Ecological observations on two mysis species, Taphromysis Louisianae Banner and T. Bowmani Bacescu from the upper reaches Of Davis Bayou, Jackson County, Mississippi.
Mysids, (Crustacea,Mysidacea) are important trophic commandants of coastal and estuarine communities in North America and throughout the world (Tattersal, 1951, Beck, 1977, Stuck et al, 1979, Perice, 1982, and Takahashi, et al, 2004.). Shallow water bays and estuaries serve as major habitats; mysids use these areas as nurseries. Although extensive studies on mysids exist, but due to difficulty in quantitative sampling because of demersal habitats and lack of uniform distribution, gaps exist on information regarding mysids (Daly et al, 1986). The current study provides some information on seasonal abundance, population composition, structure and life histories of two species of mysids, T. louisianae and T bowmani in small tidal bayou system, which opens into Biloxi Bay near the city of Ocean Springs, MS. The sampling stations where sufficient mysid collections were made are characterized by sand/silt or muddy bottoms with abundant plant detritus.
The study was carried out in the upper beaches of Davis Bayou, Southern Mississippi, Jackson County, located between lower site (30[degrees]24'91"N, 88[degrees]44'17"W and upper site (30[degrees]24'54"N, 88[degrees]45'72"W). The parts of the upper reaches of the Davis Bayo under study are extended to 1.5 km with average width of 15-20 m. Five collecting stations were selected along the study area. Davis Bayou is considered to be a mesohaline to hypohaline system (VanderKooy et al, 2000). During the study salinities at sampling sites were markedly low (0 to 1%), apparently the area under study is strictly Hypohaline. Tidal submerged vegetation at lower two stations comprised Ruppia maritime and Vallisnerja americana while at the upper three stations Junkus rumariana and Spartina aleterniflora were dominant.
MATERIALS AND METHODS
Samples were taken monthly, initially using sledge net (35 x 12 cm mouth size, 1 mm mesh openings). The sledge net was towed of 200 meters, at a depth of 1-4 m along a shore line in the midstream zone using a small boat at speed of 1-2 knots. These samples contained only few or no mysids, possibly due to low contained drastically low number of mysids; as a result a different collection method was employed using hand held kicknet (50 x 25 cm mouth area, and 1 mm mesh openings). The kicknet was towed 10-15 m along the shoreline.. The samples were preserved immediately using 5% buffered formalin for biological analyses. The surface salinity and temperatue were taken with a refractameter and centergrade thermometer. The mysid specimens were separated into different taxonomic groups and each group was sexed. Standard length (distance between base of eyestalks and mid-posterior margin of sixth abdominal segment was used as measure of body length using stereoscope microscope. Based on the development of the oostegites (brood plates) and male copulatory stylet, females and males, respectively, and other criteria were categorized as juvenile below 3.0 mm and between 3 and 4.9 mm as immature and o above 4.9 mm as mature males measured between 5.0 to 8.5 mm and mature females between 5.00-10.00 mm, Taphromysis bowmani mature males measured between 5.0-8.5 mm and mature females between 5.0-9.0mm. The developmental stages of the embryos were observed and categorized by stage. In stage I, the embryo is in the egg membrane, but has developed rudimentary antennae. During Stage II, the embryo hatches, with further development of the antennae, appendages, and later development of recognizable pigmented eyes. The Stage III goes through further growth and has developed stalked eyes before being released from the marsupium
Temperature And Salinity
Monthly surface water temperature peaked in June 2009 and showed a minor peak in October, 2009 (Figure 1). Surface temperature varied from lowest, 16[degrees]C in January, 2009 to highest 28 [degrees]C on 19 June 2009. The Seasonal and temporal fluctuations in water temperature were quite significant, while surface salinities remained lower and stable, ranging from 0% to 1% throughout the study period due to continuous freshwater inflow. Because we only made relatively few measurements, there may have been peaks of higher salinities that were missed.
Over 2400 specimens representing both the species were individually measured and categorized based upon their sexual maturity. The monthly collections showed consistent appearance of T. louisianae and T. throughout the sampling period. Americamy sis bowanni almyra and A.bahia; however, only occurred twice in our samples on 1 January 2009, and 26 March 2009, and were not used for detailed analysis. Taphromysis species showed significant fluctuations in seasonal abundance, during monthly collections. T. bowmani peaked in May and maintained significant abundance in May and June and in January. T. louisianae peaked in June and showed relatively higher monthly values as compared to T. bowmani during the major part of the study period. T. louisianae varying between 82 to 325 with mean as 130, and T. bowmani varied between 10 to 265 with mean of 101 during the study period (Figure 2). The seasonal pattern of abundance for both species of Taphromysis during the study period was synchronous.
The sledge net collections made in the subtidal zone yielded significantly fewer numbers of specimens as compared to shoreline collections indicated intertidal habitats in Davis Bayou Estuary were more favorable to mysids.
Growth And Development
Based on the development of the oostegites in the females and the copulatory stylet in males below 3.0 mm SL immature specimens were considered to have standard lengths between 3 and 4.9. Males of Taphromysis louisianae with fully developed copulatory stylets were considered as mature and measured between 5.0 to 8.5mm. Mature females of T. louisianae between 5.01-0.0 mm.
As in T. louisianae the length of mature males for T. bowmani was between 5.0-8.5 mm. In mature females the length range was between 5.0- 9.0 mm. Thus the maximum size observed for T. bowmani females was 1.0 mm less than the largest female of T. louisianae observed. The females of both T. bowmani and T. louisianae species less than 5 mm in length lacked fully developed brood plates (oostegites). In contrast a majority of the females with standard lengths greater than 5.0 mm had fully developed, interlocking, brood plates. The smallest incubating female observed was 5.0 mm SL, with the majority of the ovigerous females observed being 6 mm SL or greater.
The largest males of T. bowmani and T. Louisiana measured 8.5 mm. The fourth male pleopod showed early differentiation in endopod and expod at sizes above 4.0 mm. Besides the difference in the number of inter-furcal spines on the telson, the two species of Taphromysis can be distinguished with T. louisianae having each anterior margin of its carapace armed with a small spine, a character lacking in T. bowmani (Stuck, et al 1979) On the fourth male pleopod of T. bowmani, the shorter flagellum is distally bifid and distinctly shorter than the other flagellum; whereas, on T. louisianae the tips of both flagella are simple and flagella are subequal in length and pincer-like (Bacescu, 1961, Stuck, et al, 1979). The sympod on the antennules of adult males was characterized development of soft sensory setae (see both species, after attaining size 6.0 mm, the immature males mysids, less than 5 mm in size lacked the setae.
Population Composition And Structure
The monthly changes in the population structure were analyzed on percent basis (Figures 3A and 3B) The immature males (IM) of T. louisianae varied from 0% to 12% with mean of 5.27%, that of mature males (MM) from 10% to 54% with mean of 25%, that of immature females (IMF) from 0% to 58% with mean of 22.1%, that of mature females (MF) from 1% to 51% with mean of 27.36, that of Ovigerous females (incubating) from 1% to 46% with mean of 20.27%.
A. Taphromysis bowmani
The immature males of T. bowmani varied from 0% to 14% with mean of 4.1%, that of mature males 7% to 44% with mean of 24.9%, that of immature females 0% to 40% with mean of 17.9%, that of mature females 7% to 50% with mean of 25.5%, that of ovigerous females from 6% to 50% with mean of 27.6 The overall sex ratio including immature, mature and ovigerous mysids of T. Louisiana was 28.2% males to 71.78% females, and that of T. bowmani was 29.1% males to 70.9% females, indicating males are significantly fewer in number in Davis Bayou Estuary.
Brooding Females And Brood Size
Ovigerous females of T. louisianae and T. bowmani were present in samples throughout the study period. The numerical seasonal abundance varied markedly but because of year round presence of immature mysids and ovigerous females the study indicates reproductive activities continue year-round (Figs., 3A&B). Large ovigerous females were observed from February to May 2009; whereas, the average size of ovigerous females decreased from June to August apparently due to females becoming reproductively more active resulting in increase in number but smaller in body size (brood size larger in winter, and smaller in summer).Body size of the brooding females increased with advancing developmental stages of embryos, also noted in other studies (Mauchline, 1973, Takahashi, 2004 and Hanamura, et al, 2008). The samples contained post brooding mysids with fully expanded marsupium but no embryos present. The maximum number of eggs found in T. Louisianae was 21 and in T. bowmani was 25 during February, and in April, 2009. The egg developmental and larval stages of both species of Taphromysis under study had similar size measurements. The eggs and stage I larvae of both Taphromysis species measured 0.5 to 0.6 mm. Because of the more rapid development and shorter duration time, the presence of stage I embryos, which are enveloped in an egg membrane, was less often observed. The maximum number of stage II embryos in any female observed were 25, measured 1.0 to 1.2 mm, and had pigmented eyes. The stage III embryos had stalked eyes and well-developed appendages measured 1.3 to 1.7 mm. The maximum number of embryos the females carried were 13 (T. louisianae) and 12 (T. bowmani) during the study period. The present study noted the embryos within the same marsupium were at the same stage, except rarely stage II embryos got admixed with stage III, also reported by other studies possibly due to "adoption" of the embryos from another female (Mauchline 1973).
This study focused on Taphromysis louisianae and T. bowmani, which co-occurred as year round residents and the most prevalent species of the upper reaches of Davis Bayou. On two occasions two other mysids Americamysis almyra and A. bahia occurred in our samples at down the stream stations (Figures 1 and 2). Ecological observations on Americamysis species were not made because of their rare occurrence in our samples. Both species of Taphromysis treated here represent geographical isolate of the mysid populations of coastal estuaries and shallow water areas of Northern Gulf of Mexico (Price 1978; Daly 1986; Reeder 1992; Heard et al., 2006). Originally mysid sampling in Davis Bayou estuary was carried by an epibenthic sledge that resulted in poor samples. The shoreline intertidal collections, designed later, yielded considerably higher number of mysids. This study showed shoreline habitat was highly suitable for T. louisianae and. T. bowmani also compared favorably to other studies with Mesopodopsis orientalis in Mangrove Estuary and Heteromysis riedii in near-shore areas in Malaysia (Wittman, 2000 and Hanamura et al., 2008).
Our limited seasonal observations on the abundance of Taphromysis louisianae indicated that it is generally hypohaline species with its abundance peaking in June; whereas, that of the more euryhaline T. bowmani was highest during May (Figure 2), but the seasonality pattern showed similar trend (Figure 2). The temperature fluctuations (Figure 1) clearly affected the seasonal abundance and reproductive activities. The specimens (females) of T. louisianae and T. bowmani were larger in winter months (January to March, 09) with grand average size as 7.1 and 6.9 mm; respecetively (Table 1). In contrast in warmer months (April to July 2009) the grand average for T. louisianae and T. bowmani was 4.6 and 5.0 mm; respectively (Table 1). The study noted similiar seasonal pattern in male standard length as compared to females. T. louisianae male, had grand average 6.5 mm in winter (January to March, 2009) and 4.8 mm in warmer months (April to July, 2009) and that of T. bowmani male, had 6.9 mm in winter and 4.5 mm in warmer months.
This study indicates that temperature appears to be directly related to seasonal abundance and to fair degree inversely related to average body size. Higher temperatures appeared to cause an increase in reproductive activities with an apparent decrease in average body size (Mauchline, 1980; Wittman, 1984; Johnson et al, 2001). Present study also noted relationship in total body size of females and related fecundity. The brooding females of both species of Taphromysis species with greater body size (Table 1) in winter months had higher fecundity; and in summer months the gravid females with smaller body size had lower fecundity (Moldin, 1993).
The surface salinity was markedly low (0-1%) throughout the study period and no effect was noticed across the specterum of developing stages or the reproducetive activities. We were not in a position to sample bottom salinities which may have been higher. The year round presence of brooding females and immature mysids in most monthly samples (Fig. 3, A &B) clearly indicate T. louisianae and T. bowmani are residents and that they are sexually active throughout the year round in our study area of Davis Bayou.
Although there are extensive studies on the Mysids available, literature dealing with ecological parameters and productivity remains limited. Mysids are important source of estuarine food for many fishes and invertebrates and even as human food in southeastern countries (Mauchline, 1980). The present study presents data with the population composition, structure, abundance, fecundity and reproduction; further work to enhance the information on ecological parameters and productivity will continue.
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Mohammed Mulkana, and Richard Heard
Department of Coastal Sciences, University of Southern Mississippi, Gulf Coast Research Laboratory Ocean Springs, MS 39564.
Corresponding Author: email@example.com
Table 1: Seasonal variations in average body size (mm) of Taphromysis louisianae and T. bowmani Date T.Louisiana T.bownani 1/9/09 7.0 6.7 2/26/09 7.2 7.1 3/26/09 7.5 7.1 MEAN: 7.1 6.9 (Jan-March) 4/14/09 6.1 5.9 5/27/09 5.3 5.6 6/19/09 4.0 4.1 7/7/09 4.4 4.6 MEAN: 4.6 5.0 (Aprl-July)
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|Author:||Mulkana, Mohammed; Heard, Richard|
|Publication:||Journal of the Mississippi Academy of Sciences|
|Date:||Apr 1, 2012|
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