Diet composition and feeding strategy of larvae and juveniles of green riffle goby, Stiphodon elegans in Cimaja Estuary, Indonesia

Green riffle goby, Stiphodon elegans, is an amphidromous goby living in Cimaja River, Sukabumi. Postflexion and juvenile stages of this species migrate from the sea to the river estuary for the recruitment process both in the day and nighttime. The presence of prey strongly influences the success of the recruitment process. The present study, therefore, aims to reveal the diet preferences and feeding strategies of postflexion larvae and juveniles of the green riffle goby during the recruitment process. Postflexion larvae and juveniles of S. elegans were collected day and nighttime using a lift net from December 2020 to May 2021 in the Cimaja River estuary. A total of 13,955 larvae and juveniles were captured. For diet analysis, 455 fish larvae and juveniles were analyzed, consisting of 47 larvae (8.9-11.8 BL) and 408 juveniles (11.9-21.8 BL). The diet composition of postflexion larvae comprised five genera from three different classes of benthic microalgae and zoobenthic. In comparison, fish juveniles’ prey composition comprised 15 genera from 5 different classes of benthic microalgae and zoobenthic. The postflexion larva tends to be more specialized than the juvenile in utilizing prey. There is no significant difference in the diet composition and feeding strategy of postflexion larvae and juveniles during the day and nighttime (p>0.05).


Introduction
Amphidromous comes from two words: amphi, which means two, and dramein, which means movement.Amphidromous is a group of diadromous fish that migrate between fresh and marine waters.Adult fish spawn in freshwater at rock crevices [1].Eggs generally attach to rocks [2], and newly hatched larvae are rheoplanktonic or washed away by river currents into marine waters [3][4][5].The larvae will do their first predation in marine waters to grow and develop to a particular stage where the development of organs for swimming readiness is quite good.Postflexion larvae and juveniles will migrate back to freshwaters through river mouths.Larvae and juveniles of amphidromous fish will undergo different 1260 (2023) 012009 IOP Publishing doi:10.1088/1755-1315/1260/1/012009 2 aquatic environmental conditions during recruitment [6].
One factor that supports the successful recruitment of amphidromous fish into river ecosystems is the availability of prey.The recruitment process requires a large amount of energy [7].The food will be used as a source of material in the catabolism process to produce energy that is used in a movement against the river current [3].
Cimaja estuary is one of the largest estuaries in Palabuhanratu Bay, Sukabumi, West Java Province.The Cimaja estuary is also a potential fishing ground for larvae and juvenile fishes in Palabuhanratu Bay [8,9].The previous study stated that the level of biodiversity of larvae and juveniles of amphidromous fish recruited into the Cimaja estuary waters was slightly higher than in other estuaries of Indonesia [10].One species most commonly found recruiting into the Cimaja estuary is the green riffle goby, Stiphodon elegans [9].The high diversity of amphidromous fish that recruit indicates that the habitat of the Cimaja estuary waters is still in good condition and feasible to support the survival of amphidromous fish [10].
The coastal communities of Sukabumi capture fish larvae and juvenile amphidromous fish as a food resource.The community even catches massive amphidromous larvae and juvenile fish monthly during the dark moon phase [11].The high rate of capture fisheries for amphidromous fish has yet to be supported by a sustainable amphidromous fishery management strategy in the Cimaja estuary.The composition and feeding strategy of amphidromous larvae and juveniles must be uncovered to obtain appropriate management recommendations.Several studies on composition and feeding strategies for larvae and juveniles of Gobiidae fish have been carried out.Diet composition of fish larvae and juveniles of the Gobiidae fish in the seagrass bed of the Pulai Estuary, Johor River, Malaysia [12].Ontogenetic diet shift that occurs in larval and juvenile stages in a tidal mudflat in Tama River Estuary, Japan [13].In Indonesia, a recent study on larvae and juvenile amphidromous fish's diet composition and feeding strategy was conducted in Cisolok and Cimandiri estuaries of Palabuhanratu Bay, Sukabumi [14].Therefore, this study aims to reveal the diet composition and feeding strategy of larvae and juveniles of the green riffle goby recruited into the Cimaja estuary, Sukabumi.

Study area
Postflexion larvae and juveniles were sampled each month during the dark moon phase from December 2020 to May 2021, day and nighttime.A total of two sampling stations were established to represent the river areas still affected by seawater and estuary (Figure 1).

Sample collection
Postflexion larvae and juveniles of the green riffle goby were collected using a rectangular lift net with a mesh size of 0.2-0.48mm [15].The time for collecting fish at each research station was 1.5 hours.This duration is determined by considering the tide time and the primary momentum in the recruitment process of amphidromous fish to the river estuary waters [10].The samples were collected day and night to determine the temporal differences in food variations.

Preservation and identification
Samples of postflexion larvae and juveniles of the green riffle goby were preserved in 10% formalin solution for 3 hours of immersion.The samples were then rinsed with running water and preserved in an 80% ethanol solution [16].This preservation technique ensures that the digestive tract of amphidromous postflexion larvae and juveniles are well preserved.In addition, this preservation technique can also maintain the color pattern of melanophores, which is one of the main characteristics of morphologically identifying fish postflexion larvae and juveniles [17,18].The preserved samples were then brought to the Bio-Macro Laboratory, Department of Aquatic Resources Management, Faculty of Fisheries and Marine Sciences, IPB University for morphological identification.The identification process was done using a stereo microscope (Nikon type 104).Identification was done according to some literature [19][20][21][22].The developmental stages of amphidromous are also observed by [23].Samples of postflexion larvae and juveniles were measured for body length using a micrometer scale microscope.

Prey identification
Observation of prey for postflexion larvae and juveniles of the green riffle goby begins with surgery performed under a stereo microscope.At each sampling time, 30 individuals with the gastrointestinal tract containing food for each stage were examined.The digestive tract contents were identified under a compound binocular microscope, referring to some identification books [24][25][26].

Data analysis
The diet composition was analyzed using the index of preponderance based on the following formula [27].Vi is the percentage volume of the specific prey (%) and Oi is the percentage occurrence of the specific prey (%).
Feeding strategies were obtained by linking a specific percentage of the abundance of food organisms and the percentage frequency of occurrence using charts Costello [28] modified Amundsen (1996) [29] (Figure 2).The resulting plot provides information on the prey's importance and feeding strategy.Frequency occurrence and prey abundance using the formula [30]: where Pi is the prey-specific abundance of prey i; Si is the stomach content composed of prey i, and Sti is the total stomach content in only those fish with prey i in their stomachs.

Results and discussion
A total of 13,955 individuals of postflexion larvae and juveniles of S. elegans were collected during the sampling time.A total of 455 fish, consisting of 47 postflexion larvae and 408 juveniles, were observed for diet analysis.All food types of the green riffle goby are presented in Table 1.The diet of postflexion larvae and juveniles of the green riffle goby consisted of 13 genera from 5 different classes, namely Bacillariophyceae, Chlorophyceae, Cyanophyceae, Cilliata, and Rotifera.The Bacillariophyceae, Chlorophyceae, and Cyanophyceae belong to the phytobenthic group, while Cilliata and Rotifera belong to the zoobenthic groups.Bacillariophyceae is the class that has the highest diversity of prey types.
In another study, [30] also found that the green riffle goby tends to be herbivores, with the primary diet being benthic algae.Most fish from the Gobiidae are known to be geezers by eating benthic organisms attached to bare substrates such as rocks and plant roots [31].The several types of Gobiidae fish in Pabean Bay are generally phytobenthic, with the most dominant prey types coming from the Bacillariophyceae class, such as Gyrosigma sp., Navicula sp., Nitzschia sp., and Surirella sp.Other types of prey were found Chlorophyceae class and Cyanophyceae class.Cyanophyceae class such as Oscillatoria sp.[32].Bacillariophyceae is a phytobenthic group with a wide distribution and can adapt to various aquatic environmental conditions [33].In another study, many organisms from the Bacillariophyceae class resist extreme changes in the aquatic environment and have high reproductive power [34].
All the identified prey types can be found in the juvenile digestive tract of the green riffle goby.However, only five types of food were found in the digestive tract of the postflexion larvae of the green riffle goby.We found that the size range of food types was one of the reasons for this difference.The five types of prey found in the digestive tract of the postflexion larvae of the green riffle goby had smaller dimensions than the others (Table 2).
Postflexion larvae of the green riffle goby have a body length of 8.9-11.8mm, while in the juvenile stage, it has a body length between 11.9-21.8mm.At the larval stage, the fish's mouthwidth is generally smaller when compared to the juvenile stage.Therefore, diet preferences in the larval stage are limited by the size of their mouthwidth [13].The preponderance index value of diet composition of the postflexion larvae and juveniles of S. elegans both during the day and at night in the sixth month of sampling was highest in the type of Tabellaria followed by Surirella.Both species belong to the class Bacillariophyceae (Tables 3 and 4).Benthic algae of these two types are often found attached to rocks.The bedrock in fast-flowing river waters is a suitable growing medium for these two types of benthic algae and other species from the Bacillariophyceae class [35].This makes rocky waters and fast currents the primary habitat preferences of the green riffle goby and other gobiid fishes [36,37].
Our finding shows the high dependence of the green riffle goby's larvae and juveniles on the rocky estuary environment.In the sampling process, we found sand and rock mining activities in the estuary waters of the Cimaja River.River rocks are taken to be used as materials for construction.This activity poses a potential threat to the survival of postflexion larvae and juveniles of the green riffle goby that recruited into the Cimaja River estuary area [11].In other studies, there has been evidence of a decrease in the habitat of amphidromous fish due to mining activity [38].Furthermore, a decrease in habitat as a nursery and foraging area for larvae and juveniles of amphidromous fish of the Plecoglossus altivelis altivelis species in the Takahashi River, Okiyama (Japan) as a result of the placement of artificial buildings [39].The feeding strategy of postflexion larvae of S. elegans is generalist-specialist with the dominant prey, namely Tabellaria and Surirella.Meanwhile, juveniles of S. elegans tend to be generalist.In general, amphidromous fish larvae that recruit to estuaries have a generalist diet strategy, which this diet strategy is a form of larval adaptation that adjusts to food availability and predation ability while in nature [14].Tabellaria and Surirella were found to be quite dominant at several sampling times during the day (Figures 3 and 4).The ontogenetic diet shift in feeding strategy is believed to be related to the fish's ability to consume its food.At the postflexion larval stage, fish generally have limitations in their ability to obtain prey, such as the ability of their digestive organs and the width of their mouth openings.As for the juvenile stage, the ability of the digestive organs has developed better when compared with the postflexion larval stage.Likewise, with the width of the mouth opening.This causes fish in the juvenile stage to be more generalist [37].
In another study, the larvae and juveniles of the Gobiidae fish in the Tama River Estuary, Japan, had an ontogenetic diet shift as ontogenetic morphology occurred.The prey type found in the larval stage is copepods, but in the juvenile stage, a change in the type of prey is found to be polychaetes [13].The amphidromous Eleotridae utilizes Bacillariophyceae and Chlorophyceae in the larval stage in Cimandiri and Cisolok estuary.Meanwhile, zoobenthics, such as rotifers, are quite dominant compared to phytobenthics consumed by juveniles [14].
The development of the body from larvae, juveniles, and then adults was accompanied by a decrease in the threat of death [5].At an earlier stage, the threat of death is higher, one of which is competition for food and the ability of larvae to obtain food.In the larval stage, the feeding niche tends to narrow because it is limited by the ability to move and the mouth opening size [37].Prey-specific abundance (%)  Statistical tests showed that the composition and feeding strategy of postflexion larvae and juveniles of S. elegans were not significantly different temporally during the day and nighttime (p>0.05).Feeding activity of the postflexion larvae and juveniles of S. elegans occurs both in the day and nighttime during the recruitment or transfer process.The recruitment process for postflexion larvae and juvenile fish requires energy to swim against the current.Thus, fish larvae and juveniles need food as a catabolic material to obtain energy [3].The recruitment of fish larvae and juveniles of amphidromous fish into the river ecosystem is carried out by attaching to rocks and, at the same time, consuming phytobenthic and zoobenthic that are also attached to rocks [40].

Conclusion
The diet composition of the postflexion larva of S. elegans comprised five genera from three different classes of benthic microalgae and zoobenthic.In comparison, juveniles' prey composition comprised 15 genera from 5 different classes of benthic microalgae and zoobenthic.The postflexion larva tends to be more specialized than the juvenile in utilizing prey.There is no significant difference in the diet composition and feeding strategy of postflexion larvae and juveniles during the day and nighttime (p>0.05).

Figure 1 .
Figure 1.Sampling location for postflexion larvae and juveniles Stiphodon elegans in Cimaja estuary

Table 3 . 7 Table 4 .
Index of preponderance of the postflexion larvae and juveniles S. elegans, in Cimaja estuary at daytime from December 2020 to May 2021 Index of preponderance of the postflexion larvae and juveniles S. elegans in Cimaja estuary at nighttime from December 2020

Figure 3 .
Figure 3.The feeding strategy of the postflexion larvae S. elegans in the Cimaja estuary at day and nighttime

Table 1 .
Diet composition of postflexion larvae and juveniles the green riffle goby

Table 2 .
Diet of postflexion larvae and juveniles of the green riffle goby