Length – Weight Relationship of Bivalves and Gastropods from Mangrove Forest of Brgy. Nabago, Surigao City, Philippines

Bivalves and gastropods are among the most important food resource in coastal communities. The destructive ways of harvesting these mangrove associate fauna may cause a threat to the forest and may alter the physiological condition of these organisms. The length-weight relationship of two bivalves (Anodontia edentula and Saccostrea cuccullata) and 11 gastropods (Littoraria angulifera, Littoraria melanostoma, Littoraria scabra, Chicoreus capunicus, Tenguella musiva, Nerita exuvia, Nerita lineata, Cerithidea decollate, Terebralia palustris, and Terebralia sulcata) species collected from the mangrove forest of Barangay Nabago, Surigao City was assessed in this study. A. edentula exhibits positive allometry (b=3.7002) while S. cuccullata shows negative allometry (b=2.6145). The 11 gastropod species demonstrate a negative allometric pattern (b<3). The negative allometric pattern of these species indicates that the length increased faster than the weight, which could be attributed to the different environmental factors in the area. The data obtained in this study is essential for the monitoring of these organisms and management of the entire mangrove ecosystem in the area.


INTRODUCTION
Mangroves play potent roles in the lifecycle of various aquatic organisms.They serve as spawning, nursery, and feeding ground to many organisms including mollusks (particularly the bivalves and gastropods) which are considered the most ubiquitous macrofauna in the mangrove ecosystem (Dewiyanti & Karina, 2012).Their diversity and abundance are biological indicators of the dynamic condition in the mangal ecosystem (Macintosh et al., 2002) and for in-depth climate change adaptation and mitigation studies (Dangan-Galon & Dolorosa, 2014).
Mangroves are also significant for livelihood and for fisheries yet often neglected in research and management.Studies have shown that uncontrolled harvesting and habitat destruction threaten the molluscan population (Gomez & Mingoa-Licuanan, 2006;Jontila, Gonzales, & Dolorosa et al., 2014).Studies on length --weight relationship (LWR) in fishery management are often conducted.By studying LWR, taxonomic differences and events in the life of an aquatic organism are determined.Any deviation from the relationship indicates variation in the ecology of the habitat or physiology of the animal, or both.Locals collect the bivalves and gastropods from the mangrove forest of barangay Nabago as essential source of food and livelihood (Sajol-Degamon & Fernandez-Gamalinda, 2021).Collection of some common gastropods and bivalves are being handpicked, and some are being dug from the muddy soil destroying mangrove stands as they were being cut to ease the extraction process.Persistent cutting of mangrove trees pauses a threat to the existence of mangrove-associated fauna in the said locality.Hence, a study on the length-weight relationship of these gastropods and bivalves are conducted to determine the physiological status of the resources as well as the ecological condition of the mangrove forest in the area, and to provide the basis for policy formulation in the conservation and sustainability measures of the molluscan and mangal resources.

Study Site
Three stations were identified in the mangrove forest of Barangay Nabago, Surigao City, Philippines.Station 1 is surrounded by homes and a boat/vessel port, making it more susceptible to anthropogenic activities.More mariculture activities take place at Station 2. Several fish cages are observed in the area.Station 3 is situated away from residential areas and mariculture operations.Most fishermen gather sea cucumbers, sea urchins, sea grapes, and other marine species in the area.
A three (3) 100-m transect lines were established in each station where five 10x10 m plots with 10 m intervals were laid (Figure 1).A total of 0.45 hectares of mangrove forest served as the sampling site for collecting gastropods and bivalves subjected to length-weight relationship study.

Data Collection
Gastropods and bivalves present in the sampling stations were collected through hand picking.Bivalves firmly attached to the roots and stems of mangroves were taken using a scalpel and knife.All collected gastropods and bivalves were placed in plastic containers and brought to the laboratory for identification and measurement.Taxonomic key guides were used to identify the species.Morphometric measurement was done after all the collected samples were cleaned.Vernier calipers at 0.05 mm precision were used to measure the length, while a digital weighing scale (KERN EW) with 0.01 g sensitivity was used to measure the weight of the dried gastropods and bivalve specimens.

Data Analysis
The length-weight relationships of gastropods and bivalves were determined using the power equation: W = aL b , where W represents the total weight; a, is the intercept, indicating the initial growth; L refers to length; and b represents the relative growth rates of the variables and provides information on growth (Le Cren, 1951;Froese, 2006).The growth pattern is determined through b-values.When b is equal to 3, growth is isometric, i.e., the increase in length follows an increase in weight.When b significantly differs from 3, growth is allometric i.e. increase in length may result to a decrease in weight or vice versa.A negative allometric growth pattern is exhibited if b < 3, while a positive allometric if b > 3. Descriptive statistics and analyses of relationships were done using the statistical package of Microsoft Excel 2016 version.

Length -weight relationship of bivalves
The parameters of the length-weight relationship of bivalves (Table 1) and gastropods (Table 2) from the mangrove forest of Nabago are shown in a tabular presentation.Ninety-eight (98) bivalve individuals belonging to two families, Lucinidae and Ostreidae, were collected from the sampling stations.The total length and weight of Anodontia edentula mean 4.1220 (±0.9721) cm and 19.0100 (±19.3887)g, respectively.Saccostrea cuccullata, on the other hand, has a mean total length of 4.4548 (± 4.9066) cm and a mean total weight of 3.3125 (±0.9918) g.
The two bivalve species under separate families exhibited an allometric growth pattern.With the power equation, W=0.0144L 3.7002 and r 2 = 0.7499, A. edentula demonstrates a positive allometric growth with b-value = 3.7002 while S. cuccullata, at W=0.1513L 2.6145 and r 2 =0.9106, presents a negative allometric growth pattern having the b-value of 2.6145.This difference may be attributed to the type of bivalve they belong to and the type of substrate they live.A. edentula are infaunal burrowing bivalves residing in muddy substrates of the mangrove forest while S. cuccullata are epifaunal bivalves firmly anchored and crystallized on the roots and trunks of mangroves.
The positive allometric pattern (b > 3) considering the total length and total weight of A. edentula conforms to the findings in the study of Degamon andGamalinda (2021) Petteta et al. (2019), Perez and Santelli (2018), Yahya et al., (2018), Turra et al., (2018), Elvira and Jumawan (2017), Ramesha and Sophia (2015), Miley et al., (2012) and on the study of Park and Oh (2002).According to Perez and Santelli (2018), this growth pattern is typical to infaunal burrowing bivalves.This means that the weight of A. edentula increases faster than its shell length.
The negative allometric growth pattern (b < 3) of S. cuccullata indicates that its length increases faster than its weight.This result conforms to the study of Aban et al., (2017) on the length -weight of Perna veridis or Asian green mussel, and epifaunal bivalve from Bolinao Bay, Pangasinan, Philippines, and in the study Sundaram et al., (2011) where the b-values obtained by these authors varied from 2.4175 to 2.8616.It has to be noted that whatever growth patterns these bivalves have, there are several factors influencing them.Their growth and morphometry may be affected by both environmental factors such as current, depth, temperature, wave exposure, and sediment and biological factors like predation and burrowing abilities (Gaspar et al., 2002;Gimin et al., 2004;Babei et al., 2010 andTurra et al., 2018).

Length-weight relationship of gastropods
Table 2 presents the length-weight relationship of gastropods found in the mangrove forest of Barangay Nabago.A total of 866 individuals belonging to four (4) families were collected from the sampling sites.Both Littorinidae and Potamididae families have three species each namely Littoraria angulifera, Littoraria melanostoma, Littoraria scabra, Cerithidea decollata, Terebralia palustris and Terebralia sulcata, respectively.There are also two species under each family of Muricidae and Neritidae.These are Chicoreus capunicus and Tenguella musiva for Muricidae family and Nerita exuvia and Nerita lineata for family Neritidae.Among the 11 gastropods species, N. exuvia, N. lineata, T. palustris and T. sulcata are among the most numbered individuals.They are commonly found on the lower trunks and exposed roots of mangrove stands.
It can be gleaned from Table 2 that all gastropod species exhibit negative allometric growth patterns with b < 3.This indicates that gastropods' shell length increases faster than their weight.According to Zuschin and Stanton (2001), marine gastropods need strong shells strength to have a high capacity to live in unfavorable environmental conditions and as an adaptation to environmental stresses.The energy shell growth allocation is higher than the soft organ that shifts growth into a negative allometric pattern (Mendoza et al., 2019).As observed, cutting of mangrove stands and overharvesting of shellfishes are still practiced despite restrictions from the government agencies and local government units.

Mangrove conditions, diversity of bivalves and gastropods and LWR relationships
The mangrove environment in Stations 1 and 2 were considered to be disturbed because of its anthropogenic activities such as the nearby households and mariculture activities, respectively.Hence, Stations 1 and 2 showed lower diversity index of macroinvertebrates than Station 1 but do not directly affect the positive allometry of A. edentula since these bivalve species are infaunal burrowing (Gaspar et al., 2002;Gimin et al., 2004;Babei et al., 2010 andTurra et al., 2018) and can be collected particularly in disturbed mangrove forest with muddy substrates.The highest diversity index of macroinvertebrates in Station 3 may be attributed to its location.Among the three stations, it was situated away from maricultures and other anthropogenic activities, thus the mangrove forest was considered not disturbed.Consequently, the S. cuccullata displayed negative allometry which can be attributed to two factors.The first factor of negative allometry both in Stations 1 and 2 was due to the disturbed mangrove forests.As S. cuccullata is considered an epifaunal, its allometric growth pattern is directly affected in a disturbed mangrove environment.Moreover, the second factor affecting allometry is the wave exposure and temperature (Gaspar et al., 2002;Gimin et al., 2004;Babei et al., 2010 andTurra et al., 2018).Though Station 3 was considered a non-disturbed mangrove forest, its location is on the seaward.Hence, epifaunal bivalves may have direct wave exposure and intense heat thereby affecting their growth pattern.
It is worth to note that A. edentula were not collected in Station 3 as it has a sandy substrate and considered not disturbed mangrove forest.They can only be found and thrived in muddy substrates which were observed in Stations 1 and 2, respectively.Meanwhile, A. edentula achieved positive allometry despite inhabiting disturbed mangrove forests in Stations 1 and 2. This is due to their capacity to burrow deeply in muddy soil (Gaspar et al., 2002;Gimin et al., 2004;Babei et al., 2010;and Turra et al., 2018), which may contribute to their positive allometric growth pattern.

CONCLUSION AND RECOMMENDATION
The length-weight relationships of bivalves and gastropods from the mangrove forest of Barangay Nabago showed an allometric growth pattern.For bivalves, A. edentula exhibits positive allometry (b>3) while S. cuccullata shows negative allometry (b<3).For gastropods, all 11 species demonstrate a negative allometric pattern (b<3).These differences in the allometric growth pattern may be attributed to the environmental and biological factors currently influencing the sampling site.The results of this study may provide baseline information in the conservation and sustainable management of the mollusk resources in the area.Further studies may be conducted on reproductive growth and resource assessment for possible aquaculture cultivation and production.

Figure 1 .
Figure 1.Sampling Sites of bivalves and gastropods from the mangrove forest of Barangay Nabago

Table 2 .
Parameters of the length -weight relationship of gastropods from the mangrove forest of Barangay Nabago, Surigao City

Table 1 .
Parameters of the length -weight relationship of bivalves from the mangrove forest of Barangay Nabago, Surigao City