Rev. Biol. Trop., 46 Sup!. 5: 121-125, 1998

Salinity and temperature tolerances of the green and brown mussels, Perna viridis and Perna perna (Bivalvia: Mytilidae)

M.I. Segnini de Bravo, K.S. Chung & J.E. Pérez Institulo Oceanográfico de Venezuela, Depto. Biología Marina, Universidad de Oriente, Núcleo de Sucre, Cumaná 6101, Venezuela.

(Rec. 25-VII-1998. Rev. 21-II-1998. Acep. 5-V-1998)

Abstract: Since the appearance of the green mussel, Perna viridis, in Ihe Golfo de Paria in 1993, the habitat of the brown mussel, Perna perna, has been altered. P. viridisis driving out the P. perna mussel from its natural beds in La Esmeralda, Guatapanare and El Morro'de Chacopata, Sucre State, and al the end of 1995 it appeared in Isla Margarita, Nueva Esparta State, Venezuela. Por thal reason, a study has been carried out lO determine whether P. viridis from La Esmeralda has a hígher tolerance and adaptability than P. perna, in terrns of salinity and temperature. Low and hígh lelhal temperatures were 6°C and 37.5°C for P. viridis and 3°C and 34.5"C for P. pema. Low and high lelhal salini­ ties were O and 64%0 for P. viridis and 8 and 54%0 [or P. perna, respeclively, indicating that the green mussel has wider Iherrnohaline tolerance Iimits than the brown mussel. This may explain why P. perna has been displaced by P. viridis in less than tbree years.

Key words: Therrnohaline lolerances, habitat alteration, distribution, tropical mussels, P. viridis, P. perna.

Temperature and saIinity are among the EcoIogically tolerance may be defined as a most important environmentaI factors affecting measure of how the change of an externaI fac­ abundance, availability and distribution of tor may affect the organisms and their ability to marine organisms. These factors affect the adapt to such fluctuations. ToIerance has two metabolic rates which may be reflected in the components: a genetically determined capacity activity level of organisms. The overall range to withstand a certain range of parameters and of sea surface temperature found in a particular a phenotypic capacity, named acclimation, to Iocation in the oceans is not high, with the aver­ shift toIerance within this range as a result of age annual differences in the Atlantic and previousIy experienced conditions (Segnini & Pacific Oceans not exceeding 10°C. However, Chung 1991, Segnini et al. 1993). vertical temperature gradients in the Atlantic P. viridis is a mussel from Indo-Pacific and Pacific Oceans are severaI orders of mag­ Ocean where temperature ranges between 26 nitude greater than sea surface temperature gra­ and 28°C (Penchaszadeh & Vélez 1996). P. dients. Temperature has been shown to limit perna is a sub-tropical mussel that colonized geographical as well as vertical distribution of the Caribbean Sea severaI decades ago and coastal and oceanic species. Salinity may established itself in Venezuelan coasts, where it involve changes in embryonic development formed natural beds, probably, favoured by the depending on other environmental factors surge of sub-surface waters which have tem­ (Walsh et al. 1989), peratures up to 21°C in the North-OrientaI coast 122 REVISTA DE BIOLOGIA TROPICAL

of Venezuela (Pukuoka 1971). P. viridis has ting pump in order to provide oxygen, water been studied as indicator of pollution (Chidam­ circulation and uniform temperature. In the baram 1991, Karunasagar & Karusanagar case of CTM (maximum), heaters were used to 1992, Prakash & Rao 1993, Rajagopal et al. obtain three temperature increasing rates (0.2, 1994); as possible aquaculture species (Hanafi 0.5 and 0.8°C per min, using one, two or three et al. 1988, Sreenivasan et al. 1989, Gallardo et heaters, respectively). Por these tests, five al. 1992); distribution (Rylander et al. 1996) replicates of two mussels were used for each and as ecological monitor (Krishnakumar et al. heating rate. In the case of critical thermal 1990, Agard et al. 1992). minimum, temperature was lowered at 0.5°C This study presents the temperature and min using a cold bath obtained by adding ice sallnity tolerances and activity responses of P. cubes, regularly, to the bath. In this case, the viridis and P. perna to fluctuations which are contai-ner used had 1 1 of water. fundamental to predict their behavior, occu­ To determineCTM for higher and lower tem­ rrence or distribution. peratures, the death criterion was the mussel opening the valves and not closing them. A mor­ tality criterion was defined taking into account MATERIALSAND METHODS that stressed organism have their valves open, andwhen a metal baris introducedinto them,the Mussels (P. viridis and P. perna) were co­ mussel closes the val ves quickly. In the case of llected from natural beds at La Esmeralda, lower temperature tolerance limits (LTIL), mus­ Estado Sucre, Venezuela (10°40'35" N, sel was considered dead when the organism did 63°30'50" W). Mussels were brought to the not close the valves in 15 S. laboratory, cleaned of epibiotic organisms and Por determination of low and high lethal acclimatized for 15 days under laboratory con­ salinity, concentration was modified in 2 %0 ditions at 25°C (the same as the location where every 2 days. theywere collected); the pH was between 7 and The organisms were not fed during the 8 and oxygen 90% saturation; 600 mussels (5- whole experimental periodo 110 mm of total length, measured in antero­ posterior direction) of each species were used. In each case, two replications and one control RESULTS were carried out. They were placed in aquaria of 95 litres capacity, with a heater and tempe­ The results obtained from this study show rature control(± 0.1 oC). Theseawater used was that P. viridis has wider thermohaline tolerance filtered to remove particles bigger than l/lm limits than P. perna. None of the green mussels and treated15 minutes in an UV filter. died at temperature below 33.5°C. At this tem­ To determine upper temperature tolerance perature, P. perna had a 20% mortality rate. limits (UTTL), water temperature was When the temperature rose to 34.5°C, the mor­ increased 1°C every day from the acclimation talityrate was 50% for thisspecies. Por P. viridis temperature. Por the lower temperature tole­ it was 20% at 36's°C and 50% at 37 ,S°C. rance limits (LTTL), themussels were placed Critical thermal maximum (CTM), shown in a container (Prigid Unit Inc.) specially in Pig. 1, indicated that at the rate of designed for that purpose and temperature was 0,2°C/minute,the brown mussel (P.perna) died decreased by approximately 1°C per day. at 38.3°C and the green mussel (P. viridis) died Por determination of critical thermal maxi­ at 40.8°C. At the rate of OSC/minute, the mum and minimum (CTM), two organisms brown mussel died at 39.6°C and the green (30-48 mm total length) were taken at random; mussel at 41.8°C. At the rate of 0.8°C/minute, they were placed in a 5 1 container with 2 1 of the brown mussel died at 40's°C and the green seawater inside and an air stone with an aera- mussel at 43.1 oC (Table 1). SEGNINI DE BRAVO et al.: Salinity and temperature tolerances of Perna viridis and Perna perna 123

TABLE 1 TABLE 2 High and low lethal temperature and salinity limits Critical thennal maximum (CTMax) and minimum (CTMin) Jor Pema pema and Pema viridis at different Category Pernaperna Perna viridis heating and cooling rates UTIL(°C) 345 375 LTTL("C) 3.0 6.0 Species CTMax ("C/mín) HLS(%O) 54 64 Q2 05 Q8 LLS(%O) 8 o Perna perna 38.3±O.26 39.6±0.99 40.5±O.54 Perna viridis 4O.8±O.34 41.8±0.61 43.l ±0.55 UTTL: Upper temperature tolerance limíts

CTMin (05 °C/mín) LTTL: Lower temperature tolerance limíts Perna perna 2.4±O.08 *** HLS: High lethal salinity Perna viridis 5.4±O.l2 *** LLS: Lower lethal salinity

*** (p< 0.01)

RATE

virldis 54 62 64 68 SALlNITV CONCENTRA TION (% )

• Fig. 2. Salinity tolerance of Perna perna and Pema viridis .

HEATING RAYE ('C/min) DISCUSSION Fig. 1. Critical thermal rnaximum at tmee heating rates. The critical thermal maximum (CTM) is a method used to provide information about to­ It is useful to notice that when the critical lerance of the organisms as a answer to envi­ point was reached, the studied organisms ronmental media in an ecological and physio­ extended its foot, to move in different direc­ logical sense. Critical thermal maximum (mi­ tions until the mussel lay in the bottom of the nimum) determination is related to a fast and container. Then they drew in its foot, and after controlled heating or cooling of an organism a few seconds they opened the valves and died. from its acclimation or environmental tempera­ For lower temperatures the foot was stuck to ture until the final point where the organism the bottom of the container but when the mus­ die. sel was touched, it drew in the foot from the For the three heating rates, the lower CTM outside very slowly. value was obtained at the slow heating rate For lowering temperature, P. viridis had a (O. 2°C/minute) and the higher value at the 10% mortality rate at7°C and 100% at 6°C. On higher rateo This indicates that external water the otherhand, P. pernahad a 2% mortality rate temperature increases more quickly than inter­ at 4°C and total death (100%) at 3°C. nal body temperature not allowing an acclima­ The salinity effectis shown in Fig. 2. When tion at the lowest heating rate used. These the experimentation for lowering salinity was results do not agree with the definitiongiven by carried out P. perna showed a 15% mortality at Hutchison (1961) who suggested that heating 12%0 and 50% at 8%0.For P. viridis 2% mor­ rate should be in a magnitude that body tem­ tality was found at 40/00 salinity. At OOfoo we perature could be very close to the water tem­ observed 4% mortality. They were left in this perature where the organisms are placed. condition and after 11 days the bioassay was Chung & Acuña (1981) reportedsimilar r �sults terminated without any mortality (Table 2). for P. perna. 124 REVISTA DE BIOLOGIA TROPICAL

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