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SOME OBSERVATIONS ON THE BIOLOGY OF THE FRESHWATER PRAWN Macrobrachium lanceifrons (DANA), WITH NOTES ON THE FISHERY

by

S.B. RASALAN, M.N. DELMENDO, and T.G. REYES
Philippine Fisheries Commission
Manila, Philippines

Abstract

The paper reports some observations on the development of Macrobrachium lanceifrons (Dana), spawned in aquaria. Rearing techniques developed for this species should also be applicable to larger species of freshwater prawn.

Notes on the shrimp fishery in Laguna de Bay are also presented.

OBSERVATIONS SUR LA BIOLOGIE DE LA CREVETTE D'EAU DOUCE Macrobrachium lanceifrons (DANA), ET NOTES SUR LA PECHERIE DE CETTE ESPECE

Résumé

Les auteurs rendent compte d'observations faites sur le developpement de Macrobrachium lanceifrons (Dana) nés en aquarium. Les techniques d'élevage mises au point pour cette espèce devraient pouvoir s'appliquer également à d'autres crevettes d'eau douce de plus grande taille.

La communication renferme aussi des notes sur la pêcherie de crevettes de Laguna de Bay.

OBSERVACIONES SOBRE LA BIOLOGIA DEL LANGOSTINO DE AGUA DULCE Macrobrachium lanceifrons (DANA), Y NOTAS SOBRE LA PESQUERIA

Extracto

En este trabajo se presentan algunas observaciones sobre el desarrollo de Macrobrachium lanceifrons (Dana), cultivado en acuarios. Les técnicas de cria empleadas para esta especie pueden aplicarse también a otras especies mayores de langostinos de agua dulce.

Se dan igualmente notas sobre la pesca del camarón en la Laguna de Bay.

1 INTRODUCTION

The freshwater prawn Macrobrachium lanceifrons (Dana), locally called “hipontagunton”, is one of the important aquatic resources of Laguna de Bay, Philippines. Large specimens are especially esteemed as food by the people in the area, and it is a welcome substitute when fish becomes scarce in the market. Not only does it serve as human food but it is also an important duck food, supplementing the snails which are the major food of the ducks. This prawn is rather small compared with commercial marine species and with Macrobrachium rosenbergii (de Man), the giant freshwater prawn.

In Laguna de Bay, three families of prawns occur, namely, the Palaemonidae, represented by M. lanceifrons, the Atyidae, represented by Caridina gracilirostris (de Man), and the Penaeidae, represented by Metapenaeus affinis (H. Milne Edwards).

Of these species, M. lanceifrons is the most common, C. gracilirostris is also common but, due to their very small size, these prawns are only collected when they occur in swarms at certain periods of the year. M. affinis is rare in the lake.

Considering the importance of M. lanceifrons in the economy of the Laguna de Bay area, it is worthwhile to know something about its biology and its contribution to the economic welfare of the lakeshore communities. Information obtained on the biology of the species will provide data for management purposes.

The experimental rearing of this species was also a trial run of the techniques of rearing freshwater prawns in tanks or aquaria, with the hope of applying the same to bigger species of Macrobrachium found in freshwater areas in the country.

2 METHODS AND PROCEDURES

Specimens for identification were collected in Laguna de Bay. The work of Cowles (1914), Henderson and Matthai (1910), and Holthuis (1950), were used as reference in the identification of the species.

Live specimens of mature male and female M. lanceifrons, were placed in aquaria for biological observations. Spawning, hatching and development of the larvae were observed in the laboratory.

Production estimates were based on prawn catch data collected from motorized push nets, manual push nets and prawn brush shelters in Laguna de Bay. The number of these gears that operate around the lake was taken from the census report of the evaluation team of the Limnology Project. Weight and sex ratio were determined from the samples collected. Species composition of the prawn catches sampled were also determined.

3 DESCRIPTION OF ADULT

The taxonomy of Macrobrachium lanceifrons (Dana) was studied by Cowles (1914), who, like the majority of earlier workers, used the name Palaemon for this genus. The help of Mr. Agustin Umali, Ichthyologist, of the National Museum, was sought in the identification of the species.

The rostrum is laterally flattened, divided into an upper (wider) and lower (narrower) region by a ridge. It bears 10 to 13 dorsal teeth, covering the entire length, and 3 to 5 ventral teeth. The tip of the rostrum is slightly curved upwards in full grown individuals but straight in the young. Antennal spines are present on the carapace, which is always smooth and shiny. Each antennule bears two flagella (upper and lower); the upper consists of two rami fused in the basal part; the lower pair is single. The first and second pairs of legs are chelate; the second is usually much longer and more robust than the first. The carpus of the first pair of legs extends beyond the antennal scale, with stiff hairs sparsely distributed. The second pair of walking legs or chelipeds are equal in length in young specimens but unequal in fully grown ones; a velvety coat of hairs is present in fully grown males on the mobile finger, except on its tip. The third, fourth and fifth pairs of walking legs are provided with stiff hairs in rows.

The second abdominal somite overlaps the first and third somites. The telson has four apical spines, of which the outer pair are shorter than the inner. Four setae are present on the mid-apical margin, between the inner pair of spines. Two pairs of spines are situated on the dorsal margin of the telson.

Fully grown males can best be distinguished from fully grown females by the length and shape of the second leg or cheliped. In the male, this is longer and is provided with felted hairs on the mobile finger, (Fig. 1).

Fig. 1 Adult Macrobrachium lanceifrons (Dana);
(Female shown on top and the male below).

Adults caught in Laguna de Bay range in length from 26 to 61 mm (from tip of rostrum to tip of telson). The average length is 43.0 mm and the average weight 0.45 g. There are more females than males in all length-groups below 50 mm but all the largest specimens are males. Details of the length-frequency distribution and of variations in the sex-ratio with respect to length are given in Table I.

TABLE I

Length-frequency distribution for sexes combined and separately (with percentages of total sample) and sex-ratio of M. lanceifrons from Laguna de Bay

4 FECUNDITY

Females of M. lanceifrons start to produce eggs when about 30 mm in length. The eggs are oval in shape and average 0.66 by 0.54 mm. The specimens examined with eggs ranged from 30 to 55 mm. The average number of eggs per small female is about 179 while the number of eggs produced by large females is 781 (Table II).

TABLE II

Fecundity of Macrobrachium lanceifrons found in Laguna de Bay

Table II shows that females with eggs occur frequently in the 34 to 37 mm size group, but as they increase in size, the number of females with eggs decreases. Additional data are being collected to confirm this observation.

The data obtained indicated that females of M. lanceifrons become sexually mature at a length of 30 mm, although an unpublished report by Villamater mentions mature prawns of 20 to 35 mm. In the present paper, only females with eggs on their pleopods were considered mature.

5 LARVAL DEVELOPMENT

Adult prawns were successfully kept in aquaria for a long period without aeration. Specimens collected from the lake were sexually ripe females, some with eggs already deposited on their pleopods.

Spawning was satisfactorily achieved by selecting ripe females and pairing them with males at the ratio 1:1. A sexually ripe female can easily be determined by the presence of a green coloured mass that could be seen through the dorsal portion of the carapace, and brown-red chromatophores on the external wall of the ovary. It was found that prior to deposition of eggs on the pleopods, molting took place. After six days, eggs were already attached to the pleopods. Mating was, however, not closely observed. The eggs were hatched after 22 days of incubation at room temperature of 25° to 28°C. Ling and Merican (1961) observed that in the case of M. rosenbergii eggs hatched after about 19 days of incubation. The eggs were green in colour at the early stage of development, becoming orange when about to hatch.

Newly hatched larvae possess the following characteristics; antennal scale with 4 distinct segments at tip and with setae on anterior edge; rostrum very small and slender, curved downwards; eyes held close to head; three pairs of maxillipeds with exopods bearing four setae each; telson broad and triangular bearing 14 plumose spines; the innermost pair of spines is very small (Fig. 2).

Fig. 2 Development of the larval telson of M. lanceifrons.

The triangular form of the telson remained unchanged up to the 6th day (Fig. 2A). By the 10th day development of the uropods was completed and the telson was distinctly articulared with the last abdominal somite; it was rather narrower than previously and bore 8 pairs of spines, the innermost pair being very small (Fig. 2B). All five pairs of walking legs were well developed at this period. Larvae from the 11th to the 15th day after hatching showed little change. The uropods became larger and the telson narrowed; the terminal spines changed in size but not in number and a pair of lateral spines appeared (Fig. 2C).

Most of the young prawns that had successfully hatched died after 15 days. The cause of such mortality was not known. However, it might have been due to lack of food or to the chemical composition of the water. Both are vital in the development of the larvae. Larvae reared in lake water survived for a longer period. According to Ling and Merican (1961), young prawns of this kind need saline water at a certain stage of development.

Larvae when 27 days old had rudimentary pleopods. By the 31st day the fifth pair of walking legs was longer than the first four and the inner lateral portion of the dactyl was armed with 14 long straight spines. Legs 1 to 4 each bore 8 long terminal spines.

At the age of 45 days, 5 pairs of walking legs and pleopods were completely developed. Young prawns at 58 days had 8 dorsal and 3 ventral rostral teeth. Body length was 11 mm. At this stage development of the larvae was complete.

The young prawns are predominantly plankton consumers. They start to feed on fish flesh and vegetable matter at the age of 70 days. Prior to this period the young did not feed on such kinds of food.

Young prawns that survived were kept in the laboratory up to about 3 months. They then measured 16.2 mm long. From 87 days, molting was observed to be regular, with an interval of 10 to 11 days. At this stage they remained mostly at the bottom.

The rate of growth was observed to be rather rapid. This observation, however, needs further investigation. The fast rate of growth here may be due to sparse population in the aquaria, and inadequate samples collected.

It is possible that M. lanceifrons spawn all the year round, depending upon the availability of food. This is deduced from the fact that after about one month from spawning the same female deposited another batch of eggs on its pleopods. This female was intensively fed with chopped fish while the prawns in the other aquaria which did not spawn were fed irregularly. Prawns collected from the lake carried eggs most of the year. It is possible that they have peak periods of spawning.

The aquaria where the larvae were hatched were enclosed with lawanit boards with black paint to eliminate bright light and the distrubances created in the laboratory. The larvae were observed to be disturbed by mechanical movements. All newly hatched larvae were observed to be easily attracted to light and actively swam in groups near the walls of the aquarium. Their manner of locomotion is vertical with the head pointing downward. This movement gradually changed as they grew older.

6 THE PRAWN FISHERY

There are three major gears used in harvesting prawns in Laguna de Bay, namely, the motorized push net, the manual push net and the prawn brush shelter.

The motorized push net (Fig. 3) produces the bulk of the prawn catches in the lake. Sub-samples from catches consisted of 91.8 percent prawns, 2.5 percent fish and 5.6 percent snails by number; the corresponding weights are indicated in Table III.

Fig. 3 Motorized push net outfit.

Fig. 4 Manual push net

TABLE III

Composition of sub-sample of catch taken by motorized push net

TABLE IV

Composition of sub-sample of catch taken by manual push net

TABLE V

Composition of sub-sample of catch taken by brush shelter

The manual push nets (Fig. 4) are operated in shallow areas of the lake, mostly near the shore. This gear catches 90% prawns by number or 87.9% by weight (Table IV).

On the other hand, the brush shelter catches 85.4% prawns by number or 84.5% by weight (Table V). Caridinia gracilirostris is caught in small quantities by all three methods.

The figures presented on catch composition merely indicate selectivity of gear in favour of the prawns. There is no difference in size composition of the prawns caught.

The prawn fishery of Laguna de Bay contributes considerably to the duck farming industry which is a flourishing endeavour around the lake. If not fed to the ducks the prawns are dried and transported to other markets in the provinces.

It has been reported earlier (Rabanal, Acosta and Delmendo, 1964) that the withdrawal of prawns from the lake is about 1 ton per hectare per year. Recent observations reveal that this quantity is decreasing from year to year. An ad hoc survey made in 1968 indicated a reduction in shrimp production. A full report on this survey is being prepared.

7 REFERENCES

Cowles, R.P., 1914 Palaemonidae of the Philippine Islands. Philipp.J.Sci. 9(4): 371–9

Delmendo, M.N., 1967 An evaluation of the fishery resources of Laguna de Bay. Occ.Pap.Indo-Pacif.Fish.Coun., 67(5): 16 p.

Henderson, J.R. and G. Matthai, 1910 On certain species of Palaemon from South India. Rec.Indian.Mus., 5(4):277–305

Holthuis, L.B., 1950 The Palaemonidae. I. Subfamily Palaemoninae. The Decapoda of the Siboga Expedition. Part X. Siboga Exped., 39a9:267 p.

Ling, S.W. and A.B.O. Merican, 1961 Notes on the life and habits of the adults and larval stages of Macrobrachium rosenbergii (De Man). Proc.Indo-Pacif.Fish.Coun., 9(2):55–60

Rabanal, H.R., P.A. Acosta and M.N. Delmendo, 1964 Limnological survey of Laguna de Bay. A pilot study on aquatic productivity. IPFC/C64/TECH 46