Absfract The intertidal spider Desis marina lives in holdfasts of the kelp Durvillaea antarctica at Kean Point, Kaikoura, New Zealand. It occupies holdfasts from ...
New ZealandJournalof Zoology,1987,Vol.14: 29-42 O Crowncopyright1987 030| -4223I 87/ r 40t-0029$2.50/0
29
Popufation structureand use of Durvillaeaantarcticaholdfasts by the intertidalspider Desismarina(Araneae:Desidae) C. L. McLAY T. L. HAYWARD Departmentof Zoology University of Canterbury, Christchurch, New Zealand.
INTRODUCTION
The spider Desismarina (Hector 1877)is an unusual speciesliving in the intertidal zonearoundthe coastlinesof the North and South Islandsand also on Stewartand Chatham Islands.D. marina is the sole New Zealand representativeof the ill-defined Family Desidaewhich includesl4 speciesrecorded The intertidal spider Desis marina from the coasts of southern and easternAfrica, Absfract India, easternAsia, Japan,islandsof the lives in holdfastsof the kelp Durvillaea antarctica southern Pacific Ocean(includingAustraliaand New South at Kean Point, Kaikoura,New Zealand.It occupies Zealand),and the GalapagosIslands (Roth 1967; holdfastsfrom *20 to -71 cm below Mean Sea Roth & Brown 1976).Members of this family do Level (MSL). All holdfastsfrom < 100 to > 1000 not differ markedly from their terrestrialrelatives cm2 were used for nest building but spiderswere yet are apparentlysuccessfulin a very different more frequentlyfound in small ( < 400 cm2)hold- and Here they encounter the problems of habitat. fasts;14.50/o of holdfastscontainedone or more spimaintaining aerial respiration when periodically ders;and the meannumberof spidersper colonised submerged under seawater as well as findingfood, holdfastrangedfrom l.l to 2.5 spiders.Spiderswere contagiouslydistributed but density was not related water, and a suitable refuge from what can be a to holdfastarea.In most months there were fewer hostile environment for a terrestrial animal. In malesthan femaleswith an overallsexratio of 1.36 addition. the concentrationof ions in the environfemales per male. Although lone spiders are ment and in their prey are likely to be much higher encounteredmost frequently,the majority of the than that of their own body fluids. Moloney & population lives with one or more conspecifics. Nicolson (1984) have shown that Desis.formidaAdult femalesshow increasedcohabitation(shar- bilis has solved this problem by having a haemoing the samenest)during springand summer with lymph concentrationwhich is twice as high as malesand femalesof similar sizeoccurringtogether. terrestrial spiders and similar to that of marine Significantnumbersof juvenilesenteredthe popu- invertebrates.With the exception of Desis.forntilation during February-May and their growth sug- dabilis (see Lamoral l968a,b, l97l; Moloney & geststhat they may maturein 4-5 months and may Nicolson 1984)no speciesof this genushave been live for a further l2-18 months.Femalesgrew to closely studied and many still lack adequate a larger size than males. The percentagemoult descriptions. Forster (1970) clarified the taxonomic position increment or Dyar's Constant was 17.2o/oand it of D. marina and summarisedthe little that is tendedto decreasewith increasingpre-moult size. known about its distribution and ecology.The The comparativelylow value of Dyar's Constant, results of respirationstudiesand an investigation the problemsof mate location,and cohabitationas of how this spider managesto remain submerged a mating strategy,are discussed. for long periods (up to l9 days) have already been published(McQueen& Mclay 1983;McQueenet Keywords Desidae;Durvillaeaantarctica;Desis al. 1983). Of all the maritime spiders on New marina: population structure; intertidal spider; Zealand shoresD. marina penetratesdeepestinto Dyar'sConstant;mating strategy the intertidal zone.It is found in shells,rock crevices, and kelp holdfasts (Durvillaea antarctica (Chamisso))betweenhigh and low tides. During neap tides the spider might expectto spendmany days totally submerged.Spidersliving in kelp holdfastsbuild a silken nest which lines a cavity deep within the base of the plant. The nest containsa pocketof air (averagevolume < 2.0 cm3,Hayward Received 29 August1984,acceptedll April 1986 1982)within which the spidermust remain during
30 in order to survive. Within this nest submergence the spidermust captureits food, mate,and rear tts young.Movementsbetweennestsor establishment of new nestsare restrictedto periodswhen the tide is out; thus, D. marina spendsa considerableportion of its life within a spacethat is not a greatdeal largerthan its own body size.Adult spidersgrow length4.8 mm) to a total lengthof l7 mm (carapace and weighup to 112 mgm. As spidersare typically lerrestrialanimals,their presenieon the shore raisesquestionsabout how they are able to survive in this habitat, and what adaptationsthey have for living in an aquatic environment.We ther:eforeundertook a study of the ecologyof D. marifla concentratingon its distribution on the shore,life hislory, and behaviour. This paper deals with population structure and growth and includesdata on the use of Durt'illaea anlarctica holdfasts, spider density, seasonal changesin population structure,and growth per and adults. moult for jr-rveniles
METFIODS Our studyareawasat Kean Point, I(aikoura(4225' 3 5 " S , 1 7 3 " 4 31' 5 " 8 ) w h i c h i s l o c a t e do n t h e K a i koura Peninsula.fhe coastlineat this point is very exposedto wave action and one of the dominant featuresis the prolific growth of kelps Durvillaea antarctica and 1). v,illana. The occurrenceof D. antarcticais patchy and independentof rock type; someplants may survive as long as l0 years(Hay 1977).Field surveysshowedthat the upper limit for D. antarcticavaried (+31 cm to -56 cm with respectto MSL) from siteto site but the lowel limit was fairly constant at approximately -100 cm belowMSL (McQueen& Mclay l9B3).Desismarina occupreskelp holdfasts,living in the cavities createdby burrowing and grazinganin'ralssuch as limpetsand chitons. Kelp holdfastsare solid structureswhich cement the plants on to rocks.Largeplants have holdlasts which may exceed60 cm acrossand 5 cm in thicknessand althoughmost of the outer surfaceis continuous there are severalapertures,especiallynear the periphery,which allow water and animals to enter and leave the honey-combedinterior. Small plants offer little internal spacefor animal colonisationLrutlargeplantscan tre almosttotally hollou, inside, only adheringto the rock by the margins. Colonisationof this habitat by D. ntarina is only made possibleby the presenceof burrowing and grazinganimals.Apparently,theseanirnalsdo not attack Durvillaea willana (which lacks a honeycombed holdfast);hence,spidersdo not occur in theseplants.The presenceof D. marina on waveswept shoresis made possibleby the protection
New ZealandJournalof Zoology,1987,Vol. 14 affordedby kelp holdfasts.The spiderspinsa nest which trapsa small volume of air within the sheltered environment of the holdfast (see hg. 1, McQueen& Mclay 1983).To exit from a holdfast the spider must break out of its nest and emerge via one of the aperturesaround the periphery.The spider was found in holdfastswhich rangedfrom *20 cm aboveMSL down to *17 cm belowMSL (McQueen& Mclay i9B3). Collecling was carried out at approxirnately monthly intervalsfiom December198i to February 1983during springtidesrvhenthe holdfastswere fully exposedto the air. Typically, samplecollection required3-4 h and sometimesit wasnecessary low tides. A total to collect during two successive of 2655holdfasts\ /ereexamined.We aimedto collect at least 30-40 spidersper month. To obtain the spiders,D. antarcticaholdfastswere chipped from the rocks using a spade.The presenceof spidei-swas usually indicated by tlie white nest silk inside a cavity in the holdfast. From December l9Bl to February1982all holdfastswereretained; only holdfastscontainingnestswere subsequently, kept, those not containingnestswere countedand discarded.This may have led to a slight underestirnate ( < 5okSof the number of spiderspresent when nestswereconcealed.On beingdisturbedthe spidersusuallybecameactiveand quickly emerged from their nest. These spiderswere capturedand placedin small plasticcontainers.After removalof the fronds, the holdfastsplus the captive spiders were placedin largeplasticbags,sealed,and transported to the Edward Percival Field Station for closer inspection.Holdfast size was recordedand each was examinedfor the presenceof additional spiders.Sometimesit was sufficientto bang the holdfast sharplyon the benchto causethe spiders to come scurryingout; however,often it was necessaryto painstakinglycut up the entireholdfastrvith a knife and inspect every nook and cranny. This was especiallytrue for juvenile spiders.The spiders from each holdfast were then preservedin 700/o alcohol althoughsome were kept alive for behavioural observationsand growth studies.Holdfast area was determinedby tracing the outline on a sheetof paper and then cutting and weighingthe paper;if they were too largethe maxirnum length and breadthwere measuredand rnultipliedto give the area.The location ol'each nestwas plotted on the holdfast outline. If there was more than one nest in a holdfastthe spidersfrom eachnest were kept separatewhere possible. The data we collectedconsistedof spider size (carapacelength and total length,measuredusing an eyepiecemicrometer),sex,number per nestand numberper holdfast,presence or absence of broods, and areaof the holdfast.The data on reproduction, behaviour patterns, and the use of silk will be
Mclay & Haylvard-Population structure and use of holdfastsin Desis marina
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24681012141618 Totallbngth(mm) Fig. I The relationshipbetweeniarapacelengrh-(Cl')and total length (TL) mm for D. marina. Trianglesindicate juveniles,closedcirclesare femalesand open circlesare males(n:ti+\.'
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100-199200-299 300-399 400-499' 500-599'600-699 700-799 800-899'900-999 1000+ Holdfast area (cm')
Fig. 2 Frequencydistribution of Durvillaeaantarcticaholdfastswith (crosshatched) and without (open) Desismar ina. Holdfastareais groupedinto 100cm2 sizeclasses (n:2ll holdfasts).
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reportedin subsequentpapers.Our resuhsuseeither 'I'he relatotal length(TI-) or carapacelength(CL). variables is CIL:0.371 tionshipbetweenthesetwo + A.274TI-; or TL:-0.34 + 3.3 CL, coefficient of (r2):0.905,n:134 (seeFig. l). Total determirration length and carapacelength were the most convenient measuresof spidersize.It has beencommon irr population studies of spiclersto assign individualsto differentinstarson the basisof their size. The peculiar habitat requirementsof D. tn(trina nrade it irnpossiblefcrr us to carry out long term rearingexperirnentsin the laboratorywhich would have allowed separationof different instars. Irr of discreteinstarsrecognisaddition,the existence ableort the basisof size(e.g.,CL or TL) alonewas not apparentin our field data,Therefbre,we have populationstructureon the basis chosento analy'se of spider size and can only allocateindividuals to newly ernergedspiderlings(approx. 1.8 rnm TI-), juvenilesup to 7.9 rnm TL, and adults('fL > 7.9 rnm). We believe that this method provides an adecluateq,a)' of interpretingdata about the life cycleof this species. Spiders kept for growth measurernentswere placedin piasticcontainers(11 cm diameterby 2 cnr deep) rvhich had trvo I cm diameter holes punchedin thenr.The hole in the lid was covered with wire gaLrzefor ventilation and the other throughthe basecarrieda cotton wool stick soaked il sea water. Once in the contairlersthe spiders were kept in the dark and transportedto the Zoology l)epartment,thriversity of Canterbury,Christchurch. In the laboratory the containers were stackedalongtrayscontainingseawater so that the cotton stick rernainedmoist. Thesetrayswerekept underblackpolytheneat l5'C in a controlledtemperatureroom. Our aim was to simulatethe natural holdfastconditionsof darkness.moisture,and a temperatureresemblingthe field. We attempted to rnaintain the spidersin tank-heldholdfastsbut this did not prc)vepossiblebecausethey readily escapedand the holdfastsbeganto decaywithin 2* 3 days.
New ZealandJournalof Zoology,1987,Vol. l4 TableI
Occupancy of holdfastby Desistnartna. Holdfastarea
Intermediate Large Small Spiders (0-399cmr) (400-799cn2)( > 799cmr)Total Presenl Absent Total
t2
21
49 t2l
+J
70
lt 9 20
110 101 2tl
1 r : 7 . 8 8P, < . 0 2 5d. f : 2 .
under rock overhangs.Spiderswere found in all these kinds of holdfasts.Similarly, spiderswere found in holdfastsfi'om very exposedrocksto those inner rocks. Spiderswere also on serni-sheltered found on isolated rock stacks which are surrounded by water all year round, hence,the lack of dry accessto theserocks did not preventspider colonisation.D. antarcticaplantsoccur from *31 cm above MSL to -100 cm below MSL and D. rnarina was found to occupyholdfastsin the range fronr *20 cm to -Tl cm (McQueen& Mclay 1983).Only thoseholdfastsat the lower end of the range were not used by the spidersbecausethey were rarely exposedto the air. No spiderswere found in hclldfastsof the coexistingkelp speciesD. v,illana becausethe holdfastsof this kelp did not containany cavities. Holdfast sizewas investigated(usinga sampleof 230 holdfastscollectedDecember1981-February 1982)to determineif there was an optimal size rangeinhabitedby D. rnarina. Spiderswere found in holdfastsof all sizesfiom the smallest(5.7X 5.7 cm) to the largest(40 x 65 cm) holdfast.However, a frequencydistribution of occupancyagainstholdfast area (Fig. 2) suggests that spidersprefer holdfastslessthan 400 cm2.Intermediatesizedholdfasts containfewerspidersthan expectedand largeholdfasts( > 899 cm2)appearto containmore,but the sample size (n:12) is small and the differenceis not significant(x2test, seeTable l). All the holdfasls in this sarnplecontainedcavitiessuitablefor spider occupancyand the size rangeis representative of the shorepopulationexceptfor the newly RESLT[.,T'S settledplants which are too small to provide sufficient nest space.This patternsuggests that the host Use of holdfasts may survive much longer than the commensal l--tesis nmrina carronly utiliseDttrtillaea atittrctica spider. holdfastswhich havebeentunnelledand excavated by chitons, linrpets,and other grazers.Since virtually all holdfastsare tunnelled,rnostof them rep- Density resent suitable habitat for the spider. These Of the total of 2655holdfastsexamined,386(14.50/o) holdfastsoccuron the sunny (northernaspect)and contairredD. marina (Table 2). The percentageof in Janshaded(southernaspect)sidesof rocks,growingon holdfasts with spiders ranged from 65.70/o all angles from horizontal to vertical or growing uary to 7.7ohin October 1982(seeFig. 3). This per-
Mclay
& Hayward-Population
structure and use of holdfastsin Desli marina
33
100
Fig. 3 Percentageof Dtu'villaea arttarct ica holdfasts containing one or more Desrsmarina in different months from December l98l to February1983.The bars indicate950/o confidencelimits.
MAMJ)ASONDJF Month
centageremainedlow from May to November and tended to increaseduring the summer months. wererecordedduring the 1981Higherpercentages 82 summer than during the following summer. Changesin the proportion of holdfasts colonised was not related to changesin dispersionof the population (i.e., whether spiderswere more commonly found alone or in groups,seebelow).Mean numbers of spiders per colonised holdfast ranged from l.I-2.5 and showedpeaksin February and June 1982,declinedto a low level in August, and tended to increaseagain during the summer (Fig. Table2 Numbersof holdfasts and spiderscollected at Kaikourafrom December 198l-Februarv 1983. Spiders Holdfasts Month
with Total soidersMalesFemales Juveniles Total
December 1 8 6 4 15 35 23 January February 104 50 30 t45 26 15 March 11 April 66 6 May 156 30 9 June 8716692540 28t 21 lt July August 8281269 September t 4 4 1 3 2 1 1 1 2 0 11 October 3 6 4 2 8 13 November 4 1 3 4 0 December 1 9 3 2 8 t4 r8r 37 23 January 18 February 3 8 6 4 3 Totals
2655 386
5 13 34 lr 6 r7
l 9 43 13 6 34
l0 37 r07 39 18 60
20
14
45
t] 24 16 28 27
7 15 r7 15 13
35 52 47 66 58
1 7 8 240
22s
643
4). However, thesefluctuationsare only part of an apparent overall decline in the population density during the study period. Mean numbers of spiders per holdfast for all holdfasts collected showed a similar pattern (Fig. a). It was apparentduring samplingthat spiderswere very patchy in their distribution. Often 10-20 holdfastscould be examinedwithout finding any spiders and then in several adjacent holdfasts in successionwould be found harbouring spiders.The mean number of spidersper holdfastwas 0.24 and the variance0.597.If the distribution was random the variance/meanratio should be closeto 1.0;in fact it was 2.48, which is significantly different (P
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Mclay & Hayr,vard-Population structure and use of holdfasts in Desis marina 100
37
Fig.8 Changes in thepercentage of lone adult 1)ssisnnrina fron't f)ecenberl98l to February1983. Diamondsconnected by a dashed line indicatelone males,squares indicate femaleswiihout bror:ds and trianglesare for all fernales (rvith and without broods).
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femalesof similar size tendedto occur togetherin each nest (correlationcoefficient, r:0.43, P< .01, n:42). The mean sizeof cohabitingmaleswas3.44 mm CL (S:0.51)and cohabitinglbrnales3.39mm (S:0.59).This suggests that matingsoccurbetween spidersof similar size.Howeverthere is a scatter of points about the line of equal size(in l8 pairs (42.90/0) the male was smaller than the female,in 2l pairs (500/o) the male was larger,and in only 3 pairs (7.10/o) the sizeswere equal) suggestingthat relativesizeof rnatesis not of criticalimportance.
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MJJASONDJF Month
Fig. 9 The percentage of Desis rnarina adult females c o h a b i t i n(gi . e . ,s h a n n gthe samenest)from December Life cycle and growth l 9 8 l t o F e b r u a r y1 9 8 3 .
During the first tlvo months of our str"rdy(Decen-rber and January)the populationwasdominatedby a d u l t s( 8 - 1 1 . 9m m T L ) ( s e eF i g . l l ) . S i g n i f i c a n t numbers of juveniles enteredthe populationin Februaryand smallernumbersin March.A second, more dramatic influx of recruitsoccurredin May but juvenilesare presentin all months.The pulse of recruitmentin May can be followed in most of the subsequent months,beingprominentin August (6-1.4 mm TL), Octoberand November(7.5-8.9 € m m T L ) , J a n u a r y( 1 0 . 5 - 1 1 . 9m m T L ) , a n d F e b E ^ ruary (12-13.4mm TL). Using 8 mm as the cric terion of adulthood,D. marina probablymatures a) t ao in 4-5 months.As spiderslargerthan l5 mm TL aa 8 g were recorded,this suggests that they mav surr.ive .d (d a f u r t h e r 1 2 - 1 8 m o n t h sa s a d u l t s .U p t o 1 3 m m o TL the sizeclasses are approximately o^ equallycomaz mon ivith the two most common size classes being G y o u n ga d u l t s9 - l 1 . 9 m m T L . B e y o n dt h i s s i z et h e numbersbeginto declinerapidly(seeFig. I l). We interpret the absenceof largenumberso{'juvenile spidersas being a consequence of the largelosses that must occur during dispersal. Femalespidersgrow to a largersizethan males 0123456 (see Fig. l2). The maximum sizefor femaleswas Femalecarapace lengtfr(mm) l7 mm TL whereasthe largestmale was 15.l mm Fig. 10 Relativesizeof Desisntarina male and female TL. Most adult size classeshave fewer malesthan pairscohabitingin the samenest(n:42). The diagonal females.Growth is the result of the cumulative lineindicates equalmaleand femalecarapace length(mm). effectsof successive moult incrementsand moult E
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Mclay & Hayward-population structure and use of holdfasts in Deszsmarina F i g . l 2 P e r c e n l a g eo f n i a l e a n d l e r n a l e D a s i sn t a r i t t a i n d i f f e r e n t srze classes.The size classesare based on total length (TL): crosshatched bars are for males, open bars are for females.
39
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7.5-8.9 9.0-10.4
12.0-13.413.5-14.9 15.0-16.4 16.5-17.9 S r z ec l a s s( m m )
Fig. l3 Percentage moult increment or Dyar's Constantplotted againstpremoultcarapace length (mnr) for Desisntarina reared-in the laboratory.Diamondsare for triangles for males,and .;uveniles, squares lor lemales(a:33).
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frequencyin relation to age.Although we have no data concerning moult frequency, moult incre_ mentswere, on average,largestfor adult females ( 9 1 ? * * p e r m o u l r ,S D : 0 . 1 3 , n : I 9 ) w i t h m a l e s ( 0 . 3 7m m , S D : 0 . 1 t , n : 9 ) a n dj u v e n i l e s( 0 . 2 gm m , qD:0 12, n:5) having smallei increments.There does not appear to be any relationship between adultmoult incrementand pre-moultsize.Juven_ ilesgrowby smallerincrementsbut we assumethat theymoult more frequentlythan adultswherethe
interval betweenmoults probablyincreases as they g^etolder. The percentagemoult incrementor Dyar's Constant (see Fig. 13) is highly variable bui the averagefor juveniles(18.6010, range8.1-27.20/o) is higher than for both females(l4.go/0,range7.5_ 33.3o/o) and males (13.870,range 6.7-2LZoUo;. n, expectedthe trend is for percentagemoult incre_ ment to decreasewith increasingpre_moultsize. The averagevalue for Dyar's Constant is 17.20/o
(sD:6.68).
40 DISCUSSION Desismarinawas first reportedin New Zealandby Robson (seeHector 1877)who found it living in 'Lithodomu.sholes' (presumablyholes made by a burrowing bivalve in mudstone)along the coastline at CapeCampbell.Over the mouth of the hole the spider spun a web behind which was the nest and eggsac.Robson believed that thesenestswere under water at all times. He observedthat the spider was exceedinglypugnaciousand claimed that it was capableof killing small fish in the tide pools. More recently Forster (1970) reported the D. ntarina is found "...alwaysbetweenhigh and low tide levelswhere they occupyempty shells,cavitiesor holesin rocksbetweenstones".Forster(1970)stated that D. marina is strictly limited to rocky shorelines but one of us (C.L.M.) has collecteda specimen from Mcleods Bay, WhangareiHarbour where the spiderwas found under a bivalve shell on the mudflat at low tide in this shelteredharbour. At Kean Point, Kaikoura, however,we only found D. marina living in kelp holdfastsand never in rock crevicesor under shells.The occurrenceof D. ntarina in kelp holdfastsis made possibleby the activities of burrowing and grazing animals such as limpets and chitons.Theseanimalshollow out the interior of the holdfast,making a mazeof tunnels and creviceswhich becomen"roreextensiveas kelps grow largerand older. The fact that D. rnarina was selectivein its useof smallerholdfastssuggests that small onesmay provide more adequateshelterand food-supplyand are able to provide a more secure refuge, safe from storm hazards. Larger holdfasts are often more extensivelytunnelledand there are often largeopeningsto the outside.Thesemay allow water to surgein and out of the holdfast destroying spider nests and thereby reducing the shelter. Therefore,it appearsthat this spiderhas very flexible habitat requirementsand can live on a variety of shores from sheltered to very exposed. On exposedshoressuch as those at Kaikoura the spider must shelterin holdfasts,but on lessexposed shoresit can live in holesor crevicesamongrocks, and where there is a high degreeof shelter it can live under bivalve shellslying looselyon the bottom. Regardlessof the kind of shore that it lives on the spider must be occasionallyexposedto the air to replenishits supply of oxygen(McQueen& Mclay 1983).In addition, it must be at thesetimes that the spidersmove about to make new nestsand find mates. At Kean Point we only rarely saw juvenile spidersand sub-adultson the rocksat low tide, suggestingthat at least some dispersaloccurs by walking.It seemslikely that dispersalof juveniles occursmostly by water.The absenceof spiders outsidetheir nestssuggests that most activitiessuch as feedingand courtshipmust occur while the ani-
New ZealandJournalof Zoology,198J,Vol. l4 We probablymissedmuch malsarein their shelters. small scaledispersalactivity becausespiderscould move into adjacent holdfasts without being observed.If this occursfrequentlyit could haveled us to underestimatethe densityof spidersbecause when samplingwe looked for the nest as an indicator of the presenceof a spider.In the laboratory we observedthat spidersdislodgedfrom their nests underwater ceased all movement and floated motionlessto the surface.They remainedfloating until they drifted up against a firm surface that allowed them to climb out of the water. Forster & Forster (1973) have reportedthat D. marina feeds on amphipods. Our observations confirm this sincewe found nestswhich had freshly killed amphipodsand empty amphipod skeletons attachedto the outer surfaceofthe nest.It appears that the presenceof an amphipod on the nest silk is detectedby the spider which then proceedsto eat the prey while still separatedfrom it by the silk. We never found the remainsof prey inside the nest. In addition we have also observeda D. rnarina to consumea polychaeteworm. Both amphipodsand polychaeteworms are exceedinglyabundantin kelp holdfasts so the spider would have a large food supply at its disposal.We assumethat feedingcan occur either when the spider is submergedor exposedto the air. If feedingdoes occur through the wall of the nest during submergencethen the spider must pump enzymesinto the body of the prey and then suck them back out. At presentwe have no data on the rate of feedingof this spider. Spiderscan undergolong periodsof fastingfollowed by bursts of feeding(Turnbull 1973)and so it is possiblethat feedingonly occurswhen the nestis exposedto the air. Moloney & Nicolson (198a) found the D. formidabilis,which lives at a similar heighton South African shoresto D. ntarina, avoidedevaporative waler lossesby remainingin silk-linednestsbeneath empty limpet shellswedgedin crevicesor under stones.Furthermore, D. fonnidabilis has a haemolymph osmolaritywhich is approximately twice as high as other terrestrialspiders,and sodium and chloride ion concentrationswhich are closeto seawater. Haemolymph osmolarity is similar to that of the amphipodsand isopodsthat are presumed to be the prey of this spider.SinceD. marina lles in a similar habitatand is likely to consumesimilar prey it is anticipatedthat this spiderwill also have haemolymph of high concentration.Clearly this representsan adaptation to a diet of marine crustaceans. While submergedD. rnarina must remain in its nestto avoid being sweptaway by the waves,This implies that reorganisationof the populationstructure betweennestsmust be confinedto brief periods
Mclay & Halrvard-Population structure and use of holdfasts in Desrsmarina
4l
coincidingwith low tides.Therefore,changesin the small,activelyhunting spiders,25o/o in sallyingprepercentageof spiderscohabitingmust be discrete dators which use no web or use only the upper sureventsand if a spideris unsuccessful in moving in face of a web, and 300/ofor araneid and theridiid with a mate it must be able to return to its old nest specieswhich use the lower surfaceof a web. The or alternativelymake a new nest before the tide highest value (about 44o/o)was found in an comes in again. Cohabitation is an important ambushing thomisid speciesMisumenopus celer. mating strategyin this spider. Almost two-thirds Hencemost of the spidershave growth increments of the population was found with conspecifics. of lessthan 280/0. D. marina also seemsto be simiAdults which do not mate during a low tide or lar in this respectbecausealthoughthe Dyar's Conachievecohabitationbeforethe onsetof neaptides stant rangedfrom 6.5-33.30/0, the averagewas only are likely to becomeunreproductivefor that month. 17.2o/o. But Enders (1976) has arguedthat a very Our observationof the patchy distribution of D. rnobile animal which grows discontinuouslycan illmarina among holdfastsmay imply that matings afford to accumulate the large reservesnecessary tend to occur among spiders living in adjacent for large growth increments becausealthough its holdfasts.Our data show that there are marked 'power' remains constant during the intermoult fluctuationsin the proportions of adult spiders period its weight will increasemarkedly giving an cohabitingwith generallyhigher proportionsduring adverse'power'/weightratio. This implies that they the warmer monthsthan during the coldermonths. must moult more often and have small percentage The hypothesis that matings are more likely growth increments.However, a largelyimmobile, betweencloseneighboursis also supportedby the ambush hunter could afford to have large growth observationthat spider density is generallylow. incrementswithout impairing its ability to capture of holdfasts (or about one prey. In this respectthe small growth increments Overall, only 14.5o/o holdfast in seven)contained spidersand not all of of D. marina are enigmatic. It is a 'sit and wait' theseare potential mates.When account is taken predator which grows to a fairly large size and yet of the presenceof immature spiders and that it only has small growth increments.This may be encountersbetweenmembersof the samesex and a consequence of living in a very small nestwhich alreadymated pairs will prove fruitless,the prob- places strict limits on the availability of oxygen lem of finding a mate in a short time must be a during long periods of submergence.D. marina is It is then not a very efficientuser of oxygen(McQueen& Mclay criticalfactorin reproductivesuccess. surprising that we often found male and female 1983)but if it grows too largethen it will have to adults cohabitingwith sub-adults.We cannot tell live higher on the shorewhere periodsof submerwhich of the pair found the other member but is genceare much shorteror alternativelyfind a shelseemslikely that some adults cohabit with sub- tered spacecapableof holding a largernest.Spiders adults,waiting until their mate reachesmaturity. that chooseneststhat can accommodatetheir own If the female is more sedentaryand is locatedby growth and the arrival of mateswill be selectedfor; the male, which is usually the casewith spiders spiders that do not will be selected against (Turnbull l9l3), this may result in higher male (McQueen& Mclay 1983). mortality,the biasedsex ratio in favour of females (1.36females/male), and the absenceof largemales ACKNOWLEDGNIENTS in the population. Gertsch (1979) suggeststhat the number of Wewishto gratefullyacknowledge theassistance of Lindmoults necessaryto attain maturity in spidersis say Smith, PhilippaGordon, TraceyOsborne,Don largelyrelatedto size.Very small spiders5-6 mm McQueen, WinnieMcQueen, andSimonPollardwiththe of spiderson the shore.Jackvan Berkelalso TL moult only 4-5 times while largespiders15- collection at theEdwardPercivalField 30 mm TL may moult 10-13 times.The number providedvaluableassistance Station, Kaikoura. We indebted are to RobertJackson of moults is variable and is influenced by the who suggested this study to us. amount of nourishment, with well fed spiders undergoingfewer moults. It seemslikely that D. marina,a largespider,could undergomany moults beforedying. Our data suggestthat later moults will REFERENCES be relatively smaller than early moults. Enders Enders, F. 1976:Size,food-finding and Dyar'sconstant. Environmental (1976)analysedpublisheddata for spider growth entomology J: l-10. incrementsand comparedtheir Dyar's Constants Forster,R. R. 1970:The spidersof NewZealandpartIII. OtagoMuseumbulletin-?: l-184. incrementwas greaterthan to seeif the percentage (a 280/o figure suggestedby Hutchinson 1959, as Forster,R. R.; Forster,L. M. 1973:New Zealandspiders an introduction. Auckland, Collins.254p. allowing coexistanceof predators resulting from exploitation of sufficiently different sized prey). Gertsch, W. J. 1979:Americanspiders. New York,Van NostrandReinholdCo.2ndedition.274 p. Enders(1976) found incrementsof about 20o/oin
42 Hay. C. H. l91l: A biologicalstudy of Durtillaea antHariot and D. u'illanaLindauer arctica(Chamisso) in New Zealand. Unpublished Ph.D. Thesis, Departmentof Botany,Universityof Canterbury, Christchurch,New Zealand,264p. Hayward,T. L. 1982:Behaviouralecologyof the marine spider Desis rnarina. UnpublishedB.Sc. (Hons) thesis,Universityof Canterbury,Christchurch,New Zealand. Hector,J.1877:Note addedto the paperby C. H. Rob'Notes on a marine spider found at son entitled Cape Campbell'. Transactionsand proceedingso.f tlte Nev,ZealandInstituteI0: 299-300. Hutchinson,G. E. 1959:Homageto SantaRosalia,or why arethereso manykindsof animals? ,lnrcrican nattu'alist93 : 145-159. Lamoral,B. H. 1968a:On the speciesof the genusDesis Walckenaer,I 837 (Araneae:Amaurobiidae)found on the rocky shoresof SouthAfrica and SouthW.est Africa.Annalso-fthe Natal rnLtseutn 20: 139-150. 1968b:On the ecologyand habitatadaptations of two intertidalspiders,Desis.fonnidabllrs(O. P. Cambridge)and Amatu'obioidesafi'icanusHewitt, with at "The Island"(Komn-retjie, CapePeninsula), noteson the occurrence of two otherspiders.,4nnal of the Natal nluselutl20: l5l-193.
New Zealand Journal of Zoology,1987, Vol. l4 1971:Thesespidersare drownedeveryday. :l.f ican vildli.fe25 : 7-10. McQueen.D. J.: Mclay, C.L. 1983:How doesthe intertidal spider Desisrttarina (Hector)remain under water for sucha long time?l{en' Zealandjournal o.fzoolog, I0: 383-392. McQueen,D. J.; Pannell,L. K.; Mclay, C. L. 1983:Respirationratesfor the intertidalspiderDesisrrtarina (Hector).ltiew,Zealandjou'nal ef':oolog, I0: 393400. Moloney,C. L.; Nicolson,S. W. 1984:Water relations and haemolymphcompositionof two intertidal spiders(Order Araneae).Journal o/' exTterintantal rnarinebiologt,and ecologt,B3: 275-284. Roth,V. D. 1967:Description of the spiderlarniliesDesidae and Argyronetidae. .ltrtericannntseuntnot'ttates2292: l-9. Roth, V. D.; Brown, W. L. 1976:Other intertidalairbreathingarthropods./ri.' Marine Insects.Cheng. L. ed.,Amsterdam,North-Holland.pp. 119-150. Turnbull, A. L. 1913:Ecologyof the true spiders(Araneomorphae)..4ttnLtalreviex"o-fenlotttologl'18 : 305-348.
