Marenzelleria Viridis (Polychaeta: Spionidae): a Review of Its Reproduction

Marenzelleria viridis (Polychaeta: Spionidae): a review of its reproduction

Ralf Bochert Universität Rostock, Wissenschaftsbereich Meeresbiologie, D-18051, Rostock, Germany Accepted 6 May 1997

Key words: Marenzelleria viridis, reproductive biology, Baltic Sea, North Sea, review

Abstract

The features of the reproductive biology such as morphology of gametes, larval morphology, larval development and development of gametes of Marenzelleria viridis Type II from the Baltic were summarized. Further reproductive features of Baltic Sea populations are given and the purpose of the review is to account for the successful immigration of M. viridis into the oligohaline areas of the Baltic considered against a background of such variables as population density and structure, salinity, temperature, food availability, oxygen and sulphide levels. Gametogenesis started in spring. Fecundity of animals depended on salinity, temperature, age and size of worms. Mature oocytes contained large cortical alveoli not yet known for polychaetes. Animals spawned in autumn in all years of investigation. The pelagic larvae of M. viridis Type II were found mainly from September to November. Larval development depended on water temperature and lasted about 4 to 12 weeks. Successful larval development from egg to juvenile was not possible below salinities of 5‰, but colonization of oligohaline regions took place by larvae with more than 4 setigers or by swimming juveniles. Reproductive features of M. viridis Type II from the Baltic were compared and discussed with the results of M. viridis Type I populations from the North Sea and North America. The two M. viridis types reproduced at different time, M. viridis Type I reproduced in spring and M. viridis Type II in autumn. Both types showed also differences in larval development, gametal development and sex ratio of mature worms.

Introduction

Our knowledge about the reproduction of Marenzelleria viridis (Verrill, 1873) Type I and II (for classi- fication of Type I and Type II see Bastrop et al., 1997) prior to its immigration into European coastal waters was restricted to a few studies from North American populations. A comprehensive description of the life stages and early development of a M. viridis population from Lawrencetown (Nova Scotia, Canada) was published by George (1966). Since then, little additional information has become available about gravid and non-gravid animals, juveniles and pelagic larvae (Boesch et al., 1976; Holland et al., 1980; Dauer et al., 1980, 1982; Jordan & Sutton, 1984; Simon, 1968; Whitlatch, 1977). These studies led to the conclusion that the polytelic species M. viridis has a well defined breeding season in North America in spring. After this spionid worm had immigrated into European coastal waters, Atkins et al. (1987) and Essink & Kleef (1993) confirmed that M. viridis populations in the estuaries of the Tay (Scotland) and Dollard (Netherlands) spawned in spring. Larval development is successful only at Sali- nities of 5‰ and above, and the larvae live pelagically until adopting a benthic life mode after passing the 10-setiger stage (George, 1966). Newcomers, like M. viridis, trying to integrate themselves into existing aquatic communities, must either occupy free niches or, by virtue of their ecological potency, displace previously established species of the community with similar demands on the environment. However, only 10% of invaders estab- lish themselves successfully (Lodge, 1993). The production of fertile offspring is obviously one prere- quisite for the successful invasion of a new habitat by a species. Therefore, when M. viridis was dis- covered in the Darss-Zingst Bodden Chain (DZBC) in 1985, the question of whether it would be able to reproduce in Baltic coastal waters or would be forced into extinction by abiotic or biotic conditions became of immediate importance. The DZBC is a brackish water estuary in the southern Baltic colonised by relatively few invertebrates compared with marine areas (Arndt, 1988, 1989). Until the end of the 1980s, the macrozoobenthos of its oli-gohaline (salinity <5‰) part was dominated by chironomids and oligochaetes (Arndt, 1988). M. viridis successfully continued its immigration into the innermost oligohaline parts of the DZBC (Zettler, 1996). The purpose of the present publication is to shed some light on abiotic and biotic factors that enabled this success.

Structure/morphology of gametes

Polychaetes vary widely in their reproductive modes (Schroeder & Hermans, 1975; Wilson, 1991). This diversity is not only reflected in the different modes of gamete release (from free spawning to sperm transfer by spermatophores or copulation) and patterns of early development (pelagic and benthic larval development or direct development), but also in the structure of gametes and especially the morphology of the sperm. So far, the spermatozoa of a little over 100 polychaete species have been studied in detail, and each possesses species-specific features (Jamieson & Rouse, 1989). M. viridis lacks gonads in the first 35 to 40 and last 20 to 25 segments, depending on total animal length. The two pairs of gonads per segment develop on the ventro-lateral side of the greatly ramified nephridial blood vessels of each epitoke segment. The testes represent a simple germinal epithelium and are located below the peritoneum (Bochert, 1996a). The ovaries are enveloped additionally by a thin layer of follicle cells (Bochert, 1996b). Mature gametes of M. viridis exhibit characteristic morphological features. The spermatozoon The mature spermatozoon of M. viridis has a simple structure. The sperm head and midpiece are about 5 µm in length. The structure corresponds to earlier descriptions of 'primitive' sperms (Franzen, 1956). According to the studies of Jamieson & Rouse (1989) the spermatozoa of M. viridis can be classified as ect-aquasperm because fertilization is external (George, 1966). The acrosome of the mature spermatozoon of M. viridis has a very complex structure and is unlike that of other ectaquasperm investigated to date. It is embedded in a deep invagination of the nucleus and is almost completely enclosed by the nucleus (Bochert, 1996a). Another striking feature of the mature spermatozoon of M. viridis is the presence of a subacrosomal space in the form of cisternae. Analogous structures have been described as short, unlinked regular invaginations in Hydroides hexagonus (Serpulidae) (Colwin & Colwin, 1961a, b) or as a reticular system in Sabella penicillum (Sabellidae) (Graebner & Kryvi, 1973a, b) and are also found in several species of Polydora (Rice, 1981). An acrosomal reaction can be assumed to take place in the spermatozoon of M. viridis during fertilization as described for H. hexagonus (Colwin & Colwin, 196lb) on account of similar morphological structures. The egg Early developing oocytes are intimately associated with the perivascular cells of blood vessels. There are two possible pathways for the uptake of exogenous precursors by growing oocytes. Low molecular weight precursors may enter the oocytes via the abundant spherical surface granules of the microvilli tips. Endocytotic vesicles along the oolemma indicate the probable uptake of large molecular weight precursors via perivascular cells of blood vessels and the egg envelope (Bochert, 1996b). The mature oocytes are discus-shaped and about 155 to 175 µm in diameter. The ooplasm is sur- rounded by a vitelline envelope about 6 µm thick, which corresponds to the length of the sperm head (see above). A typical feature of the mature oocyte of M. viridis is the appearance of 10 to 18 cortical alveoli each up to 26 µm in size (Bochert, 1996b). The occurrence of large cortical alveoli in M. viridis is unique among polychaete eggs investigated to date. Cortical organelles are prominent within the oocytes of several polychaete species, but they are small organelles 0.5-2 µm in size (for review see Eckelbarger, 1988). Large cortical organelles are known from the penaid shrimps Penaeus setiferus and P. aztecus as cortical rods up to 40 µm in length (Clark & Lynn, 1977) and are also present in the eggs of the lancelet Branchiostoma (Amphioxus) caribaeum, lampreys and many teleost fishes (Guraya, 1967, 1982). Alveoli in the oocyte of teleost Abramis brama are about 21 µm in size and may contain up to three middle alveoli, which themselves contain a few inner alveoli 1-2.5 µm in size (Arndt, 1960). These are similar to the cortical alveoli in M. viridis (Bochert, 1996b), which also contain a few inner alveoli. The chemical composition of the outer and middle cortical alveoli in A. brama differ slightly, the outer cortical alveoli contain glycoproteins with neutral and acid mucopolysaccharide components, whereas acid mucopolysaccharides are lacking in middle alveoli (Arndt, 1960). The cortical organelles in M. viridis probably have a function during fertilization process as described for several species by Guraya (1982).

Larval development The larval development of M. viridis is entirely pelagic (Bochert & Bick, 1995) (Figure 1). The fertilized egg develops into a pre-trochophore which is capable of small swimming movements by means of apical cilia. As growth proceeds larval bristles appear. The 3-setiger larva is a critical stage in larval development when substantial morphological changes take place. The yolk reserves have been exhausted, and the larvae become exclusively planktotrophic. Larval development takes place inside the former vitelline membrane, which is restructured to form the larval cuticle as has been described for Phragmatopoma lapidosa (Eckelbarger & Chia, 1978). The larvae grow continuously until they reach the metamorphic stage with a length of 16 to 17-setigers (Bochert & Bick, 1995), in contrast to larvae of North American animals, which grow only to th 10 to 11-setiger stage (George, 1966). A highly significant correlation (r2 =0.972) exists between the number of setigers (x) and larval length (у; µm) and is described by the function у = 87.22x-37.72 (Bochert & Bick, 1995).

Up to the 10-setiger stage, the morphological development of larvae from a North American population (George, 1966) is indistinguishable from that of larvae from the Baltic Sea (Bochert & Bick, 1995) or the Dollard (personal observation). The M. viridis larvae of the North American population metamorphosed after reaching the 10-setiger stage, and the largest larvae observed had 13 segments (George, 1966). The larvae in the Baltic Sea (Bochert & Bick, 1995) and the Dollard estuary (personal observation) metamorphosed after reaching the 15-setiger stage at the earliest, and the largest pelagic larvae observed had 23 setigers. This plasticity in larval development is hard to explain. The differences between the Baltic and North American populations in the number of setigers at the onset of metamorphosis might be genetically determined (Bastrop et al., 1997). On the other hand, the larvae from the Baltic and Dollard populations belong to several types, but have identical larval development modes. The observed variability in larval growth might also be due to a lack of suitable substrate inducing the larvae to postpone settlement and build new segments, as has been reported for the larvae of Sabellaria alveolata (Day & Wilson, 1934). The larvae of M. viridis Type II are newcomers among the meroplanktonic spionid larvae of the Balt- ic. Knowledge of a number of morphological features of the pelagic larvae is essential for correct species identification at this early ontogenetic stage (Hannerz, 1956; Plate, 1992; Bochert, 1996c) (Table 1).

Influence of salinity and temperature on gametogenesis

M. viridis divided into groups of adult animals (>4 cm in length), which reached already maturity at least once and juveniles (<2 cm in length), which never became sexually mature so far, were collected for a laboratory experiment at one station of the DZBC in May 1995 (see Bochert, 1996c). Juvenile and adult animals were cultured simultaneously at two different temperatures (cooling room: 10°C and room temperature: about 21°C) and seven salinities (0.5, 1.5, 3, 5, 10, 25 and 30‰). The photo period was 12:12 (light:dark) and moonlight was simulated by means of a 1 W filament lamp for 5 dark periods. The experiment was ended after 5 months and 20 animals from each treatment were used for analysis. For analysis 'juveniles' were equated with '0-group animals', because it took place a development from inmature to sexually mature worms. The stage of ripeness of males was ascertained and the diameters of 60 to 95 oocytes from each female were measured. The Kruskal-Wallis H-test was used to detect differences between the oocyte diameters for each treatment. Null hypotheses (H0) were rejected at the 1% level of significance, whereafter differences between the various pairs were detected at the 1% level of error by the Mann-Whitney U-test. Box-and-Whisker-plots were used to summarise the data distribution for each treatment (median value, 50% data box, extreme values and outliers).

Temperature The gametes of M. viridis did not develop at a constant temperature of 10°C. Only 6 adult animals (males) bore gametes (Table 2). The percentage of gamete bearing animals was much higher at a temperature of 20° C, 121 of the 125 adult M. viridis and 90 of the 140 0-group animals bore gametes of various developmental stages (Table 2).

Salinity (at 20 °C) The mean oocyte diameter was larger than 150 µm (ripeness, see Bochert 1996c) at salinities of 1.5 to 10‰ (Figure 2). At a salinity of 0.5‰ the mean oocyte diameter was 138 µm in 0-group animals and only 101 µm in adult M. viridis. Mean oocyte diameters also decreased at salinities >10‰, being only 85 µm (0-group) and 116 µm (adults) at a salinity of 25‰. Mean oocyte diameters were smallest at a salinity of 30‰: 31 µm (0-group) and 85 µm (adult M. viridis) (Figure 2).

Males achieved maturity at salinities of 3.0 to 10‰ (Figure 3). The percentage of males ready to spawn was highest (75-100%) at a salinity of 10‰. Salinities lower and higher than 10‰ had a negative effect on sperm development. Thirty-eight to 100% of males reached the spermatocyte-stage at salinities of 3,5 and 25‰. Spermatogenesis reached only spermatogonia-stage in 12 to 62% of the animals at salinities of 0.5 and 1.5‰ and even in 90 to 100% at salinities of 30‰ (Figure 3). A salinity of 10‰ proved favourable for gametal growth in M. viridis, whereas salinities ≥25‰ and ≤5‰ affected it negatively (Figures 2, 3), as shown by the longer time needed to reach maturity. M. viridis is able to tolerate salinities down to 0.03‰ at 10 °C for 72 h (Fritzsche, 1995). M. viridis is an osmoconforming species, but is able to regulate the osmolality of the body over the osmolality of the surrounding water at salinities <8- 10‰ (Fritzsche, 1995).

Male and female M. viridis failed to reach maturity within 5 months at salinities >25‰, and the gametes were significantly less ripe at a salinity of 30‰ than at salinities of 0.5 and 1.5‰ (Figures 2, 3). It seems likely that M. viridis does not have enough energy to spare for gamete development under polyhaline conditions. Such living conditions obviously represent the ecological pessimum, and this may indicate that M. viridis Type II is a genuine brackish water species.

Influence of salinity and temperature on larval development In a series of experiments, the planktonic larvae of M. viridis were exposed to various combinations of salinity (S = 0.6, 2.5, 5, 10 and 20‰) and temperature (T = 5, 10 and 20 °C) from the one-setiger stage to the onset of metamorphosis (16 to 17-setiger stage) (Bochert et al., 1996a). One-setiger larvae were unable to complete their development to metamorphosis at salinities below 5‰. Larvae failed to develop beyond the 3-setiger stage and survived for only 2-3 weeks at salinities of 2.5‰ and at S = 5‰ (T = 5 °C). At these low salinities the larvae presumably cannot provide the energy needed for osmotic and ionic regulation and, simultaneously, developmental processes. Metamorphosis to the benthic life mode was possible at salinities of 10 and 20‰. Development from larvae to juveniles was most rapid (4-5 weeks) at 20°C. At 10°C, development to this stage took 5-7 weeks, and at 5 °C it lasted 2½ to 3 months. Larval development was more rapid at 10‰ than at 20‰ (Bochert et al., 1996a). Our results confirm George's (1966) observations concerning a North American M. viridis population in an estuarine habitat with salinity fluctuations between 1 and 32‰. The rates of larval development did not vary discernibly at salinities above 10‰, and no growth was recorded at salinities below 5‰ (Geor- ge, 1966). The salinity tolerance (LC50 < 1‰) of M. viridis larvae is only slightly lower than that of juvenile and adult animals (LC50 < 0.1‰) (Bochert et al., 1996a). Larvae with 4 to 5 setigers were placed in an aquarium containing water with a salinity of 3.5‰. These animals were able to metamorphose and survive as benthic juveniles even at this low salinity at temperatures = 10°C. Pelagic larvae migrate mainly by riding the currents. Once they have reached the 4-setiger stage, larvae of M. viridis that drift into oligohaline regions where salinities do not drop below 3.5‰ are able to continue their development up to metamorphosis and adopt a benthic life mode.

Field observations

Gametogenesis The reproduction of M. viridis in the DZBC was investigated from March 1992 to April 1995. Animals were collected once a month to ascertain the reproductive stage (Bochert et al., 1996b). Gamete formation started between mid-May and mid-June in all three years of the study. Gametes were found in over 60% of the animals in July. Gamete development lasted about 4-5 months, the population starting to spawn in late September (Figure 9a). This stands in sharp contrast to the reported spring spawning season of North American and North Sea M. viridis populations (see above). Animals with ripe gametes continued to be found in small numbers until December to February (Bochert et al., 1996b). Slight seasonal and temporal differences in the proportion of spawning animals at the various stations and in spawning time were observed from year to year. Fecundity Fecundity of females was determined by counting the eggs of three segments every 10th to 15th segment (Bochert, 1996c). Eggs were found in small numbers from the 28th to 38th segment onwards. The egg number per segment then increased rapidly with increasing number of body segments. The highest egg numbers/segment in the females we examines, with one exception during this study, in the segments just before the mid body (Figure 4). The highest number was 448 eggs/segment. Eggs were never found in the last 18 to 31 segments of the body. Therefore more than 60% of the body segments were fertile. Taking the number of body segments as 100%, eggs were found in between 16-26% and 65-85%, starting at the prostomium (Figure 4). Calculations showed that the number of eggs per female varied between 10 000 and 46 000 eggs/ female (Bochert, 1996c). Females with more than 200 segments produced two to three times as many eggs as females with 117 to 163 body segments (Figure 4). The number of eggs found in 0-group animals did not correlate with body width (number of segments) (Bochert, 1996c). For instance at 20°C and 10‰, two worms with a width > 1.2 mm bore only 3 and 5 mature eggs (Figure 5). In contrast, three M. viridis < 1.16 mm in width produced more than 67 mostly ripe eggs.

Larval development Plankton samples were taken from March 1992 to April 1995, and the densities of the pelagic larvae were counted. The recruitment of juvenile worms was ascertained by benthos samples (Bochert et al., 1996b). Larvae appeared in the plankton in low abundances from early September onwards in all years of the investigation. Maximum larval densities were highest in 1992 (about 22 x 106 ind. m-3), reached 1 x 106 ind. m-3 in 1993 and only 314000 ind. m-3 in 1994. Total larval abundances decreased in 1992 and 1994 to less than 1000 ind. m-3 by mid-December, and the last larvae from this season were found in the plankton in mid-February 1993. In contrast, larval densities in 1993 reached their maximum of almost 1 x 106 ind. m-3 in late December. They then remained high (10000 to 40000 ind. m-3) until early March 1994 (Figure 6).

Additionally, horizontal and vertical distribution of pelagic larvae of M. viridis was studied in the DZBC in October 1992 (Bochert et al., 1994). Larvae were horizontally not regularly distributed and patchiness was found. This was caused by currents in the investigation area. There was no evidence for preference of M. viridis larvae for any water depth (max. water depth 5.3 m) (Bochert et al., 1994). This result was independent of time of day and lunar periodicity. The successful development of M. viridis larvae was documented by the appearance of 10-setiger stages in abundances exceeding 100000 ind. m-3 in 1992. These animals were well on the way towards the 16 to 17-setiger stage at which they assume the benthic life mode of juveniles (see above). The im- pact of low salinity on larval development was shown by the small number (maximum about 13 000 ind. m-3) of larvae that reached metamorphosis in 1994 (Bochert et al., 1996b). Beside seasonal variations in larval occurrence and density, there are clear signs of spatial variability. Larval densities were noticeably lower at the innermost oligohaline stations in the DZBC (Bochert et al., 1996b), where the low salinity (<5‰) had suppressed successful larval development (see above). The first juvenile animals were found in mid-October, whereafter their abundance increased rapidly to peak between the end of October and the end of November (Bochert et al., 1996b). 500 to 3000 juveniles m-2 (one exception 1994: 30000 ind. m-2) colonized the sediment in early spring. The innermost oligohaline stations in the DZBC were colonized in 1995 by swimming juveniles. Further factors influencing reproduction

Oxygen deficiency Oxygen can be scarce in the DZBC. Oxygen saturation was below 50% on several occasions in July and August 1994 and 1995 just as the gametes were developed (Schlungbaum & Baudler, 1996). Below 20 kPa (about 100% oxygen saturation), M. viridis gradually reduces its metabolic activity as the oxygen partial pressure decreases (Fritzsche, 1995). On such occasions, therefore, the animals must devote a greater share of their resources to compensating for oxygen depletion periods at the expense of invest- ment in gametes.

Hydrogen sulphide The occurrence of toxic hydrogen sulphide during summer stagnation periods is another major stress factor to which benthic animals colonizing shallow tideless polytrophic waters such as the DZBC are exposed. Hydrogen sulphide reached concentrations of 2-6 mmol 1-1 in the sediment and 250 µmol in the pelagial in 1995 (Schneider, 1996). The survival time of M. viridis starts to decrease at hydrogen sulphide concentrations of only 0.7 mmol 1-1 (Bochert A et al., 1997), so this environmental factor will also affect gamete production.

Population density Densities of M. viridis increased from about 1000 in 1992 through 8000 in 1993 to about 28 000 ind. m-2 in 1994 at an inner oligohaline station in the DZBC. The increase during the same period at an outer station in the DZBC was much lower: from about 5000 ind. m-2 in 1992 to 8000 ind. m-2 in 1994 (Zettler, 1996). Higher population densities increase intraspecific competition for food and space, and restricted access to food will naturally also affect the gamete production. Higher population densities of M. viridis may prolong the time needed for gametal development. Zettler (1996) reported a general increase in the abundances of macrobenthic species since 1987, but was unable to show any significant relationship between M. viridis and other macrobenthic species. Hence, interspecific relationships seem to have only a slight influence on reproductive success.

Population structure Besides having to maintain their metabolic activity as adults, juvenile animals must invest considerable amounts of the energy in growth processes, so these two stages may differ in their physiological responses to environmental conditions (Fritzsche, 1995, Bochert A et al., 1997). Seasonal effects on the structure of the M. viridis population in the DZBC were visible. Only adult animals were found in 1992. The proportion of juveniles then increased, depending on location, from 43-83% in 1993 to 55-89% in 1994 (Zettler, 1996). Juvenile M. viridis (0-group) seem to need a body width of 0.9 mm (about 40 segments) before gamete development can begin as animals lacking gametes were significantly smaller (p < 0.01) than those with gametes (Figure 7). The results of a laboratory study have shown that even juvenile M. viridis (0-group) are able to reach maturity in the first year of life (Figures 2, 3, 5), but the proportion of gamete-bearing animals among adult M. viridis was higher than among 0-group animals (Table 2, Figure 8). The percentage of 0-group animals without gametes at the end of the experiment tended to increase with decreasing salinity <5‰ (Figure 8). Thus, seasonal and spatial variability in reproductive success may result from a larger proportion of juvenile M. viridis in the population, such as 1994, when larvae were found in the plankton until spring (Bochert et al., 1996b).

Food Larvae of M. viridis are planktotrophic, and benthic stages feed on suspended and deposited material (Burckhardt et al., 1997; Dauer et al., 1981). The growth and survival rates of M. viridis larvae depend on food quality and quantity (Burckhardt, personal communications). Therefore, the great variability in larval densities observed in several years during the investigation may have been caused by changes in food conditions. Substrate turnover by adult M. viridis was on average 6.2 times higher (far wide of the mark set by the van 't Hoff-rule) at 20°C than at 10°C (Burckhardt, personal communications). The lower substrate turnover at 10°C might partly explain the low ability of M. viridis to build gametes at these temperatures (Table 2).

Predators Predators have a slight influence on reproductive success in the DZBC. Juvenile and small fishes consu- med only about 150 larvae m-3 d-1 in 1992 (Bochert, 1996c). Benthic M. viridis are well protected against fish predation by their deep burrows (Zettler et al., 1994). However several fish species feed on benthic M. viridis which leave the sediment (Winkler & Debus, 1997). However, this predation has little effect on account of the high densities of larvae and benthic animals. Sublethal effects such as the loss of palps needed for feeding activity would influence only the individual fitness of worms.

North Sea (Type I) vs Baltic (Type II) populations

Gametogenesis was studied in a M. viridis (Type I) population in the Dollard (Ems estuary, The Nether- lands) from January 1994 to February 1995. Early gametal stages were found in 75% of the animals at the end of May and in about 90% at the begining of July in 1994 (Figure 9b). Male and female gametes were indistinguishable until the beginning of October (Bochert R et al., 1997). This was about 6 weeks later than in a M. viridis (Type II) population from the DZBC. The first mature animal in the Dollard was found in November 1994 (Figure 9b), and the proportion of worms ready to spawn then increased to about 20% by mid-December. All investigated worms bore ripe gametes at the beginning of February 1995. Although about 95% of the investigated animals were mature at the beginning of January 1994, no outburst of spawning was observed until March 1994 (Figure 9b).

The sex ratio varied clearly during the study. Males and females were found in equal numbers in early October 1994, but males were twice as abundant as females in mid-November in the same year. At maturity in March 1994 and February 1995, the sex ratio of M. viridis Type I (♀♀ : ♂♂) was 1:2.3 and 1:2.8, respectively (Table 3). The different spawning seasons of M. viridis from Baltic and North Sea estuaries could be genetically determined (Bastrop et al., 1997) or caused by exogenous factors. The physical properties of the M. viridis habitats in the brackish waters of the Southern Baltic differ substantially from those in other regions. The Tay (UK), Dollard (Netherlands) and Lawrencetown (Canada) estuaries are, in contrast to the DZBC, tidal regions in which the mud flats are exposed at low tide and considerable salinity fluctuations are reported (George, 1966; Essink & Kleef, 1993; Atkins et al., 1987). Mud flats exposed by tidal action are characterized by oxygen depletion in the interstitial water (Schottler et al., 1984; Grieshaber et al., 1994). M. viridis responses to hypoxic stress by reducing its metabolic activity, although its metabolism remains fully aerobic down to an oxygen partial pressure of 2 kPa (Fritzsche, 1995). Additional hyposmotic stress leads to a further reduction in metabolic activity. Conditions appear to be more favourable for gametal development in the DZBC than in mud flats. It seems possible that M. viridis becomes sexually ripe and can spawn before the water is too cold for normal larval development. A study by George (1966) showed that almost 100% of a North American population were ready to spawn by early January, and I have confirmed this by observation of a M. viridis (Type I) population in the Dollard estuary (see above). Animals that colonize the bottom in spring, like worms of the Dollard- population, must first grow to a body size sufficient for gametal development (Figure 7). Owing to their environment, these animals are unable to achieve sexual ripeness by next autumn. Similarly, stress during gamete production delays gametal development in older animals, so that sexual maturity is achieved two or three months later. It would presumably not be meaningful for these ripe animals to spawn as soon as they become ripe in winter since larvae would have little chance of survival under such conditions (Bochert et al., 1996a). The higher environmental stress exposure of M. viridis is also reflected in the sex ratio, which is about 1:1 in the DZBC (Bochert, 1996, Bochert R et al., 1997). In contrast, females of a Dollard population probably died during gametal development, so that males outnumbered females by about 3:1 when spawning began (Table 3).

The difference in spawning season of M. viridis Type I and Type II may be genetically determined, but as reproductive success depends on many biotic and abiotic variables it may also be caused by environmental factors.

Conclusions

The reproductive success of M. viridis in the DZBC as measured in terms of number of progeny is governed by various factors. Salinity, temperature, food conditions, population density and structure, oxygen and hydrogen sulphide levels affect both gamete formation and larval development. Their long pelagic phase gives the larvae plenty of opportunity to disperse. Like the adults, the larvae possess powerful physiological adaptation mechanisms which enable them to survive stress situations better than other species (Bochert A et al., 1997; Fritzsche, 1995). This has enabled M. viridis to spread and colonize a wide range of habitats, especially in oligohaline areas. M. viridis has become an important element of the fauna in the DZBC and, we can be sure that it has secured for itself a permanent place among the macrozoobenthos of Baltic coastal waters. The difference in spawning season of M. viridis Type I and Type II may be genetically determined, but as reproductive success depends on many biotic and abiotic variables it may also be caused by environmental factors.