Morphometric Assessment and Condition Factor of the Mangrove Oyster from a Tropical Mangrove Swamp

The size composition, growth pattern and condition factor of One thousand, two hundred and sixty (1260) mangrove oyster - Crassostrea tulipa ( C. tulipa ) from the mangrove swamp, Lagos Lagoon were studied using recommended morphometric methods. The total shell length ranged from 1.5 cm to 18.4 cm, maximum width ranged from 1.5 cm – 10.4 cm and the total weight was between 1.92 g – 132.1 g. The oysters showed a unimodal size distribution. Also, there is strong significant correlation between the shell total length and the total flesh weight suggesting that an increase in length gives a corresponding increase in weight of an individual oyster. The total length-total weight and total weight-flesh weight relationships were positively correlated with calculated “b” of 0.5826 and 0.5887 for combined sexes respectively. Negative allometric growth observed was verified from the b < 3 value. The condition factor of the oysters ranged between 6.5 and 10.5 for both female and male species indicating that the species are in good condition despite reported contaminations of the lagoon water.


Introduction
Awareness of some quantitative aspects of the fin or shell fishes such as length-weight relationship is imperative in studying fish biology and ecology. The investigating and managing of the fisheries species often times entail the utilization of fundamental tools and biometric interactions to convert data collected in the field into appropriate indexes for evaluation purposes (Ecoutin and Albaret, 2003;Turker et al., 2018;Akinjogunla and Moruf, 2019). Growth activity depends on sexual category, stages of development and ecological factors such as salinity, water temperature parameters, food quantity and quality (Akinjogunla & Moruf, 2018). One of the most universally accepted tools in conversion of any fishery data is the length-weight relationships. Length and weight relationship (LWR -W=aLb) is a fundamental indicator for stock assessment (Akinjogunla and Moruf, 2019), the conversion of growth equation in length into a growth equation in weight and is essential for the computation of biomass of a fishery population (Moutopoulos and Stergiou, 2002). The condition factor which shows the degree or state of wellbeing of the fish (fin fish and shell fish) in their habitat is expressed by 'coefficient of condition'. The Co-efficient of condition is a measure of various ecological and biological factors such as gonad stages, degree of fitness and suitability of the environment with regards to the feeding condition (Gomiero and Braga, 2005).
The mangrove oysters of the genus Crassostrea are bivalve molluscs, belonging to Phylum Mollusca, Family Ostreidae, Order Dysonta and Class Lamellibranchiate (Miossec et al., 2009). They are nutritious, commercially obtainable in some parts of Nigeria (Western and Southern parts), possesses exoskeleton (outer skeleton) called shells on both sides of the body, which functions as defensive layers for the animal. They are comparatively a cheap source of animal protein and the shells have been affirmed as good feedstuff in animal feed formulation (Moruf and Akinjogunla, 2018). Fisheries management strategies need to be implemented if there is still continuous collection of bivalves' molluscs from their natural habitats if we do not want them to go into extinction in the nearest future (Kumar et al., 2018). The comprehension of growth pattern of these commercially important molluscs will be needed in developing sustainable aquaculture and fishery management practices.
Various studies have been carried out regarding the mangrove oysters in Nigeria waters but limited information on the species found in the Lagos Lagoon. In Lagos Lagoon, its shores and adjacent creeks are surrounded in undeveloped areas by mangrove swamps with trees. Although human (erecting houses, cutting down mangrove trees) and industrial (construction of jetties, harbors, etc) activities along the shores have distorted the flora in many places, the fauna inhabiting the estuary have remained distinctive of the West Africa mangrove swamps. During the dry season and at low ebbs, the oysters being attached to various substrates (stones, plastics, mangrove branches, roots, etc) are exposed.
In Nigeria, a lot of information exists on the environmental factors affecting Oyster larvae settlement in Lagos Lagoon (Ajana 1978), fishery of the mangrove oyster (Ajana, 1980), cultivation and potential of the mangrove oyster (Afinowi, 1985), fishery and culture potential of the mangrove oyster (Ansa and Bashir, 2007), growth performance of Mangrove Oyster in Badagry Creek, Lagos (Ajani, 2008) and the use of the mangrove oyster (Crassostrea gasar) as a bio-indicator for chemical element contamination (Wangboje et al., 2014). Other research works on shellfishes in Nigerian waters include seasonal variation of heavy metals in selected sea foods from Buguma and Ekerekana Creeks, Niger Delta (Akinrotimi et al., 2015); Effect of size on proximate composition and heavy metal content of the Mangrove Oyster, Crassostrea gasar from the Andoni River, Nigeria (Woke et al., 2016) and Proximate composition and mineral contents of Mangrove Oyster (Crassostrea gasar) from Lagos Lagoon (Akinjogunla et al., 2017). Also documented are reports from Akinjogunla and Lawal-Are, 2020 on the Seasonal assessment of the impacts of heavy metal deposits in Crassostrea gasar (Adanson, 1757) from the mangrove swamp of the Lagos Lagoon, Lagos, Nigeria) and Akinjogunla et al., 2021 on the biochemical profile of the mangrove oyster -Crassostrea gasar (Adanson, 1757) from the Mangrove Swamps, South-West, Nigeria) but no documented literature exist on the morphometric assessment (length-weight relationship, growth pattern, condition factor) of the Mangrove oysters from Lagos Lagoon despite its economic, nutritional importance and future potentials of the species.
The increasing awareness and need of mollusk as food in western and southern part of Nigeria, together with the dearth of information on growth dimensions of mangrove oysters in Nigeria has prompted this study. This study was embarked on aiming at providing valuable information on the growth patterns and the state of wellbeing of the mangrove oysters from the Lagos Lagoon, Lagos State, Nigeria where such study was lacking. Hence, the present contribution aims to balance for this lack of information and gives important inputs into fishery management practices upon which the sustenance, management and development of the resources could be achieved.

Description of Study Area
The Lagos Lagoon lies between Latitude 6 26' -637' N and Longitude 3 23' -4 20' E in the western part of Nigeria, covering a surface area of 208 km (Akinjogunla & Lawal-Are, 2020). Two seasons (dry season between December and May and wet season between June and November) exists in the lagoon which causes fluctuation in physico-chemical parameters (Lawal-Are and Akinjogunla, 2012).
Agala creek where the sampled oysters were collected from lies between latitude N05° 41'. 2° 66' and longitude E07° 09'. 9°01'7. It receives industrial and domestic wastes with fecal matters and oil spills as it is surrounded by residential and industrial layouts with the presence of ships and boats at the jetty.

Sampling and samples collection
Sampling was carried out on monthly basis between July 2019 and February 2020, consisting of wet season months (August -October) and dry season months (November-January). A total of One thousand, two hundred and sixty (1260) C. Plate 1. Crassostrea tulipa (exposed Oyster flesh in its shell) X 10MP Plate 2. Oysters attachment to the roots exposed at low tides X 10MP Plate 3. Crassostrea tulipa with the Vernier caliper for measuring MgX 8.0MP tulipa (Plate 1) were handpicked randomly from the roots of the mangroves (Plate 2) at low tides with the assistance of artisanal fishermen. The collected samples were immediately preserved in troughs filled with ice and transported to the Department of Marine Sciences Post-Graduate laboratory, University of Lagos, Nigeria where they were kept for analysis.

Morphometric Measurements
Four (4) measurable morphometric characters were used in this study. These morphometric characters were measured and recorded in proformas for each oyster. The length and width were measured in centimeter (cm) using a Grip Vernier Caliper of 0.5 cm precision (Plate 3) while the weights (total weight and flesh weight) were measured to the nearest 0.01 in gram (g) using Electronic Compact Scale Atoms -110C.

2.4 Growth Patterns of C. tulipa a) Shell Width/ Length -Width Frequency Distributions
The statistical width / lengthweight relationships were established using the parabolic equation: (1) (Pauly 1983) Where: 'W' and 'L' are the independent and dependent variables of allometric parameters, respectively. W is total weight of the oyster's shell in gram (g); L is length of the oyster's shell in centimetre (cm); a is rate of change of weight with length (intercept); and b is weight at unit length (slope) The equation 1 above and data obtained were transformed into natural logarithms and this gave a linear (straight line) relationship Log W = log a + b (Log L) (2) (Parsons, 1988)

b) Condition Factor (CF):
The condition factor (CF) of the oysters indicates the state of general wellness of the oyster and was estimated from the relation: where CF is Condition Factor; Ls is Length of the oyster (g); and Ws is Weight of the oyster (cm)

Statistical Analysis
Data generated were presented as descriptive statistics using Microsoft Excel 2010 and SPSS software while coefficient of determination (r 2 ) was used to determine quality of linear regression. Lines and scattered graphs were used to depict trends in the distribution and relationships between length, width and weights of Crassostrea tulipa using Minitab 14. The log of lengths and weights were obtained and plotted in order to establish the relationship between them.

Total length -Maximum Width relationship of Crassotrea tulipa
The size of the mangrove oyster from Agala creek ranged from 1.5 cm -18.4 cm total length, maximum width of 1.5cm -10.4 cm and between 1.92 g and 132.1 g body weight. The values of shell morphometric (length and width) of C. tulipa are presented in Table 1 and illustrated in Figures  1 and 2. The total length frequency of C. tulipa polygon (Figure 1) showed that size group 5.5 cm -6.4 cm was more abundant with 20.5% (369) and small size of 1.5 cm -7.4 cm (Table 1) with 66.3% (1203) while the maximum width frequency polygon ( Figure 2) showed that size group 4.5 cm -5.4 cm was most abundant with 31.8% (573) and medium size of 4.5 cm -7.4 cm (Table 1) with 49.6% (894).

Log Total Length / Log Total Weight Distribution of Crassostrea tulipa
The relationships between Log Total length and Log Total weight of C. tulipa in the regression equation is illustrated in Figure 3. C. tulipa exhibited positive allometric growth. Medium correlation value of r 2 = 0.5826 is indication that an increase in the total length of the species gave a corresponding increase in the body weight.

Log Maximum -Width / Log Total Weight Distribution of C. tulipa
The relationship between the maximum width and total weight of C. tulipa is illustated in Figure 4 for the combined sexes. C. tulipa exhibited positive allometric growth in the mangrove swamp. The correlation value of 0.0756 is an indication that an increase in the maximum width of the oysters gave a corresponding increase in the body weight. Male: Log Y = 0.6253 + 0.9205 Log X n = 800, R2 = 0.1016 Female: Log Y = 0.4084 + 1.0825 Log X n = 460, R2 = 0.031 Combined sexes: Log Y = 0.5657 + 0.9671 Log X n = 1260, R2 = 0.0756

Flesh weight / Total weight relationship of Crassostrea tulipa
The relationship between the weight of the soft part (flesh) and total weight of C. tulipa are presented in Figure 5.
Medium correlation value of 0.5887 showed a strong indication that an increase in the body weight of the species gave a corresponding increase in the weight of soft part (flesh).

Condition factor of C. tulipa
The condition factor values (K-values) of males and females C. tulipa combined together (Table 2) were greater than 10 (˃ 10), implying that the C. tulipa were healthy and the K-value of C. tulipa (males) was higher than C. tulipa (females). The K-value of C. tulipa (males, n = 800) was 10.5, while C. tulipa (females, n = 460) with size group of 7.5 -12.4cm had K-value of 6.5 and C. tulipa (females, n = 20) with size group of 12.5-12.4cm had K-value of 8.1.

Discussion
Length Weight Relationship (LWR) is defined as a means to determine or evaluate the weight gain of most fishes (fin, shell and other invertebrates) and tends to be Isometric when the 'b' value is equal to 3.0 while it is allometric when the 'b' is determined to be greater (positive allometry) or lesser (negative allometry) than 3.0 (Bagenal and Tesch, 1978). The Length -Weight Relationship is one of the benchmarks used to generate justifiable biological information. It establishes the mathematical parabola between the two parameters (length and weight), so that unidentified parameter can be readily computed from the known parameter (Ayoade and Ikulala, 2007). The connections between shells length and other proportions (width, height, weight) in bivalves could predict the outline of shell growth or its proportional changes as reported by De -Paula and Silveira (2009).
The Length frequency and width frequency relationship showed that size groups 5.5 -6.4 cm and 4.5 -5.4 cm respectively were more abundant in the creek, thus indicating that small size oysters were predominant. From this study, the results of the Log Total Length -Log Total weight relationship showed the "b"   value of C. tulipa from Agala with a medium correlation value (0.5826) indicating that the creek is moderately suitable for this species. The present results indicated a strong positive correlation between the various shell dimensions and their relation to the shell length, reflecting a general increase in total length with increase in shell width. These results were in conformity with the studies of T. fuscatus from different lagoons in Nigeria (Jamabo et al., 2009 (Port-Harcourt); Moruf and Lawal-Are, 2015 (Lagos Lagoon). The reports from this study and in comparison with related literatures cited above could be attributed to the environment, rations or water characteristics of the area where these populations were collected as cited by Addadi et al., (2006) and Dauphin et al., (2008). Further observations of the total length / total weight, maximum width / total weight and flesh weight / total weight relationships on logarithms showed that growth of the C. tulipa from the Lagos Lagoon was positively allometric with the values of b (regression coefficients) being positive. The correlation coefficients (r) of the mangrove oysters, (0.0756), though low, indicated positive correlation between their Log maximum width and Log total weights. The positive "b" value of 0.5887 obtained for the flesh weight and total (flesh and both shells) weight relationship implies that there is a strong indication that the body weights of the oysters increase with increase in body length. Similar positive allometric growth patterns have been reported by previous studies such as Abowei and Ezekiel (2013) who worked on Chrysichthys nigrodigitatus from Amassoma flood plains. Similar result have been drawn by Fafioye and Oluajo (2005) who worked on five (5) finfish species collected from the Epe Lagoon. Unlike this positive value of "b" reported from this work and other cited references above, negative allometric growth has also been reported by Offem et al., (2009) who researched on forty six (46) important fishes in a tropical flood river; Kumolu-Johnson and Ndimele (2010) who worked on twenty-one (21) fish species from Ologe Lagoon in Lagos State and Kareem et al., (2015) who reported on the data on Chrysichthys nigrodigitatus and Schilbe mystus from the Erelu Lake in Oyo State, Nigeria.
In this study, it was observed that specimens with similar total lengths tend to exhibit the same body weights which are related with the condition factor of the species. The condition factor (CF) which is used to determine the state of the habitat and general well being of the oysters varied according to the numerous sizes of the species collected from Lagos Lagoon. The high mean "K" value of 10.5 obtained from this study showed that the species evaluated were in favorable environmental conditions even though the mangrove swamps are daily loaded with the influx of domestic and industrial effluents from surrounding jetties, shipping companies and residential buildings. From this study also, it was observed that the CF decreased as the oysters increased in size and weight. Etim and Taege (1993) used the condition factor to depict the well-being of Egeria radiata of the Cross River at Itu creek. The mean condition factor for different sizes in this study ranged from 6.5 to 10.4. It was noticed that the values changed with increase in length.
The (K) values were highest in females (7.5 -18.4 cm) of C. tulipa from Agala (14.6) compared to 10.5 reported in males species with size bracket 1.5 -7.4 cm. Similar result to these observations was reported by Lawal et al., (2010). This implied that the female species of the mangrove oysters did enjoy a better environmental conditions and robustness since their CF values were found to be higher than the males. Condition factor (K) has also been strongly correlated with reproductive phase in aquatic organisms in other water bodies (Ugwumba, 1990) and also on the presence and availability of food, foraging behavior of the species and their reliance on reserved or stored food energy (Moruf and Lawal-Are, 2015). It could as well be attributed to food deprivation or their (in) ability to adapt to the highly unpredictable polluted environments (Saliu, 1997). These factors are known to impact the growth data and could lead to high or low  (Kamaruddin et al., 2011;Kareem et al., 2015).

Conclusions
Current study provided information on the morphometric assessment and condition factor of a commercially important mollusc, Crassostrea tulipa in Agala mangrove swamp off the Lagos Lagoon which is the first documented report of its kind. This is expected to serve as an orientation point for future investigations. The condition factor values of the species in this study suggest that the state of the mangrove swamp is beneficial to the mollusc in terms of availability, quality and quantity of food resources. In addendum, they might have developed a tactic to help in adapting to a habitat that receives residues from crude oil refining companies, shipping activities and domestic wastes. Residues deposited into these areas are flushed out at high tides, thereby creating a favorable environment for the species. Further studies with longer research periods need to be carried out to justify existing conclusions. Also, there must be enforcement measures taken to reduce the deposit of toxic waste on the aquatic ecosystem because this mangrove swamp produces a valuable shellfish species for local population.