DISTRIBUTION, HABITAT AND MORPHOLOGY OF THE CARIBBEAN CORAL- AND ROCK-BORING BIVALVE,LITHOPHAGA BISULCATA (d'ORBIGNY) (MYTILIDAE: LITHOPHAGINAE)

Lithophaga bisulcata is the most common Caribbean and Atlanticlithophagine and is the only species of the genus known to occurfrequently in both living and dead coral. The abundance in livingcorals is non-random and variable. Most common hosts are Siderastereasiderea and Stephanocoenia michelini. The bivalves are moreabundant in their preferred hosts than in dead coral. Individualsfrom the two habitats are indistinguishable in shell shape,musculature and size of boring and posterior pallia! glands,indicating a single population. Boreholes differ in the twohabitats with respect to size and lining. Linings are formedat the "inactive" end of the burrow; therefore living coralinhabitants line the anterior end of the burrow and dead coralborers line the posterior end. Recruitment rates are unknownin dead coral but were very low in living coral (Received 9 June 1987;     

长爪沙鼠体蚤和巢蚤数量研究

1982～1996年对内蒙古自治区鄂托克前旗长爪沙鼠Meriones unguiculatus体蚤和巢蚤的数量进行了调查和分析,得到如下结果。获体蚤15种,同型客蚤指名亚种Xenopsylla conformis conformls (50.8%) 和秃病蚤蒙冀亚种Nosopsyllus laveiceps kuzenkovi (40.6%)为优势种。获巢蚤15种,秃病蚤蒙冀亚种(74.3%)为优势种;盔状新蚤Neopsylla galea (11.9%)和叶状切唇蚤突高亚种Coptopsyllus lamelliferardua (8.1%)为常见种。年巢蚤指数的均值是年体蚤指数的6.92倍。体蚤指数与巢蚤指数不相关(P>0,05),体染蚤率与巢染蚤率不相关(P>0.05)。不同年份 的体蚤和巢蚤多样性比较稳定。连续两年春季或秋季秃病蚤巢蚤指数大于10只后,可能流行动物鼠疫。     

长爪沙鼠寄生蚤指数和气象因子关系的研究

根据内蒙古自治区土默特平原1983～1985年长爪沙鼠 Meriones unguiculatus巢蚤、体蚤、洞干蚤指数和6项气象资料进行分析,得到如下结果。① 共获蚤11种, 其中秃病蚤蒙冀亚种Nosopsyllus laeviceps kuzenkovi(67.50%)是优势种, 二齿新蚤Neopsylla bidentatiformis(22.65%)为次优势种。② 3种蚤指数的均值差异显著(P<0.0001)。③ 体蚤与洞干蚤指数相关显著(P<0.05), 模型为(洞干蚤指数)=0.0049+0.0248(体蚤指数), 巢蚤与体蚤、巢蚤与洞干蚤指数的相关不显著(P>0.25)。④ 沙鼠密度与3种蚤指数的相关均不显著(P>0.10)。⑤ 在巢蚤中,月温度是影响巢秃病蚤唯一的气象因子(P<0.05)。⑥ 分别求出鼠体的秃病蚤和同形客蚤指名亚种Xenopsylla conformis conformis与气象因子的最优回归子集(P<0.003、P<0.05), 洞干的秃病蚤和二齿新蚤与气象因子的最优回归子集(P<0.0007、P<0.01), 月蒸发量是影响秃病蚤的最重要因子。⑦ 春季与冬季、夏季与冬季巢蚤指数差异显著(P<0.05); 春季与冬季、夏季与冬季体蚤指数差异显著(P<0.05); 春季与冬季、夏季与秋季、夏季与冬季洞干蚤指数差异显著(P<0.05)。     

Revision of Sternaspis Otto, 1821 (Polychaeta,?Sternaspidae)

To the memory of William Ronald Sendall Sternaspid polychaetes are common and often abundant in soft bottoms in the world oceans. Some authors suggest that only one species should be recognized, whereas others regard a few species as widely distributed in many seas and variable depths from the low intertidal to about 4400 m. There are some problems with species delineation and the distinctive ventro-caudal shield has been disregarded or barely used for identifying species. In order to clarify these issues, the ventral shield is evaluated in specimens from the same locality and its diagnostic potential is confirmed. On this basis, a revision of Sternaspis Otto, 1821 (Polychaeta: Sternaspidae) is presented based upon type materials, or material collected from type localities. The sternaspid body, introvert hooks and shield show three distinct patterns, two genera have seven abdominal segments and tapered introvert hooks, and one genus has eight abdominal segments and spatulate introvert hooks. The ventro-caudal shield has three different patterns: stiff with ribs, and sometimes concentric lines, stiff with feebly-defined ribs but no concentric lines, and soft with firmly adhered sediment particles. Sternaspis is restricted to include species with seven abdominal segments, falcate introvert hooks, and stiff shields, often exhibiting radial ribs, concentric lines or both. Sternaspis includes, besides the type species, Sternaspis thalassemoides Otto, 1821 from the Mediterranean Sea, Sternaspis affinis Stimpson, 1864 from the Northeastern Pacific, Sternaspis africana Augener, 1918, stat. n. from Western Africa, Sternaspis andamanensis sp. n. from the Andaman Sea, Sternaspis costata von Marenzeller, 1879 from Japan, Sternaspis fossor Stimpson, 1853 from the Northwestern Atlantic, Sternaspis islandica Malmgren, 1867 from Iceland, Sternaspis maior Chamberlin, 1919 from the Gulf of California, Sternaspis princeps Selenka, 1885 from New Zealand, Sternaspis rietschi Caullery, 1944 from abyssal depths around Indonesia, Sternaspis scutata (Ranzani, 1817) from the Mediterranean Sea, Sternaspis spinosa Sluiter, 1882 from Indonesia, and Sternaspis thorsoni sp. n. from the Iranian Gulf. Two genera are newly proposed to incorporate the remaining species: Caulleryaspis and Petersenaspis. Caulleryaspis gen. n. is defined by the presence of falcate introvert hooks, seven abdominal segments, and soft shields with sediment particles firmly adhered on them; it includes two species: Caulleryaspis gudmundssoni sp. n. from Iceland and Caulleryaspis laevis (Caullery, 1944) comb. n. from Indonesia. Petersenaspis gen. n. is defined by the presence of spatulate introvert hooks, eight abdominal segments, and stiff shields with poorly defined ribs but no concentric line; it includes Petersenaspis capillata (Nonato, 1966) from Brazil and Petersenaspis palpallatoci sp. n. from the Philippines. Neotypes are proposed for eight species: Sternaspis thalassemoides, Sternaspis affinis, Sternaspis africana, Sternaspis costata, Sternaspis fossor, Sternaspis maior, Sternaspis scutata and Sternaspis spinosa, to stabilize these species-group names, and a lectotype is designated for Sternaspis laevis which is transferred to Caulleryaspis gen. n. The geographic range of most species appears to be much smaller than previously indicated, and for some species additional material in good condition is needed to clarify their distributions. Keys to genera and to all species are also included.     

A Comparative Study of Bivalves Which Bore Mainly by Mechanical Means

This account of the boring mechanisms of those bivalve groupswhich bore mainly by mechanical means attempts to show partlyby reference to published accounts of boring and partly fromour own recent observations of certain characteristics of theboring process in the Pholadidae and Petricolidae, that in contrastto the movements of burrowing forms from which originally allthe boring movements derive, the process of boring makes fewdemands on the hydrodynamic system of the bivalve. The characteristicsof the boring process are closely related to the movements inmodern forms having epifaunal or infaunal habits, supportingthe suggestions of Yonge (1963) concerning the origin of thishabit in the Bivalvia. In all groups in which boringis mechanical,the shell forms the boring tool. However, in those groups inwhich boring has its origin in the epifaunal habit, the majorforce applied to the shell in abrading the burrow isprovidedby contractions of the pedal or byssal retractor muscles. Inthe Adesmacea alone, where boring has been derived from a deepburrowing habit, the adductor muscles provide the major forcein abrasion, and the basic digging cycle has become specializedby the addition of the rocking action of the valves which succeedsretraction. In the former group the ligament is retained andprovides the strong outward force with which the shell is heldagainst the wall of the burrow. In the latter group, the ligamentis reduced, allowing the valves to rock, but here the reciprocalaction of the adductors allows the valves to diverge anteriorlyas the large posterior retractor muscle contracts. In the morespecialized species, water pressure plays a minor role, themaximum pressures recorded being associated with actions subordinateto those involved primarily in abrasion, such as rotation inthe burrow or expulsion of debris from the burrow aspseudofeces.The least specialized borers, such as Petricola, resemble burrowingforms in the importance of the hydrodynamic role of the bodyfluids. In all groups there is a tendency for hypertrophy totake place in the muscles which produce the main boring effect,and for their action to be applied with maximum mechanical advantageagainst a fulcrum provided in most cases by the foot.     

黄鼠洞干蚤和宿主密度的年间动态关系

根据内蒙古自治区正镶白旗乌宁巴图苏木1981～1993年达乌尔黄鼠Citellus dauriCUS密度和洞干蚤指数监测资料进行分析,得到如下结果。共检洞干蚤9种,其中方形黄鼠蚤蒙古亚种Citellophilus tesquorum mongolicus (66.0%)为优势蚤种,光亮额蚤Frontopsylla lu-culenta(23.6%)为次优势蚤种,阿巴盖新蚤Neopsylla abagaitui和二齿新蚤N。Bidentatifor-mis为常见种,余为少见种。鼠密度与洞干蚤指数关系显著(PPP<0.05),关系为洞干蚤指数=0.2709+0.0504体蚤指数。洞干蚤和巢蚤指数的关系是显著的(P<0.07),关系为洞干蚤指数＝0.27652+0.00348巢蚤指数。三种蚤指数之间有如下近似 关系：巢蚤指数：体蚤指数：洞干蚤指数：100：10：1。     

Hunting Behavior in the Ctenizidae

The most primitive way of hunting in Mygalomorph spiders seemsto be the free roaming and catching of encountered prey Theraphosidae.The trap-door spiders Ctenizidae, Actinopodidae and Barychelidawhich are entirely sedentary, lie in wait behind the trap-doorand leap at prey that happens to pass close to the door. Somespecies spin radial silk threads outside the door which functionas stumbleor signal-lines, and some Australian species use grassblades and other litter in the same way. A further evolutionarystep leads to species which do not build a trap-door but crowntheir burrow by a funnel-shaped web. The Dipluridae finallyare real web-builders, which depend on a sheet-web to catchtheir prey. Generally they do not dig a burrow but hide in asmall retreat from which a funnel-web leads to the net. With a few exceptions Ctenizidae are entirely nocturnal. Theirrhythm of activity has been analyzed. The "Zeitgeber" is thedaylight during the last half hour before sunset. Most trapdoorspiders never leave their burrow during their whole life. Theyneed three to four years from hatching to become adults. Adultmales die during or at the end of the restricted mating-season;they take no food when adult. Females, which undergo post-adultmolts, can probably live for 15–20 years. Nemesia caementaria, like most other species, hunts during thewhole night. The mean time of activity is about Si/o hours,consisting of periods of lying in wait and periods of intermediaterests. The spiders make an average of three leaps per nightto catch a prey, but only about 10% of all bounds are successful.A hungry animal, lurking in vain, shows unmotivated leaps. The effects of light, moisture, and temperature on hunting activityare analyzed. Ctenizidae hunt during autumn, winter, and springbut interrupt their activity in summer for an estivation whichlasts usually two months. The females of some species capture the male after mating andeat him; others never attack him. This difference in behaviorlias repercussions on reproduction. Among certain species the young nymphs of the third instar refuseall food until they have made their own burrow and can huntby themselves. The young of other species stay with the motherfor one year and leave her at the fifth or sixth instar to maketheir own burrow. The young of Nemesia caementaria can remainin the burrow of the mother until they are almost adult, i.e.,for two or three years, and feed on the prey the mother hascaught. The behavior of Ctenizidae can be grouped. Some species lurkbehind the closed or almost closed trap-door. Another groupopens the trap-door and puts the pedipalpi and the two anteriorpairs of legs radially out onto the rim of the burrow, whilethe cephalothorax is hidden behind the trap-door. A third groupspins threads of silk radially about the entrance and uses themas signal-lines. A fourth group can come out and pursue theprey, then drags it to the trap-door and into the burrow. The nature of the prey depends on the bio tope. The Ctenizidaefeed almost exclusively on insects, mainly ants and beetles;in the laboratory crickets are accepted as prey. Ctenizidae have no tarsal organ. They have different types oftrichobothria, transversal and longitudinal slit-organs, andlyriform organs. With these three kinds of sense-organs, thefunctions of which are not clearly understood, the trap-doorspiders are able to perceive the approach of prey, to judgeits distance from the trap-door, and to locate it in direction.They seem to have an organ of smell, since certain groups ofinsects are repulsive to them. Sight is not used for hunting.     

哈尔滨郊区人为鼠疫疫源地蚤类种群动态分析

1982～1999年对黑龙江省哈尔滨郊区人为鼠疫疫源地达乌尔黄鼠Spermophilus dauricus巢蚤、体蚤、洞干蚤指数和染蚤率进行了调查和分析。共获蚤9种,其中方形黄鼠蚤松江亚种Citellophius tesquorum sungaris是优势种(89.39%),其次为二齿新蚤Neopsylla bidentatiformis (10.37%)。3类蚤指数、染蚤率的均值差异均显著(P<0.01)。巢蚤与体蚤指数相关显著(P<0.05)。巢蚤指数∶体蚤指数∶洞干蚤指数≈650∶140∶1; 巢染蚤率∶体染蚤率∶洞干染蚤率≈165∶88∶1。     

An Electron Microscope Study of Cysts in the Saprolegniaceae

The structure of the zoospore cysts of various members of theSaprolegniaceae has been studied by electron microscopy. InSaprolegnia ferax, S. dioica, in Isoachlya eccentrica and I.unispora the primary cysts were smooth, whilst the secondarycysts bore stalked double-headed hooks. In S. parasitica thesecondary cysts bore tufts of longer hooks. In Protoachlya,Achlya, and Brevilegnia neither type of cyst bore hooks. InDictyuchus sterile the secondary cysts bore large spiny projections.The primary cysts in many species bore tufts of radiating hairs,and it is suggested that these are the remains of ciliated flagella.     

黄鼠巢和宿主密度的年间动态关系

根据内蒙古自治区正镶白旗乌宁巴图苏木1981～1993年达乌尔黄鼠Citellus dauricus密度和巢蚤指数监测资料进行分析,得到如下结果。共检巢蚤10种,其中方形黄鼠蚤蒙古亚种Citellophilus tesquorum mongolicus(45.4%)为优势蚤种,光亮额蚤Frontopsylla lucu lenta(25.0%)和阿巴盖新蚤Neopsylla abagaitui(21.3%)为次优势蚤种。二齿新蚤N.Bidentatiformis和角尖眼蚤Ophthalmopsylla praefecta为常见种,余为少见种。鼠密度与巢蚤指数 的关系不显著(p>0.10)、与巢染蚤率关系显著(p<0.05),不同年份的蚤类多样性和均匀性比较稳定。巢蚤和体蚤指数的关系是显著的(PP<0.005)。     

REVIEW OF THE GENUS DENDRODORIS EHRENBERG, 1831 (GASTROPODA: NUDIBRANCHIA) IN THE ATLANTIC OCEAN

Six valid species of the nudibranch genus Dendrodons Ehrenberg,1831 inhabit the Atlantic Ocean, including the Mediterraneanand Caribbean Seas. Dendrodons lumbata (Cuvier, 1804), Dendrodonsgrandiflora (Rapp, 1827), Dendrodons nigra (Stimpson, 1855)(immigrant from the Red Sea), Dendrodons krebsu (Mörch,1863), Dendrodoru senegalensis Bouchet, 1975 and Dendrodonswarta Marcus & Gallagher, 1976. Additional data about thebiology and geographical distribution of these species are presented.New evidence suggests that other species assigned to the genusDendrodons, Dendrodons racemosa Pruvot-Fol, 1951 and Dendrodonsminima Pruvot-Fol, 1951, must be included in the genus DoriopsillaBergh, 1880. Three new species of Dendrodoris are describedfrom the Northeastern Atlantic and West Africa;Dendrodons angolensis,Dendrodoris guineana and Dendrodons herytra. The variable external morphology makes species recognition difficult.Instead, the diagnostic characters utilised to separate speciesare the shape of the male cirrus hooks, the structure of thereproductive system and features of the egg-mass. (Received 25 April 1995; accepted 1 August 1995)     

OXYNOE AZUROPUNCTATA, N. SP., A NEW SACOGLOSSAN FROM THE FLORIDA KEYS (MOLLUSCA: OPISTHOBRANCHIA)

A new species of Oxynoe from the Florida Keys is described.It differs from the other Caribbean Oxynoe, O. antillarum Mörch,1863, in color, number and size of papillae, size increase ofradular teeth, diet, and type of development. The type of developmentis extended lecithotrophic. The larvae hatch about 4 weeks afterthe eggs have been deposited, and metamorphosis is completedafter about one additional week. Oxynoe azuropunctata n.sp.feeds on Caulerpa paspaloides, C. cupressoides and C. sertularioides.Details of the feeding process in Oxynoe are described for thefirst time. The junction of the oral tube and the pharynx isequipped with a complex muscular buccal apparatus, part of whichis everted during feeding. The protrusible parts of the buccalapparatus completely surround part of an algal filament, whichis then slit with the radula, and the cell sap is sucked out.The protrusible parts are retracted after the filament has beenreleased. ^*Present address: Ellebjergvej 22, 1 t.h., DK-2450 CopenhagenSV, Denmark (Received 13 November 1979;     

锡林郭勒草原布氏田鼠体蚤和巢蚤数量动态及蚤类群落演替

1989～1998年对内蒙古阿巴嘎旗那仁宝力格苏木布氏田鼠Microtus brandti体蚤和巢蚤的数量进行了调查和分析,所得体蚤包括3科7属9种, 其中优势种有: 原双蚤田野亚种Amphipsylla primaris mitis占41.84%, 近代新蚤东方亚种Neopsylla pleskei orientalis占41.18%, 光亮额蚤Frontopsylla luculenta占13.18%。所得巢蚤包括2科6属9种, 其中优势种近代新蚤占74.94%; 光亮额蚤占10.04%, 原双蚤占8.20%, 宽圆纤蚤Rhadinopsylla rothschildi占6.44%, 是常见种。田鼠体的原双蚤、近代新蚤、光亮额蚤和体蚤指数间呈正相关(P<0.05)。田鼠巢的近代新蚤、光亮额蚤、宽圆纤蚤和巢蚤指数间正相关显著(P<0.01)。春季巢蚤指数∶春季体蚤指数≈5∶1, 秋季巢蚤指数∶秋季体蚤指数≈75∶1。近代新蚤既是体型蚤又是巢型蚤, 为田鼠的优势蚤种; 原双蚤数量占体蚤的 41.84%, 是体型蚤; 光亮额蚤为体蚤和巢蚤中的常见种。     

Salinity--stress and Desiccation in Intertidal Worms

SYNOPSIS. Intertidal worms (oligochaetes, polychaetes, sipunculids)inhabiting beaches and tidal fiats both on the open sea coastand in estuaries may be exposed to significant tidal as wellas seasonal variations in salinity. However, there are veryfew measurements of the actual variations in salinity encounteredby worms in nature. Behavior (irrigation of burrow, verticalmovements in burrows, migration in gradients of salinity)maybe important in determining to which of the available salinitiesin a tidal cycle the worms may be exposed. In response to rapidlowering of salinity, worms gain water and lose salts, theseprocesses combining in diluting the internal fluids. Internalilution occurs more slowly in euryhaline species than in stenohalinespecies. Worms fully adapted to salinities lower than 30 % seawater may be hyperosmotic (demonstrated for six species of Nereidae).Mechanisms involved in hyperosmotic regulation include activetransport of salts(demonstrated in Nereis diversicolor), reductionofthe permeability of the body surface to salts and perhapsto water, and perhaps production of hypo—osmotic urine.Sipunculids can tolerate considerable loss of water rom dehydrationand concomitant increases in osmotic concentration of the bodyfluids. It is suggested that worms exposed to significant tidalvariations in salinity may seldom be in osmotic equilibriumwith their external medium.     

Communities of constructional lips and cup reef rims in Bermuda

Constructional lips and cup reef rims are little studied reef features typical of very turbulent conditions. The relatively low diversity community of these structures consists of three components, and algal mat dominated by Herposiphonia secunda, an encrusting calcareous biota characterised by crustose coralline algae and the vermetid gastropod Dendropoma corrodens, which together are the main hermatypic components, and an endolithic component with very abundant boring sponges, polychaetes, crustaceans and sipunculids. In northerly areas of Bermuda, the hydrozoan Millepora alcicornis is a prominent member of the community while to the south the urchin Echinometra lucunter is abundant. On the south side of Bermuda the community traps sediment which is deposited in the void spaces. The environment shows reduced grazing and this may account for the presence of several relict and rare species.     

Ecological Aspects of Some Coral-Boring Gastropods and Bivalves of the Northwestern Red Sea

More than 17 molluscan species were obtained from burrows incoral substrata at Al-Ghardaga (Hurghada on maps) on the RedSea coast, six of which in particular bore into livingcolonies.The species reported in this paper belong to the families Mytilidae,Coralliophilidae, and Gastrochaenidae. The direction of boringin living corals is to the outside, the borers keeping pacewith the growing coral layer to maintain their burrows open.Coral growth is generally of a higher rate than that of borers,and burrows are accordingly mostly much larger than their inhabitants.There is evidence in such cases that burrows form initiallyby growth of coral around the settling young. Boring of Lithophagaspecies is mostly due to the abrasive action of the shell whichmoves straight and posteroventrally without any rotation. Incoralliophilids,boring is also executed mechanically by the turning movementsof the shell. Boring in dead coral is directed inwards, andburrows are nearly as large as the borers. The latter avoidthe blocking of their burrows (e.g., by a living coral incrustation)either by great siphonal extension (Rocellaria) or the freeends of the shell may be strengthened to maintain the capabilityof boring in the opposite direction (Lithophaga laevigata).Both L. luevigata and Modiola chmamomeus bore mainly mechanicallyby the rocking movements of the shell. Chemical boring is stilla possibility,particularly in the posterior narrow region ofburrows of Modiola lodging the extended pallial siphons whichare deprived of any effective mechanical devices for boring.Therole of boring algae in rarefying bored coral material hasalso to be included as an indirect chemical factor.     

Shell-Boring Mechanism of the Gastropod, Purpura (Thais) lapillus: a Physiological Demonstration of the Role of Carbonic Anhydrase in the Dissolution of CaCO3

Measurement of carbonic anhydrase (CA) activity in homogenatesof accessory boring organs of the muricid gastropod,Purpuralapillus, by Meldrum and Roughton's manometric method showedthat CA is always present in boring as well as in inactive ABOs,but in variable amounts. Tests by the same method on whole ABOsin an isotonic solution were negative, proving that CA remainsintracellularly. Experiments in vivo on inhibition and activation demonstratedclearly that CA is responsible for demineralization of the valvesof lamellibranchs by Purpura:(a) at low concentrations of Diamox,partial inhibition of the enzymeoccurred, that is, the numberof complete holes decreased or disappeared and etchings increased;and at higher concentrations full inhibition took place. Inhibitionis reversed when snails are replaced in normal sea water; (b)action of pure CO₂ or mixtures of CO₂ and O₂ accelerated boring:in the optimal mixture, three times more boreholes were producedby snails than in the controls, and in about half the time.Under these conditions the reaction catalyzed by CA goes tothe right with hydration of CO₂ and is accompanied by releaseof H⁺ ions; in the presence of an increased concentration ofCO₂, the reaction is intensified and results in an additionalrelease of H⁺ ions. Consequently, destruction of CaCO₃ by theABO of Purpura in sea water enriched with CO₂ is accelerated. To identify the nature of the exchanged ion, we tested the effectof NaCl and KC1 on the boring mechanism in vivo by increasingthe amount of these salts in the seawater of snails with prey.In this case augmentation of the boring activity was noticed.These results suggest that the boring activity is accompaniedby complex ionic exchanges.     

Can one species determine the structure of the benthic community on a temperate rocky reef? The case of the long-spined sea-urchin Diadema antillarum (Echinodermata: Echinoidea) in the eastern Atlantic

We sampled 36 coastal rocky reefs throughout the overall Canarian Archipelago and consider (1) the daily macroalgal consumption of the long-spined sea urchin Diadema antillarum and (2) the daily net production of macroalgae along temperate rocky-substrates, to provide evidence that Diadema antillarum plays an important role in the structure of the shallow benthic environment of the eastern Atlantic. D. antillarum was found to be the main key-herbivore species, as it controls by its own the algal assemblages, with negligible contribution of other grazing species.     

Precopulatory mate assessment in relation to body size in the earthworm Lumbricus terrestris: avoidance of dangerous liaisons?

In the earthworm Lumbricus terrestris L., mating occurs on the soil surface, but partners remain anchored in their burrow andmating is preceded by repeated mutual burrow visits betweenneighbors. This study focuses on body size as one possibletrait that earthworms may assess during these burrow visits.Size-related mate choice is predicted to result in size-assortativemating, which we found in one field sample (n = 90 pairs),but not in a second (n = 102). We discovered that when mates separate, one of them can be pulled out of its burrow. Thiswas more likely for small individuals or those mating acrosswide distances. In a subsequent greenhouse experiment, we allowedfocal individuals to mate with two neighbors of different sizes.Relative size affected neither mating rate nor primary preference,but focals mated sooner with the same-sized neighbor than witha differently sized one. Small focals visited large neighborsmore often than small ones. We conclude that size influencesmate choice as well as the outcome of mating and discuss howthe "tug-of-war" that ends a mating contributes to this result.Precopulatory visits may involve assessment as well as enticementto lure the partner closer to the individual's own burrow,in order to minimize the risk when mating with a partner thatis large or far away.