The Aplexa hypnorum coenosis in Zuid-Beveland

The

_Aplexa

_hypnorum

coenosis in Zuid-Beveland

by

C. den Hartog

(Hydrobiological Institute, division Delta-research, Yerseke, Holland).

METHODS:

The method of the surface-census for the

_{investigation}

of mollusc communities _was recommended

_by

MORZER-BRUYNS

₍₁₉₄₇₎

in his thesis. In accordance with

_this,

all molluscs from _{small square}

_sample

plots

are

collected,

separated

into

_living

and dead ones,

counted,

and

particularities,

if _any, are noted down. For the

_study

of water

molluscs this method is _not

very

adequate.

It is

possible,

however, to

obtain mud and sand

_samples

from the bottom

_by

means of a

grab.

In _{stagnant fresh} water the

Ekman-grab

especially

is _very useful. The

_{grab samples}

have to be sieved and

_subsequently

the obtained molluscs and other animalscan be

counted,

measured and

preserved.

The surface-census _as well _as the

grab-method

are useless when we want to survey the animals

living

between the

waterplants.

There-fore,

I

applied

another

quantitative

sampling

method,

which is

The

_ecological

demands of

_Aplexa

_{hypnorum (L.,}

_{1758) seem to} differ

_considerably

from those of mostotherwater snails. This

_species

lives almost

_always

_on

_light

_to

_moderately

_clayey

bottoms in small ditches which

_{completely dry}

out

during

thesummer. The numberof

species

whichareabletosurvive such circumstances is

_relatively

small. It is not

_{surprising, therefore,}

that the habitat in which A.

_hypnorum

lives is characterized

by

a poor,but very

outstanding

assemblage

of

species.

Thecommon occurrence of the

species

in the former island of Zuid-Beveland(DEN

HARTOG,

1963)

enabled me to starta

quanti-tative

investigation

on its

_biocoenosis,

and to compare my results with those of other

_{investigators.}

My

thanks are due to Messrs. L. DE WOLF and A.

_J.

_J.

SANDEE for their invaluable assistance with the fieldwork of this

investi-gation.

Basteria, Vol. 27, No. 3 en

4,

1963 50

already

in use

by English limnologists.

In an

homogeneous

biotope

it is

_possible

to obtain

quantitative

data

by

collecting

the animals

during

a determined time-unit. MANN (1955) used this method for

his

_quantitative

studies on leeches, and REYNOLDSON (1958)

applied

it for

sampling

triclads.

_Although

these

_{investigators}

recommended the time-unit of an hour,the luxuriance of the water-snail fauna in Zuid-Beveland is sufficientto allow the

investigator

30 minutes for

collecting.

There are several

_objections

which can be _put forward

_against

such a time-census. In the first

place

the method is not very exact.

Thenumbers collected within half an hour have no absolute value

as a result of the fact that the person of the collector becomes a

factor. His _{tempo is} not

always

thesame. In anopen

vegetation

he will _no doubt work _more

accurately

than, for

_example,

in a ditch

where he is hindered

_by

_high

reed or the

_sharp

stalks of

_sedges.

Weather conditions also

_play

a part. The effect of wind on the water surface is _very inconvenient for

collecting

in

_ponds

and

larger

water areas.

Cloudy

weather

greatly

influences the

trans-parency of the water.

Optimal

results can be obtained

only

when the weather is fair. _Moreover, the data obtained

by

different

collec-tors _may be

_{insufficiently}

_comparable

as a consequence of

differ-ences in

eye-sight, working

tempo, handiness and accuracy. Theresults which were obtained

during

the

investigation

of the life

cycle

of

Aplexa

hypnorum

(DEN

HARTOG & DE WOLF,

1962)

with the

help

of the time-census method are, however,

sufficiently

consistent for confidence in its

_{applicability}

for

_coenological

studies.

The

surveys

given

here have been

kept

as

homogeneous

as

possi

ble. The molluscs _were collected

by

Mr. L.DE WOLF while the other

creeping

invertebrateswere collected at thesame time

by

Mr. A.

_J.

J.

SANDEE. In table 1 the data_are

_{given only}

for those _groups for which _a reliable

_picture

is obtained. The

_species

which live in the mud and can not be collected

_by

the time-census

_method,

as e.g. tubificids and

micro-organisms

(Ostracoda,

Cladocera,

Copepoda,

rotifers and small

_{turbellarians),}

_areomitted. Water insects also_were

not studied. The

_description

of the

_Aplexa

coenosis, is thus far from

complete.

For a real

quantitative

inventory

of _a

_biocoenosis,

the habitat hastobe

sampled

using

several methods andateamof

special-ists must be available for the identification of the material. This

_is,

however, a

Utopian

scheme. For

practical

reasons

coenological

studies haveto be limited to one or a few taxonomic _{groups with}

FAUNISTIC COMPOSITION:

The

_Aplexa

coenosis _was

_surveyed

in 12 localities in the

_period

from

May

29th to

June

5th, 1962. The results _are

_given

in Table 1 on p.

52/53.

The nomenclatureof HEUKELS & VAN OOSTSTROOM, Flora _van

_Nederland,

edition

_14,

_1956,

is followed for botanical

names.

In the 12 localities a total of 3422

living

molluscs was counted.

The bulk consists of 3

_{species only,}

_namely

Aplexa

hypnorum

with 1125

_specimens

or

32.9%,

Lymnaea

ovata with 1641

specimens

or

48.0%,

and

Lymnaea

palustris

with 606

_specimens

or 17.7%.

Only

50

_specimens

_or

_1.4%

of the total number

_belong

to other

species.

The average number of

_species

in a

_sample

is 3.67.

The coenosis is well-characterized

_{by Aplexa hypnorum,}

which is

not

only

faithfulto the

biotope

of the

_temporarily

dry

ditches,

but is also oneof the dominant snails in it. Thetwo

species

of

_Lymnaea,

L. ovata and L.

palustris,

which also dominate in this

biotope,

are

ubiquitous

and _occur in all

_eutrophic

fresh and

oligohaline, standing

and

_{slowly running}

_waters.

_They

_are useless for the characterization of the

community. Planorbis leucostoma,

although

not common in

Zuid-Beveland,

seems tobe afaithful

species

of the

Aplexa

commun-ity

as it has been found there

_{only together}

with A.

hypnorum.

As the

temporarily

dry

ditches and the

permanently

water-con-taining

ditches _are

_mostly

_{in open communication with each} other, an

exchange

of

_species

_{may take}

_place.

_{This may}

_explain

the occasional

occurrence, in the

Aplexa

coenosis of _some

species

which are not

resistantto a more or less

protracted drought,

e.g. Planorbis

planor-bis,

Physa

fontinalis, Lymnaea

stagnalis,

and

_Sphaerium

_corneum.

On the other

_hand,

afew

_specimens

of

_Aplexa

_hypnorum

may

some-times be found in the

Lymnaea ovata-Planorbis

vortex coenosis.

Among

the other animal _groups the small

_{flatworms, belonging}

to the

Neorhabdocoela, Dalyellia viridis

and Phaenocora

_unipunctata

may be

regarded

as faithful

species

of the

Aplexa

coenosis. These

small flatworms were found sometimes without A.

hypnorum,

but

always

in similar habitats.

Further

companion

species belong

to themore

ubiquitous aquatic

animals. Asellus

_Chlorohydra

_{aquaticus, Dugesia}

lugubris

and

viridis-sima_are the _more

_{frequent species}

and the_occurrence of

Glossipho-nia

_complanata,

_Polycelis

tenuis and Dendrocoelum lacteum is

spor-adic.

52 Basteria, Vol. 27, No. 3 en

4,

1963

Aplexa hypnorum (L.)

Planorbis leucostoma Mill.

Lymnaea ovata (Drap.) Lymnaeapalustris (Mull.) Planorbisplanorbis (L.) Lymnaea stagnalis (L.) Physafontinalis (L.) Sphaerium corneum (L.)

Total number of specimens

Turbellaria:

Dugesia lugubris (Schmidt) Polycelis tenuis Ijima

Dendrocoelum lacteum (O.F.M.) Phaenocora unipunctata (Oerst.) Dalyellia viridis (G. Shaw)

Hirudinea:

Glossiphonia complanata (L.) Haemopis sanguisuga L.

Crustacea

Asellus aquaticus (L.)

Gammarus duebeni Lillj

Hydrozoa

Chlorohydra viridissima (Pall.) Pelmatohydra oligactis (Pall.)

Fishes: Pungitius pungitius (L.) 1 2 3 4 5 6 789 10 11 12 T 169 273 146 13 80 14 37 25 66123 117 62 1125 38 646 395 47 8 80113 56 16 102 96 44 1641 — — 5 187 80101 191 16 3 2 2 19 606 1 7 — 2— — — — 10 4 4 _ _ _ i__ _ _ _ i 212 919547 255 179 195 34699 98 227 220 1253422 X _ l _ _ _ 6— 4 — — — — 24 36 — — — — 1 — 38 — — — — 12 — 79 ₂₃ 17 — —±75

---12-6---

2--16---1----16 1 — — 1 8 — 1— 5 14 -_ 7 — — ___ + — — + + + +—₊ — _{+ +} + + + + + + + — TABLE 1. coenosis in Zuid-Beveland

Aplexa

hypnorum

The 1 2 3 4 5 6 7 8 9 10 11 12 T Aplexa hypnorum (L.) 169 273 146 13 80 14 37 25 66 123 117 62 1125 Planorbis leucostoma Mill. — — — — — — 5 — 13 — — — 18

Lymnaea ovata (Drap.) 38 646 395 47 8 80 113 56 16 102 96 44 1641

Lymnaea palustris (Müll.) — — 5 187 80 101 191 16 3 2 2 19 606

Planorbis planorbis (L.) 1 — — 7 — — — 2— — — — 10

Lymnaea stagnalis (L.) 4 _ _ 4

Physa fontinalis (L.) 1 — — 1

Sphaerium corneum (L.) — — 1 — 11 — — 5 — 17

Total number of specimens 212919 547255 179 195 346 99 98 227 220 1253422

Turbellaria:

Dugesia lugubris (Schmidt) 1 — 1 — — — 6— 4 — — —

Polycelis tenuis Ijima — — — — — — — 8— — — —

Dendrocoelum lacteum (O.F.M.) — — — — — — 13 — — —

Phaenocora unipunctata (Oerst.) — 24 36 — — — — 1 — 38 — —

Dalyellia viridis (G. Shaw) — — 12 — _{79 23} 17 — —±75

Hirudinea:

Ghssiphonia complanata (L.) — — — 12 — 6 — — —

Haemopis sanguisuga L. 2 — — 26 — — — 1— — — —

Crustacea :

Asellus aquaticus (L.) 16 1 1 8 1 5 14 —

Gammarus duebeni Lillj. — 7

Hydrozoa:

Chlorobydra viridissima (Pall.) + — — ₊ + _{+ +}—₊ — _{+ +}

Pelmatokydra oligactis (Pall.) + — — — —

— — —

Fishes:

LEGENDA

1. Goes, ditch nearthe Television tower. _{Sparse vegetation}with Phrag-mites communis,Agrostis stoloniferaand Glyceria fluitans;29-V-1962.

2. Wolphaartsdijk, ditch _alongthe Nieuwe Veerweg near the junction

withthe Bolleweg. Closed _vegetation ofPhragmites communis; 29-V-1962.

3. Nieuwdorp, ditch along the Noord-Kraayertse Weg. Scattered vege-tation of Scirpus maritimus, Alopecurus geniculatus, Equisetum palustre and Lysimachia nummularia; 30-V-1962.

4. Ditch _along the main road Heinkenszand - 's Heerenbroek. Closed

vegetation of Agrostis stolonifera and Glyceria fluitans, with

inter-spersed Phragmitescommunis, Juncusarticulatus, Equisetumfluviatile, Galium palustre, Mentha aquatica, Lysimachia nummularia and Callitriche obtusangula; 30-V-1962.

5. Ovezande,ditch in the south-western part of the Louisepolder.

Vege-tation of_{Polygonum amphibium, Agrostis stolonifera,} Mentha

aqua-tica, Lysimachia nummularia and Scirpus lacustris _ssp. glaucus;

30-V-1962.

6. _{Hoedekenskerke,} ditch in the _{Oosterzwakepolder, opposite} the Nieu-we Hoondertpolder. Open vegetation of Alisma plantago-aquatica, Equisetum palustre, Mentha aquatica, Lemna minor and L. gibba; 4-VI-1962.

7. Kapelle, ditch _along the _Hillewerfweg near the junction with the Plasweg. Closed vegetationof_Phragmites communis interspersedwith some plants ofRanunculus sceleratus, R. baudotii and Callitriche obtusangula;29-V-1962.

8. Biezelinge,ditch between „Veldzicht" and „Smokkelhoek". Vegetation

ofPhragmites communis _(dominant), Phalaris _arundinacea, Agrostis stolonifera and Callitricheobtusangula; 4-VI-1962.

9. Kapelle, den Bok, ditch. Vegetation of Phragmites communis and

Typhalatifolia; 4-VI-1962.

10.

_Phragmites

Yerseke,ditch in the Molenpolder. Vegetation of

commu-nis; l-VI-1962 (Den Hartog & De Wolf, 1962).

11. _{Krabbendijke,} ditch along the _railway in the Nieuw Krabbendijkse polder. Scattered _{plant growth} ofPhragmites communis, Mentha

aquatica,NasturtiumVeronica catenata, Rumex crispus, cf.

micro-phyllum,Lysimachia nummularia, Callitriche obtusangula,

Ranun-culus baudotiiand a closed _layer of the mossAcrocladium cuspidatum; 5-VI-1962.

12. Ditch alongthe Frederikadijk east of Krabbendijke. Open vegetation ofPhragmitescommunis,

Nasturtium

cf.

microphyllum

and Ranunculus baudotii; 5-VI-1962.

Basteria,

Vol.

_27,

No. 3 en

4,

1963

54

Haemopis sanguisuga

tolerates desiccation _very

_well,

but _this spe-cies is more common

along

larger

bodies of water. No

doubt,

this very voracious leech preys on the snails in the

Aplexa

coenosis. The

stickleback

Pungitius

pungitius

is a common inhabitant of the

Aplexa ditches, but

it leaves thembefore the

beginning

of the sum-mer

drought.

It is remarkable that the brackish-water fauna is so

_scantily

repre-sented in the

Aplexa

habitats. Gammarus duebenihas been found

only

occasionally,

while _{among the}

_{micro-organisms}

collected and identified

_by

me,

only

Macrostomum hamatum Luther may be

regard-ed as a brackish-water

species.

RELATIONSHIPBETWEEN THEAPLEXA COENOSISAND THE VEGETATION:

Although

no

phytocoenological

surveys were

made,

it is still clear from

my notes on the

accompanying

vegetation,

that the

Aplexa-coenosis is not bound to_any

special

plant community.

In 5 of the 12 localities

_vegetation

was scanty, inanother 5

Phrag-mites communis was the dominant

species.

In the

remaining

two

localities

_Agrostis

_stolonifera

and

_Polygonum

_amphibium

were

re-spectively

the main

_species.

The

_vegetations

from these localities

were too sparse tobe identified with certain

plant associations,

but the

accompanying

plants

belong

to 3 _groups:

1.

_Species

of the

_{Agropyro-Rumicion,}

the alliance which is cha-racteristic for stress zones between

contrasting regimes

of envi-ronment, ih this case for the transition zonebetween wetand

dry

(VAN LEEUWEN, 1958). Some of these

plants

are able to

withstand

_protracted

inundations

_(Agrostis

_stolonifera,

Polygo-num

_amphibium,

Mentha

aquatica),

while others can colonize these

_periodically

dry

places

by

creeping

shoots (

Lysimachia

nummularia).

The

_vegetations

of the _surveys 3,

4,

5, 8 and 11

may be considered to be

_examples

of this alliance.

2.

Species

of the

_{Glycerieto-Sparganion,}

the alliance of the

edges

of

ditches,

in which the water flows _very

gently

(Glyceria

fluitans,

Alisma

_{plantago-aquatica, Nasturtium}

cf.

_{microphyllum).}

3.

Species

of the

_{Callitricho-Batrachion,}

the alliance of

_amphibious

water

plants,

which is characteristic for shallow and

tempor-arily

dry

ponds

and ditches

_(Ranunculus

_baudotii,

Callitriche

obtusangula).

Aplexa hypnorum

doesnot occur in luxuriant

_waterplant

commun-ities nor in

well-developed

communitiesof the

Agropyro-Rumicion.

In fact the

species

inhabits those ditches whose bottoms are

just

the lower limit of the

_{Agropyro-Rumicion}

and

just

the upper limit of the Callitricho-Batrachion. The habitat of the

Aplexa

coenosis is

on the _average

_just

too wet for the

Agropyro-Rumicion

and

re-sults ina

fragmentary

development

of this

alliance,

as

only

itsmost

eurytope

representatives

intrude into the ditches. On the other

hand,

the bottom is

_mostly

_dry

for too

long

a time to enable successful

establishment of the Callitricho-Batrachion.

Moreover,

the

develop-ment of the

_vegetations

is

_gready

influenced

by

the

_{yearly cleaning}

of the ditches.

SOME RECORDSOF THE APLEXA COENOSIS IN OTHER REGIONS:

The

_aquatic

mollusc fauna of Zuid-Beveland is _poor, and the coexistence of

Aplexa

hypnorum

and Planorbis leucostoma could be a local

phenomenon.

From observations made in other areas and

from records in literature it _appears, however,that both

_species

have been found often

_together.

The results of the

investigations

made in Zuid-Beveland may

be

considered tobe

representative

for the

south-western part of the Netherlands.

In the island of Goeree-Overflakkee A.

_hypnorum,

P. leucostoma and the flatworrfi Phaenocora

_unipunctata

have been found

together

in a

_{temporarily dry}

ditch _near Dirksland. HENRARD

₍₁₉₄₆₎

record-ed A.

_hypnorum

and P. leucostoma as

_concurring

in Voorne, and

according

to him this combination

may be found in several

places

in the Netherlands. In the island of Schouwen I found

only

P.

leuco-stoma in

temporarily

dry ditches,

and could not find A.

hypnorum,

although

this

_species

_was recorded

_by

KUIPER

(1944).

In a moist

dune

valley

on this island BUTOT

_(n.p.)

_recently

discovered a few

dead

_specimens

of A.

hypnorum

together

with P. leucostoma, some

of whichwere alive.

On the Pleistocene soils in the eastern and southern parts of the country the combination of A.

hypnorum

and P. leucostoma is also

widely

distributed,

but there

_Lymnaea

_glabra

(Mull.) seems to

join

the

_community,

and also some Pisidium

species

may occur. KUIPER

(1949)

found A.

_hypnorum

and L.

glabra

together

in a

drainage-furrow near Duizel

(province

of Noord-Brabant). In the

surround-ings

of the

_Mantinger

Bos in the

_province

of Drente A.

_hypnorum,

P.

leucostoma,

L.

glabra

and a few other

species

were recorded as

occurring together

(KUIPER, 1952).Another

_example

of the

Basteria, Vol. _27, No. 3 en

4,

1963 56

from De

_Klomp

_(province

of

_Gelderland).

It

is, however,

question-able whether L.

_glabra

is atrue memberof the

_Aplexa

coenosis. The life

_cycles

of A.

_hypnorum

and L.

_glabra

are in some respects very

similar,

but the patterns of distribution in the Netherlands are very different.

_Moreover,

in the

_regions

where both

_species

occur,

they

arefoundmore often apart than

together.

The coexistence of A.

_hypnorum

and P. leucostoma has been ob-served also in other

_European

countries. LAIS

(1926)

mentioned someinstances from the north-eastern_partof France, NELSON

(1880)

and BOYCOTT

₍₁₉₃₆₎

recorded the combinationfrom Great Britain and HAESSLEIN

_(1956,

_I960)

found both

species

often

_together

in

Wiirttemberg

(South

Germany).

Although

the coexistence of these

species

isacommon

feature,

the

_ecological

tolerance of P. leucostoma is

_considerably

wider than that of A.

_hypnorum,

and it occurs as well

in habitats where the latternever hasbeenfound. It is notuncommon,

for

example,

in small eulittoralcreeklets in the fresh-water tidal area

of the rivers Rhine and Meuse

(Biesbosch).

So P. leucostoma is

_only

locally

faithful _to the

Aplexa

coenosis.

_However,

_regionally

it has

to be

_regarded

as a characteristic

species

of temporary and unstable

aquatic

habitats. HAESSLEIN

₍₁₉₅₆₎

described from the

_Nordlinger

Ries in

_Wiirttemburg

aValvata

pulchella

_association,

which he

con-sidered tobe characteristic for river

_valleys

and lowland marshes. He

regarded

Valvata

_pulchella

Studer

₍₌

V. macrostoma

Steenbuch)

Aplexa hypnorum

and

Segmentina

nitida(Miill.) as faithful

species

of this

_community,

and

_according

to his table Planorbis leucostoma is

extremely

abundantin it.

_Although

this Valvata

_pulchella-

association is

_certainly

_not identical with the

Aplexa

coenosis,

described from the south-western part of the

Netherlands,

both communities_are

close-ly

related in

ecological

respect. The main difference in faunistic

com-position,

is the

_high

abundance of Valvata

_pulchella,

which in the Netherlandsis_rare and limitedtoafew localities in the

proximity

of

the rivers Rhine and Waal. Other differences_are the abundance of

some

Sphaeriidae

and the constant occurrence of

Bithynia

tentaculata (L.) and Planorbis contortus (L.). These features indicate that the

dry period

must be rather short.

In the

_region

of the

_Pegnitz

HAESSLEIN

_(I960)

found

Aplexa

hypnorum

and Planorbis leucostoma also _as members of theRadix peregra association,which he consideredtobe characteristic for

mount-ainous marshes. This association does _{not seem to} be

very homo-geneous. Several surveys of it

published

by

HAESSLEIN

_(I960)

show

only

slight

differences with the _surveys made

_by

me of the

_Aplexa

THE APLEXA COENOSIS DURING THESUMMER DROUGHT:

The surveys of the

Aplexa

habitats in Zuid-Bevelandwere made

just

before the

_beginning

of the summer

drought.

When the

_dry

period

starts,

Lymnaea

ovata, L.

palustris

and A.

hypnorum

assemble in the lowest

places

of the

ditches,

and hide under all kinds of

ob-jects, pressing

the aperture of the shell closeto the substrate.

_Many

of them die.

I

kept

some A.

hypnorum

in a small water-filled

tank,

which was

allowed to

dry

out in the _sun. After 3 weeks

_drought

50% of the

specimens

were _yet

_{alive, especially}

the smaller ones. This

observa-tion

agrees very well with the result of field

investigations

carried out in the

_Molenpolder

atYerseke _(DEN HARTOG &DE WOLF,

1962).

In I960 a numberof small animals survived a

drought

period

of _ca. 2 months,but not one

large specimen

was sosuccessful. In 1961and

1962,

when the

_drought

lasted for

_respectively

3 and 4 months not a

single specimen

survived. Inever observed that the animals

dug

them-selves in the bottom of the

ditch,

as has been recorded

_by

VAN

BENT-HEM

JUTTING

(1933,

p. 190), FROMMING

(1956,

p.

152),

ADAM

(I960,

p.

163)

and BUTOT

(1962,

p. 120). CLESSIN (1872)

thought

that

perhaps

the animals hid in the

mud,

as he could not find them

when the ditcheswere

dry.

I have seen, however, several

specimens

which had

_slightly

sunk into the soft mud

_surface,

but _{not one}

Aplexa

was found

by sifting

mud

_samples.

I

think, therefore,

that the

species

overcomes the

_{drought period}

in the form of _{its eggs.}

Among

L. ovata also _many

_specimens

become victims of the

drought.

In Yerseke a month after the

beginning

of the

dry period

I

collected a

large

number of L. ovata. It

_appeared

that of these

_only

10%

was still

alive,

and

_they

_were

_mostly

the smaller individuals.

However,

this

_species

isabletosurvivea

drought period

of 4months, since it_waspresent

_during

the whole

_{drought period}

from the middle of

_June

_tothe end of October 1962._{The young}

_specimens

wereseen

actively

creeping

and

_climbing

_against

the

plants,

even when

_they

were

exposed

tosunshine.

Young

L.

_palustris

alsocan survive a

drought

of 4 months,but the losses in individualsseem even

larger

than in L.ovata.

P. leucostoma is rare in Zuid-Beveland. I haveno observations on

the method of aestivation in this

_species.

It seems toclose its

apert-urewitha mucose membrane (FROMMING,

1956).

During

the

drought

the

_{hydrobiotope changes}

intoa land

_biotope,

enabling

land animals to colonize the

dry

bottoms of the ditches. Near Yerseke I noted the

_{following slugs}

and snails in the

_{dry Aplexa}

Basteria,

Vol.

27,

No. 3 en

4,

1963 58

biotope:

Limax maximus

_L.,

Deroceras reticulatum

_(Mull.),

D. laeve

(Miill),

Succinea cf.

_elegans

_Risso,

_Cochlicopa

lubrica

_{(Mull.), Cepaea}

nemoralis(L.) and Arianta arbustorum(L.).

ACOMMUNITY OF OUTCASTS?

In his work on the

ecology

of the British fresh-water molluscs

BOYCOTT

₍₁₉₃₆₎

_paid

much attentionto the

_ecological

_{group of the}

species

which are

always

or

nearly

always

found in habitats which

seem very unfavourable to

aquatic

animals. He

_thought

that these

species

had been driven

by

pressure of

competition

from the

_"good"

localities into

_{"ecological slums",}

i.e. small bodies of water which

_dry

up in summer, or which are for other reasons not very suitableto

aquatic

molluscs. He gaveas

examples Lymnaea

glabra

andPlanorbis leucostoma

_(sub

nomine P.

_spirorbis).

In free nature L.

_glabra

has been found

_only

in

_{temporarily dry,}

soft-water localities. Inan

_aquarium

it _grows and

reproduces

in hard water as well.

According

to

BOY-COTT P. leucostoma has "a distinct addictionto

drying ponds,

marshes and other bad

_places",

while it is absent from _permanent

_aquatic

habitats.

_However,

in

_larger

_places

where

_competition

between the

species

isnot so severeas is thecasein small

ponds,

P. leucostoma may

be foundalso, _e.g.

along

the banks of lakes. _BOYCOTT,

_supposing

that

Aplexa hypnorum

(sub

nomine

Physa

hypnorum)

could tolerate bad conditions

_although

it

preferred

better_{circumstances,}believed that this

species

in fact did not

belong

to the above group. Here,

according

to me, he made an error of

thought.

The

_species

which are driven into the slums

_{by competitive}

seclusion do_not

_{necessarily prefer}

such

places

as ahabitat. In many cases thereverse will be thecase. These

species

can maintain themselves

_only

_by

their

_capacity

to live under unfavourablecircumstances, where

_they

_are not

subjected

to compe-tition, or areabletocompete withsuccess. The "bad

places"

must be seen as

refugia

for these

_species.

_Although

these

_species

have under natural circumstances

_certainly

their

_optimum

in such stations, it is

questionable

whether

_{rhey prefer}

them.

The

_competition

_hypothesis

of BOYCOTT

(1936)

is not

proved.

He did notmentioneven which

_species

exercise the

_"competitive

press-ure". It is known from

_experience

_{with several taxonomical groups that}

competition

has to be seen as an

_equilibrium

between two

ecologic-ally equivalent

species.

The

_{competing species}

are thus

_mostly

con-geners, or

species

with

very similar

adaptations.

They

do not have

necessarily

to be hostile toeach other but

availability

of

_food,

rate

the factors that

_mostly

decide which

_species

will

finally

survive in

a certain

biotope.

Thus

competition

between two

species

is not a

struggle,

which is

_pursued

with

_changing

success, but in contrast it

proceeds

according

to fixed laws and the result in _every

_place

is

usually

a

foregone

conclusion. When L.

glabra

and P. leucostomaare considered in the

light

of the above

_{generalizations}

it

appears that

they

mustbe in

_competition

_respectively

with other

_Lymnaea

_species

and with other small Planorbis

species.

Further it is

improbable

that A.

_hypnorum

_{competes with}

Lymnaea

species,

as it

usually

coexists with them.

_However,

_species

of rhe _genus

_Physa

_may exercise

com-petitive

pressure. In this connection it is

_interesting

that BOYCOTT

(1936)

did not

give

any record of rhe coexistence of A.

hypnorum

and

_{Physa fontinalis, although}

he recorded A.

_hypnorum

from

sever-al

_"good

_places"

with

_species

combinations which maybe referred to to the

_Lymnaea

ovata —Planorbis vortex coenosis and the

Lymnaea

stagnalis

-Planorbis corneus coenosis (HONER,

1963).

In Zuid-Beveland Ionce found two

specimens

of A.

hypnorum

in a

perma-nently

water-containing

ditch _near

_Ovezande,

_together

with

num-erous Planorbis vortex, P.

planorbis, Lymnaea

ovataand L.

_palustris,

thus in the

_Lymnaea

ovata —Planorbis vortex coenosis. These

speci-mensof A.

hypnorum

were from a

quantitative point

of view not

important,

andcame no doubt from an

adjacent

Aplexa

coenosis.

The remark of BOYCOTT

(1936)

that the stress of

competition

in

spacious

habitats should be less than in small habitats also has tobe

proved.

The occasional occurrence of P. leucostoma and A.

hyp-norum

along

the banks of lakes _{can not} be

_regarded

_as

_proof,

since

a lake is not a

homogeneous

habitat,

but consists of several sub-habitats. I

myself

have found A.

hypnorum

only

once

along

the banks of a

large

body

of water, viz.

along

the Groote Gat _near

Oostburg

in Zeeuwsch-Vlaanderen. This brackish-water broad shows

anannual

_salinity

fluctuation from 0.5 to 7.8°/oo CI' and toconsider itas a

single

habitat is a seriousmistake. In the broad itself A.

hyp-norum is

absent,

but thenarrow

strip

between the wide

_reed-fringe

and the

_dike,

which insummer is

_completely

_dry,

is a suitable habi-tat for the

_species.

_During

the time that the

_strip

is

submerged,

salinity

is

_usually

low

_enough

to make theoccurrence of A.

hypno-rum

possible.

The

_Aplexa

coenosis consists of a number of

ubiquitous

_species,

tolerantto unfavourable circumstances and resistant to a

protracted

drought

period.

According

to the

_{argumentation}

of BOYCOTT the faithful

_species

of the coenosis have to be

_regarded

as outcasts, as

they

are driven in this extreme habitat

owing

to their

inferiority

in the

_competition

with other

_species.

_However,

I wonder whether

Basteria, Vol. _27, No. 3 en

4,

1963 60

competition

is the

only

factor

_determining

the occurrence of A.

hypnorum

in

_temporarily

_dry

ditches. In Zuid-Beveland several

shallow,

permanently

wet fresh-water ditches occur and these are

only

populated

by

Lymnaea

ovata;

although

A.

hypnorum

would be free from

_competitive

stress in these

ditches,

I did notsucceed in

finding

even one

specimen.

There _{are some reasons} to _supposethat A.

_hypnorum

shows some

preference

for the

_temporarily

_dry

habitats. In the first

_place

the

species

lives

_just

at the lower limit of the

_{Agropyro-Rumicion,}

and

just

at the upper limit of

Callitricho-Batrachion,

thus it has a very

narrow vertical range. Its absence from the Callitricho-Batrachion

can not be

explained

by

the

_competition

factor,

as in these vegeta-tions

_only

L. ovata and L.

palustris

occur.

Furthermore, NEKRASSOW (1929) found that the lamellation of the spawns and

egg-membranes

of A.

hypnorum

is muchmore

compli-cated than in the spawns of

Physa fontinalis

He

_regarded

this

feat-ure as an

adaptation

to the environment, as it would increase the

resistance of _{the eggs}

_against

desiccation.

A

study

of the life

cycle

of A.

_hypnorum

atYerseke (DEN HARTOG

& DE WOLF,

1962)

showed that

_species

is _very well

_adapted

to the

annual

_cycle

of the habitat, as each summer the

_original

_generation

falls victim to the extremes of the environment, but the

_species

survives in the _new

_generation

_{of eggs.}

Therefore,

it seemsto me, that A.

hypnorum

may notbe

regarded

as anoutcast, which is driven in unfavourable stations

_by

compe-tition On _{the contrary, it}

_prefers

the

_"ecological

slums". The

_Aplexa

coenosis is notan

assemblage

of

species

which have been driven

to-gether

as the result of

competitive

seclusion,

but has to be seen as a

community

which is

well-adapted

to unfavourable environmental circumstances.

REFERENCES

ADAM,

W., I960.

_Mollusques

I.

_Mollusques

terrestres et dulcicoles.

Faune de

Belgique,

p. 162-163.

BENTHEM

_{JUTTING, T.,}

_VAN, 1933. Mollusca

₍₁₎

A.

_Gastropoda

proso-branchia et

pulmonata.

Fauna _van Nederland vol. 7, p. 189-191.

BOYCOTT,

A.

_E.,

1936.The habitats of fresh-water mollusca in Britain.

J.

Anim. Ecol. vol.

_5,

p. 116-186.

BUTOT, L.

_{J. M.,}

1962.

Malacologisch

onderzoek _op Goeree.

Jaarb.

Wetensch. Gen. Goeree-Overflakkee

_1961,

p. 142-172. CLESSIN,

S.,

1872. Die Lebensweise der

_Physa

_hypnorum.

_Korresp.

Blatt Zool.-mineral. Ver.

_Regensburg

vol.

_26,

_p. 170-171.

FROMMING,

E., 1956.

Biologie

der

_{mitteleuropaischen}

Siisswasser-schnecken._Berlin,

p. 1-313.

HARTOG, C. DEN, 1963.Thedistributionof the snail

_{Aplexa hypnorum}

in Zuid-Beveland in relation to soil and

_salinity.

Basteria vol. _27,

p. 8-17.

HARTOG, C. DEN, &L. DE WOLF, 1962. The life

_cycle

of the water

snail

_{Aplexa hypnorum.}

Basteria vol.

26,

p.61-72.

HAESSLEIN,

L., 1956.Mollusken und

_{Molluskengesellschaften}

der

Ge-wasser der

Nordlinger

Ries.

Jb.

Ver. Vaterl. Naturk.

Wiirt-temberg,

vol._{Ill, p.} 174-199.

— I960.Weichtierfauna derLandschaften_ander

Pegnitz.

Abh.

Naturhist. Ges.

_Niirnberg,

vol. 29 nr.

2,

p. 1-148.

HENRARD, J.

B., 1946.

Bijdrage

tot dekennis der molluskenfaunavan

Oostvoorne. Basteria vol. 10, p. 25-32.

HONER,

M.R., 1963.Freshwater larval Trematodes in the Netherlands:

a

synecological

study

of their occurrence. Thesis, Utrecht,

p. 1-116.

KuiPER,

J.

G.

_J.,

1944.

_Bijdrage

totde kennisvande niet-mariene

mol-luskenvan de Provincie Zeeland. Basteria vol. 9, p. 1-29.

— 1949- Enkele vondsten_vanzeldzame slakken in

Limburg

_en

Noord-Brabant.

Corresp.

bl. Ned. Mai. Ver. no. 31, p. 216-219.

_{(mimeographed).}

— 1952. De excursie _naar het

Mantinger

bos (20 mei 1951).

Corresp.

bl. Ned. Mai. Ver. no.

43,

p. 376-377.

(mimeo-graphed).

— 1955.

Lymnaea glabra (Miiller). Corresp.

bl. Ned. Mai. Ver.

Basteria, Vol.

_27,

No. 3 en

4,

1963

62

LAIS, R.,

1926. ZurMolluskenfaunades Gebieteszwischen Maasund Mosel. Arch. Molluskenk. vol. 58, p.25-36.

LEEXJWEN, C. _{G. VAN,} 1958.

Enige opmerkingen

over het

Agropyro-Rumicion

_crispi

Nordh. '40 in Nederland.

_Corresp.

bl. Flor.

Vegetatie

Onderz. Nederlandno. 11, p. 117-123

(mimeo-graphed).

MANN, K.

_H.,

1955. The

_ecology

of the British fresh-water leeches.

J.

Anim. Ecol. vol. 24,_p.

98-119-MORZER BRUYNS, M. F., 1947. Over

_{levensgemeenschappen.}

_Thesis,

Utrecht,p. 1-195

NEKRASSOW,

A. D., 1929.

Vergleichende

Morphologie

der Laiche von

Siisswassergastropoden.

Z.

_Morph.

Okol. Tiere vol.3, p. 1-35. NELSON, W., 1880. On the association ofLimnaea glabra,

Physa

hyp-norum and Planorbis

spirorbis. Journ.

of Conch, vol. 3, p. 115-116.

REYNOLDSON,T. _B., 1958. The

_{quantitative ecology}

of

_{lake-dwelling}

Triclads in northern Britain. Oikos vol. _{9, p.}94-138.

Samenvatting

De

Aplexa

coenose, welke karakteristiek is voorin dezomer

droog-vallende

_slootjes

_op min of _meer

_kleihoudende

_bodems,

werd be-schreven aan de hand van 12 opnamen,

gemaakt

op

Zuid-Beveland

volgens

de

_{tijdcensus-methode.}

De coenose wordt

hoofdzakelijk

sa-mengesteld

door drie

_{soorten waterslakken,}

de kensoort

Aplexa

hypnorum (L.)

en de beide

_{ubiquisten Lymnaea}

ovata

(Drap.)

en L.

palustris

(Müll.). Een tweede locaal bruikbare kensoort Planorbis leucostoma Mill. is slechts zeer spaarzaam

vertegenwoordigd

op Zuid-Beveland. Twee andere kensoorten _{konden worden aangewezen onder} de

_{vertegenwoordigers}

_van andere

diergroepen,

n.l. de beide

platwor-men Phaenocora

unipunctata

(Oerst.)

en

Dalyellia viridis (G.

Shaw).

Decoenoseis

_{wijd verspreid}

in

_Nederland,

alsmede in West- en

Mid-den-Europa

ende Britse eilanden.

Er werd

geen

binding

waargenomen van de

Aplexa- coenose

aan

enige

plantengemeenschap.

De coenose komt slechts daar tot

ont-wikkeling,

waar de slootbodem

juist

met de

benedengrens

van het

Agropyro-Rumicion

samenvalt,

maar ook te

lang droogvalt,

om een

behoorlijke ontwikkeling

van een tot het Callitricho-Batrachion beho-rende

_{waterplantengemeenschap}

_mogelijk

temaken.

In het lichtvande

concurrentie-hypothese

vanBOYCOTT

(1936)

zou

de

_Aplexa

_{coenose opgevat} moeten worden als _een

_gezelschap

van

weinig kieskeurige

ubiquisten,

die resistent

zijn

tegeneen voor

water-slakken uiterst

_{ongunstige milieufactor}

als

_uitdroging.

De

kensoor-ten zouden slechts tot deze

_ongunstige

_biotoop

_{beperkt zijn,}

omdat

ze zich elders ten

gevolge

van concurrentiemet anderesoorten niet

kunnen handhaven. Het is echterzeer

waarschijnlijk,

dat A.

hypnorum

een zekere

preferentie

heeft voor

langdurig

droogvallende slootjes,

want ze ontbreekt in het

Callitricho-Batrachion,

waar de

milieu-omstandigheden

minderextreem

zijn

enwaar

ze evenmin

concurrentie metanderesoorten behoeftte vrezen. De

levenscyclus

van dezesoort

is zeer

goed

aangepast aan de

bijzondere

milieu-omstandigheden

en

de

_eipakketjes

vertonen

speciale lamellaire

structuren, die als

adapta-ties aaneen

langdurige droogteperiode

kunnen wordenopgevat.