell
size at 31 ppt. These findings suggest that differences in density and size of mucous cells could be an effect of
osmotic and ionic regulation of H. diversicolor. It appears that mucous cells proliferate at the least favorable
salinity level (40 ppt) compared with the salinity level suitable for growth (31 ppt). In non-optimal salinities, a
high level of mucous cell production may be needed to maintain the ionic concentration of the blood in the
anisosmotic extracellular regulation of some mollusks (Florkin 1966), which may also happen in H.
Acknowledgements Thanks to Kyushu Electric Company for research funds and Kagoshima Mariculture Society in Tarumizu
City, Kagoshima, Japan, for individuals of Haliotis diversicolor. We are also grateful for the inputs of peers that improved the
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