Micronephthys minuta Theel, 1879 - one of the most common polychaets of the White Sea. Numerous papers (Lvova, 1976, 1978; Chivilev, 1981, 1982) and Ph.D.Theses (Lvova, 1981; Chivilev, 1983) deal with this species, students investigate it while field training. Unfortunately till now two species: Micronephthys minuta and M.neotena have been mixed here. The main aim of present article is to clear situation with taxonomic composition of the genus Micronephthys within the White. Above this we tried to find difference in their biology. This task was facilitated by fact that both species are abundant here. Finding of new species of the genus Micronephthys in the White Sea made unclear which species belong data on biology of M. minuta here. So, besides pure taxonomy we count useful to give summary on biology of both White Sea species, including where possible data from other parts of their areals.
Nephthys minuta Theel, 1879:28-29, Pl.2, fig.18; Zatsepin, 1948:122, table.XXX, 12 (partim); Uschakov, 1955:217 (partim).
Micronephthys minuta - Tzetlin, 1980:25 (partim); Jirkov, 1989:74, fig.15.4 (partim) - non Pettibone, 1963:188-19O, fig.47b,c.
Micronephthys sp.aff. minuta - Fournier, Pocklington, 1984:261.
Material: 123 specimens; 18-270 ¬.
Prostomium: the worms with non-everted pharynx have sub-square prostomium, with the slightly festooned anterior margin. Antennae insert near anterolateral angles of prostomium, direct forward. They arise from one basis, conical, with bulbous base, just the same in size, broadly separated. Peristomium is not increased and flattened, the structure of prostomium and peristomium corresponds to group II (Ohwada, 1985a). Large nuchal organs arise dorsal just posterior to prostomium, can become invisible due fixation. Everted pharynx with 18 bifid, radially flattened terminal papillae, there are two smooth intervals in mid-dorsal and midventral positions. 18...20 rows of subterminal papillae. Subterminal papillae conical, small, decrease in size towards proximal end of pharynx with coefficient about 2: the first papilla is twice bigger the second one, the second is twice bigger the third and so on. There is one long unpaired median dorsal papilla. Drew pharynx reaches S-10.
S-1: notopodium with low conical acicular lobe, neuropodium truncated conical, both rami of parapodium slightly protruded on the basal part of parapodium, drawing together. Both ventral and notopodial cirri are present, starting from S-1, but on the S-1 they are very little and can be seen only after the staining.
Presetal and postsetal lamellae on both rami of parapodia of the small worms (less than 3-4 mm) are invisible even after staining. The bigger ones have rudimentary lamellae. Acicular lobes are conical. Acicula strong, tapering with fine recurved tips directed away from the interramal space. There are distinct transversal notches on these tips. The surface of the interramal space and branchiae wrinkled. Branchiae occupy about one third of the interramal space, straight or slightly involved, sometimes on the more development setigers tapered at the distal part. Branchiae are first present on S- 6...S-9 and at the worms with length more than 3 mm they finish on S-10...S-14. So therefore branchiae present on 3...9 setigers. The number of the last BS slightly (coefficient of the Pearson's correlation 0,691) but significantly (probability of difference from zero is more than 99,99%) positively correlates with the total number of setigers.
The total number of BS also slightly (coefficient of the Pearson's correlation 0,607) but significantly (probability of difference from zero is more than 99,99%) positively correlates with the total number of setigers.
Setae are of 3 types: barred, capillary and serrated. At the same time setae of each types can be the long and the short ones. The short setae meet more rarely than the long ones, they can be the tips of growing setae.
All barred setae present in preacicular position, they start on notopodium of 1 setiger and on neuropodium of 2 setiger. They replace by smooth ones at 2...3 setigers after the last BS. Their maximum number per one parapodia is 3...11. The short barred setae are present together with the long ones, but very rarely, not on all setigers and all worms.
Serrated setae present in postacicular position few setigers earlier than branchiae, maximum number 10. The short serrated setae present, extremely rare. Capillary setae start on S-1. They always present in postacicular position. The number of long capillary and long serrated setae together is about 10...20 and approximately the same on the first and medium setigers. It is necessary to point out that using our microscopes we cannot in any case to differ clear this both types of setae. Perhaps the part of capillary setae is really serrated, but their notches are closed by the stem of the seta.
On the setigers with well developed branchia capillary setae generally present inner than acicula (in the interramal space). There 3...5 capillary setae always present outgoing from almost one point and forming the compact fascicle. In addition the separate capillary setae may be in the other parts of postacicular position. There are about 10 serrated setae on any of these well developed setigers and they generally present outward from acicula and don't form the distinct compact fascicle as capillary ones. The number of short capillary setae approximately the same on all setigers (1..3). The more worms the more number of setae.
Entire animals looked over are reached at length of 8 mm, 31 setigers, according to Theel (1879) up to 16 mm and 30 setigers. Preserved worms colourless. Living ones: on the dorsal side between the parapodia of S-3 and S-4 there are two black pigment points in the lower part of the brown loop (the brain). This spots dispose on the pharynx and appear through the transparent covers. The blood-vessel begins between these spots (more correct it finishes here because the stream of blood starts from the pigidium). The blood-vessel goes down till the pigidium dorso- medially, it has some loops. There is a wide dirty-green stripe with two blood-vessels by side along the ventral part of worm. The blood-vessels drop in the branchiae also. Prostomium may be somewhat green, the other body pale-rose. The pigidium with the long ventral cirrus. There are no any rings on the pigidium, as in M.neotena.
Remarks. 1. Despite M. minuta has been described from the Barents sea, there are only 4 specimens from open part of the sea (69+55'N, 42+00'E, 125 m; 72+36'N, 50+41'E, 162 m and 74+30'N, 50+23'E, 136-138 m), they have branchia on S-7...S-13.
2. Micronephthys sp. aff. minuta sensu Fournier, Pocklington, 1984, judging to their remark, agrees well with our description of M. minuta s.str.<
3. M. minuta sensu Pettibone, 1963 differs by distinctly later start of branchia (on S-12...S-14), so it is not nor M. minuta s.str., neither M.neotena.
Distribution. Our data: the Barents Sea, the White
Sea, North Spetsbergen; Fournier, Pocklington (1984) found species
in Nova Scotia (Canada).
Aglaophamus neotenus Noyes, 1980:106-116, fig.1-3; Grenon, 1981:111-112.
Nephthys minuta - Zatsepin, 1948:122, tabl. XXX, 12 (partim); Uschakov, 1955:217 (partim) - non Theel, 1879.
Micronephthys minuta - Tsetlin, 1980:25 (partim); Jirkov, 1989:74, fig. 15.4 (partim).
Material: 112 specimens from the White Sea; 3-9 m.
Description. Prostomium: the worms with non-everted pharynx have slightly elongated, rectangular prostomium, with the slightly festooned anterior margin, anterolaterally distinctly convex, laterally straight. Antennae arise near anterolateral angles of prostomium direct forward and sideways, they arise from one basis, conical, with bulbous base, just the same in size, broadly separated. Peristomium is not increased and flattened. The structure of prostomium and peristomium corresponds to group II (Ohwada, 1985a). Nuchal organs arise dorsal just posterior to prostomium, can become invisible due fixation. Everted pharynx with 18 bifid, radially flattened terminal papillae. There are two smooth intervals in mid-dorsal and midventral positions. There is one long unpaired median dorsal papilla. Drew pharynx reaches S- 10.
There are ca. 16 rows of subterminal papillae in the middle part of pharynx, in the distal part 20 subterminal papillae present, additional papillae are in the end of distally branched rows or they are isolated, situate between rows. Rows are short, maximum 6 papillae present. Surface of pharynx otherwise smooth. S-1: neuropodium with low conical acicular lobe, notopodium truncated conical, both rami of parapodium slightly protruded on the basal part of parapodium, drawing together.
Both ventral and notopodial cirri are present starting from S-1, on the S-1 they are very small and can be seen only after the staining.
Presetal and postsetal lamellae on both rami of parapodia are rudimentary. Acicular lobes are conical.
Acicula strong, tapering with fine recurved tips directed away from the interramal space. There are distinct transversal notches on these tips.
The surface of the interramal space and branchiae wrinkled. Branchiae occupy about one third of the interramal space. Branchiae straight or slightly involved, broadly rounded distally. Branchiae start from S-5...S-7 and finished on S-12...S-21 in the worms longer 3 mm. So therefore branchiae present on 7...15 setigers. The number of the last BS slightly (coefficient of the Pearson's correlation 0,402) but significantly (probability of difference from zero is more than 99,9%) positively correlates with the total number of setigers.
Setae are of 3 types: barred, capillary, and serrated. At the same time setae of each types may be the long and the short ones. The short setae meet more rarely than the long ones, they can be tips of growing setae. Barred setae presented in preacicular position only, they start on notopodia of S-1 and on neuropodia of S-2. They replaced by smooth ones at 2...3 setigers after the last setiger with branchia. Their maximum number is 10. The short barred setae present together with the long ones, but very rarely, not on all setigers and all worms. Serrated setae start in postacicular position 1-2 setigers before branchiae, maximum number 13. The short serrated seta was observed only once. Capillary setae start at the S-1 above and below the acicula. The number of long capillary setae reaches maximum in the anterior part of worms (maximum number 8). Development of branchiae within the anterior part of the body accompanies with the replacement of long capillary setae by the serrated setae. The long capillary setae practically absent on the setigers with the well developed branchiae (maximum number 1...2). The number of short capillary setae is approximately the same on all setigers (1...3). The more worms the more number of setae. In the spawning time setae become 1,5 times longer.
Entire animals looked over are reached at length of 11 mm (according to Chivilev - till 13 mm), 38 setigers. In living worms on the dorsal side between the parapodia of S-3 there are two black pigment points (they fade in the preservatives). There is the belt of the little reddish-brown spots in the middle of the pigidium.
Remark. Ohwada (1985a) has re-investigated the paratypes A.neotenus in NM 47166 (collected in Wentworth Point, Damariscotta River, Maine, USA) and discovered that there 20-22 rows of subterminal papillae.
Distribution. The White Sea, Atlantic coasts of North
Previous investigation of variation in branchiae of Micronephthys (Jirkov,1989) shows that shallow-water (4-8 m) and deep-water (> 30 m) populations sharply differ in the White Sea; specimens from the Barents Sea did not differ from the deep-water ones from the White Sea. However, pharyngeal structure was not investigated, it did not allow to make definite conclusion concerning taxonomic status of those populations. It was supposed only, that they belong to different species. Present more detail study shows that they are really two different species (M. neotena and M. minuta s.str. respectively). The reason for misidentification is probably connected with fact that author of M. minuta - Theel - have not mentioned number of BS-1. At the same time he pictured parapodium of S-10, this fact many authors (Zatsepin, 1948; Uschakov, 1955; Tzetlin, 1980) counted as mention as it is S-1. In reality, as it was shown earlier (Jirkov,1989) there is no one specimen of Micronephthys with so late start of branchiae, vice versa in 9 worms S-10 was the last segment. The next misunderstanding (the source we failed to trace) is putting into diagnosis of the genus Micronephthys character "absence of branchiae" (Day, 1973, Noyes, 1980, Ohwada, 1985). In Russia, judging to investigated collections, appear a habit to identify all small Nephtyidae and juvenils of large species as M. minuta. On the other hand, foreign authors, judging to fact that genus did not mentioned (and till now do not mention) from regions where it inhabit (for example the North Sea) have a habit to identify Micronephthys as juvenils of large Nephtyidae. Dr.M.Petersen (pers. comm.) support such a habit. As result M. minuta for a long time count as endemic of Russian seas, but it is not mentioned in the Barents Sea (probably due losing while sieving).
Despite presence of overlapping in amplitudes of number of BS-1 between M. minuta s.str. and M. neotena (fig.3), we recommend this very character for using while identifying of numerous worms. This character can be easily seen under stereo compound microscope in each worm and does not require preparing slides, it allows to identify even living worms without any charm. Naturally this is the character of population, so single specimen of any can not be identify using this character only, but usually there is no problem to collect a lot due high density.
Total number of BS is also diagnostic for the species (fig. 4). This character relates with total number of segments and can be described by equation: BS=a+bS. Coefficients a and b are significally different in M. minuta s.str. (a = -5,3+2,4, b = 0,42+0,09) and M.neotena (a = 3,1+1,9, b = 0,22+0,06). From these equations can be seen that difference is greater in smaller worms with smaller total number of setigers.
The number of last BS is also diagnostic for the species (fig. 5). As previous one, this character connects with total number of segments and can be described by equation: BS=a+bS. Coefficients a and b are significally different in M. minuta s.str. (a = 3,6+1,5, b = 0,31+0,06) and M.neotena (a = 8,5+1,8, b = 0,21+0,06). From these equations can be seen the smaller total number of setigers the bigger difference.
Comparison of equations shows that total number of BS is more diagnostic than number of last BS, as characters difference is significant within greater intervals of setigers number.
It can be explain that total number of BS from one side (anterior) depends on species character "number of BS-1", which does not relate with total number of S, from the other side (posterior) depends on connecting with total number of specific character "number of last BS". As in M. minuta branchia start later and finish earlier, interspecific difference in total number of BS is more than in number of BS-1.
For more sure pharynx structure can be used. In M. minuta subterminal papillae from 20, rarely 18 rows. In M. neotena the same 20 rows present distally, but proximally part of rows joined or otherwise disappear and in the middle part only near 16 rows present.
M.neotena differs from M. minuta by presence of band of red- brownish spots in the middle of pigidium.
Prostomial shape can be used as additional character
in worms with pharynx inside. M. minuta has almost square
prostomium, M.neotena slightly elongated, rectangular.
Published data on biology of M. minuta in the White Sea, in reality mainly belong to M. neotena, but their biology probably are very similar. Judging to investigated collections, M. minuta replaces M. neotena deeper in the White Sea, the shallowest finds of M. minuta were made at 18 m, while the deepest ones of M. neotena - at 10 m. It allows to state for sure that M. neotena inhabits upper water mass (summer temperature up to 18ø'), while M. minuta s.str. - middle and deep ones (summer temperature up to 3-4ø'). This separation is very sharp: both species did not find in the same sample never, despite distance between places of collection sometimes did not exceed few kilometres. Moreover, as it will be shown below, settlement of juveniles can be in the same place, but only one species survive. So sharp separation allows with certainty to suppose which species really means in date on biology of "M. minuta" from the White Sea. "M. minuta" mentioned by Russians from other Russians seas, but as there were no any data on morphology, it is absolutely impossible to understand which species researcher deal with, so we did no use these data. Accordingly to (Lvova, 1981) population density of M. minuta at 40 m can achieve 1000 ex./m2; M.neotena - up to 8 000 ex./m2 at the depth of 5-7 m. Fournier, Pocklington (1984) reported species from the similar localities. There is no precise data on depth and temperature, they lay between 9-195 m and below 6+C respectively, species found below thermocline salinity 21-26 %o. As to M. neotena, in the White Sea this species inhabits on rich organic soft sediment from tidal zone. Population density up to 8000 sp./sq.m at depth 5-7 m (Lvova, 1981; Chivilev, 1981). Shallower the density decrease due decreasing density of juvenils (Lvova, 1981). We did not found species deeper 10 ¬. Along Atlantic coasts of America it inhabits the same depth: from tidal zone to 8-10 m (Noyes, 1980; Grenon, 1981).
M. neotena inhabits mainly upper (2.5-4 cm) sediment layer, where it actively moved with speed up to 20 cm/sec. (Lvova, 1981), in aquarium it can be seen swimming.
Everywhere within areal of M. neotena the winter temperature is 0ø' and lower, summer - up to 15-20ø' and more.
Species tolerates large salinity fluctuation: in the experiments worms from the White sea have limits of gametes and larvae toleration - 15-35, adults - 10-40 (Lvova, 1981). Along Atlantic coast of America it was found at salinity varies from 12 to normal (Noyes, 1980; Grenon, 1981).
Duration of life. Oogenesis begins in worms of 4 months old, while width is 0.5mm and more (length 1.45 mm and more) (Chivilev, 1983). Accordingly to other data (Lvova, 1981) worms become mature two years old, length 20-30 mm and 20-22 S. In the White Sea spawning portioned for near one month, begins at the end of May - June at 4-8°C (Lvova, 1976, 1978, 1981; Chivilev, 1981, 1983). Near Man, where much warmer spawning takes places in late March - beginning of May (Noyes, 1980). Each female produces from 250-400 oocytes (small worms, 6 mm length) up to 9000-10000 (large worms, 12-13 mm length) (Lvova, 1981). Oocytes 95 mk in diameter (Chivilev, 1981). Larval development have been described in details by Noyes (1980) and Lvova (1981). Pelagic development near Man finishes in June (Noyes, 1980). Analysis of size structure of population of Micronephthys spp. in the White Sea, at 5 m (Chivilev, 1981) allows to evaluate two peaks of settlement first one in the middle of summer, second - in the autumn. More exactly peak time can not be located: summer peak was absent March 04 and present June 21; autumn peak was absent September 15 and present October 10. the high of peaks is approximately the same. However, judging to data on local population oogenesis, spawning takes place only once, before summer peak (Chivilev, 1981). Thus the resource of autumn peak is unclear. It is very tempting to conclude that autumn peak belongs to M. minuta s.str. However our data show that the picture is more complicated. In July 27, 1991 in neighbouring population at 102 m depth M. minuta s.str. has oocytes in body cavity, but in July 24 at 18 m depth they were absent. Probably at different depth (in different water masses) spawning takes place in different time: in the middle water mass at the same time as spawning of M.neotena, whereas deeper takes place later, and this very part of the M. minuta population is the source of autumn peak of settlement of Micronephthys spp., while M. minuta from shallower depth has spawning at the same time as M.neotena.
Resuming biological data we can suppose, that both
investigated species are r-strategers, they seem to be widely
distributed, but are rare not only due losing petite worms while
collecting and wrong identification (as juveniles), but due low
density as well. They are abundant in marginal biotops only, where
species diversity is low and biotic relations are weak (the White
sea, estuaries etc.). Available data allow to suppose that M.
minuta - arctic species, M.neotena - north-boreal. The last
species seems to penetrate to the south in European waters up
to the North Sea, where it is replaced by similar morphologically
and biologically Micronephthys sp. nov., description of
it is in preparing. The last species should inhabits the same
localities as other two: widely distributed, but common only in
marginal biotops. Areals of all three species are shown in Figure
4. Despite the areals are overlapping, species are not co-existing.
Authors thank Dr. A.B.Tzetlin and anonymous reviewer for valuable advises, and Dr. A.I.Azovsky for help in mathematician aspects. The research have been supported by Russia Fund of Basic Researches, Government program "Biodiversity" and Soros foundation.
[Literature cited not included. Please see original text]