A field and laboratory study on factors affecting polymorphism in the rotifer Keratella tropica

Summary We present data on polymorphism of the rotifer Keratella tropica from an outdoor, mesoscale experiment and a series of laboratory induction assays. In the outdoor experiment the biomass of planktonic crustaceans was noticeably depressed by larval fish predation, and associated with this depression K. tropica underwent a striking reduction of caudal spines. Subsequent laboratory studies showed that the positive association between crustacean biomass and caudal spine length was a cause and effect relationship. This is the first record of rotifer morphological change as an indirect effect of fish predation. In laboratory experiments filtrates of monospecific cultures of a cyclopoid copepod, a calanoid copepod and 5 cladocerans induced a remarkable spine development. Morphological induction showed a direct relationship with the concentration of crustaceans, both under field and laboratory conditions. Long spines were found to be strong deterrents against small predators (Acanthocyclops robustus copepodites), but were useless against large ones (females of the same species). The morphotypes of K. tropica obtained by experimental induction from a single clone encompass much of the worldwide variation of the species.     

Cyclomorphosis in Keratella tropica (Apstein) of Lake Asejire,Nigeria

Of one thousand, one hundred and four (1104), Keratella tropica collected with a 55 m mesh plankton net from the surface of Lake Asejire from February 1974 to February 1976 lorica length and width, and the lengths of the posterior spines were measured.Lake Asejire K. tropica are among the smallest in Africa. Variations in the patterns of spine length development were independent of season as individuals without left posterior spines and with relatively long left and right posterior spines occurred as commonly in the rainy as in the dry season months.Correlations between K. tropica dimensions and thirty environmental factors underscore the importance of physical environmental factors and point to biological and genetic factors as determinants of the patterns of cyclomorphosis in K. tropica.     

Effects of an Ostracod (Cypris pubera) on the Rotifer Keratella tropica: Predation and Reduced Spine Development

This study investigates two different effects of an ostracod on a rotifer. The rotifer Keratella tropica and the ostracod Cypris pubera were cultured on the alga Cryptomonas erosa. Adult ostracods (2.2 mm body length) significantly reduced the population growth rate (r day^–1) of K. tropica from 0.42 to 0.13. Individuals of this size ingested live rotifers and produced fecal pellets with rotifer loricas. Smaller ostracods (both 0.59 and 1.61 mm body lengths) did not affect K. tropica 's population growth rate. Surprisingly, C. pubera significantly inhibited spine development in K. tropica. Rotifers cultured with juvenile, non‐predatory ostracods had similar lorica lengths but right and left posterior spines that were 10 and 30% shorter, respectively. The spine reduction induced by the ostracod kairomone is in striking contrast to the spine elongation induced in this rotifer by kairomones from copepods, cladocerans and Asplanchna. In shallow ecosystems, large ostracods that swim in the plankton may be important predators of rotifers. In addition, the presence of ostracods in plankton communities may be one of many factors affecting rotifer spine development. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Induction of different defences by two enemies in the rotifer Keratella tropica: response priority and sensitivity to enemy density

1. Keratella tropica has qualitatively distinct spine‐development responses to kairomones released by the predatory rotifer Asplanchna and the cladoceran interference competitor Daphnia. Asplanchna induces a fourfold lengthening of the right posterior spine (to c.100 μm), a shortening or loss of the left posterior spine and a lengthening of two pairs of anterior spines. Daphnia induces a moderate elongation of both right and left posterior spines. This study tests three hypotheses regarding the sensitivity of these responses to enemy density, and the response priority when both enemies are present. 2. First, since K. tropica and Brachionus calyciflorus have similarly pronounced and effective spine‐development responses to Asplanchna, with no appreciable demographic cost, they should be similarly sensitive to Asplanchna density. This was the case. Both showed an exponential response to increasing Asplanchna density, well described by an asymptotic exponential regression model, and exhibited 50% maximal spine development at statistically similar Asplanchna densities–2.5 and 1.7 μg dry weight L^?1 (1.8 and 1.2 individuals L^?1), respectively. Strong selection for these Asplanchna‐induced responses clearly has led to a coupling of exuberant and effective morphological defence with an unrivalled sensitivity to predator density. 3. Second, since K. tropica’s response to Daphnia is much less pronounced and effective than its response to Asplanchna, it should be less sensitive to Daphnia density. This hypothesis was supported. Spine development increased linearly with increasing Daphnia density and was 50% maximal at 454 μg dry weight Daphnia L^?1, a biomass density 180 times greater than that inducing a comparable response to Asplanchna. 4. Third, since K. tropica’s response to Daphnia does not reduce Asplanchna predation, K. tropica should respond to Asplanchna when both enemies are present at densities sufficient to induce spine development. This was the case. The presence of Daphnia neither reduced nor increased the length of the right posterior spine; it only limited the extent to which the left spine was reduced or lost.     

Asymmetry and variation in keratella tropica

Keratella tropica usually has two posterior spines. Both may vary in length, but the left is always shorter than the right. Sometimes the left spine is absent. Spine length varies with the length of the lorica, but there are other, external, influences. Three separate lines of evidence indicate that factors which promote spine length are related in some way to the presence of calanoid copepods.     

Predator-specific inducible defenses in the rotifer Keratella tropica

1. A Patagonian strain of Keratella tropica has very different induced morphological responses to two predators – the carnivorous rotifer Asplanchna brightwelli and the interference competitor Daphnia pulex. Asplanchna induces the most exuberant morph. Compared to the basic morph, it has a fourfold longer right posterolateral spine (up to c. 115 μm), greatly elongated anterolateral and anterosubmedian spines, and no left posterolateral spine. Transitional morphs have an incompletely developed right posterolateral spine and a reduced left posterolateral spine. Daphnia induces moderate development of both posterior spines but no elongation of any anterior spines. Induction of these morphs by Asplanchna and Daphnia is mediated by kairomones. 2. The Asplanchna‐induced morph is much better defended against large (0.9 mm) Asplanchna than either the basic or Daphnia‐induced morph. The long right posterolateral spine usually prevents capture or ingestion. The Asplanchna‐ and Daphnia‐induced morphs are similarly susceptible to interference from large (3 mm) D. pulex. 3. Life‐table experiments with cohorts of the basic and Asplanchna‐induced morphs at 5 × 10³ and 2 × 10⁴ cells of Cryptomoas erosa per millilitre indicate little or no cost of the induced defense. Lifetime fecundity (13–15 offspring per female) did not differ significantly between morphs. The mean intrinsic rate of natural increase (r_m day⁻¹) of the induced morph was very slightly but significantly lower than that of the basic morph at the lower food concentration (0.46 versus 0.48) but no different from it at the higher food concentration (0.53 versus 0.54). However, spine development may involve undetermined allocation costs and environmental costs relating to interactions with other organisms. 4. It is not clear why K. tropica has separate induced responses to Asplanchna and Daphnia. Moderate spine development probably reduces damage or ingestion by small (<1.5 mm) daphniids, as in other species of Keratella, but further development may confer no protection against larger ones. Thus, the ratio of benefit to cost with daphniids (and other cladocerans) may be highest for intermediate spine development. In contrast, much greater spine development seems necessary for effective defense against Asplanchna. The more moderate response to Daphnia also may reflect less likely spatial and temporal overlap.     

Morphological variation of <Emphasis Type="Italic">Keratella cochlearis</Emphasis> (Gosse) in Myanmar (Burma) in relation to zooplankton community structure

Keratella cochlearis was present in 27 of 35 water bodies sampled in Myanmar, and was the most abundant rotifer in 10. Measurements of lorica length and posterior spine length from 20 localities showed that posterior spine length varied both with lorica length and with the composition of the crustacean zooplankton. Long spines were associated with dominance by Heliodiaptomus. The shortest spines were found in samples dominated by cladocerans or cyclopoid copepods. Posterior spine length was positively correlated with the number of diaptomid copepods. Forms without posterior spines were found in 17 localities. The lorica lengths of these spineless forms were generally similar to those of co-occurring spined forms (r = 0.68), but in a few samples the loricas of the spineless forms were significantly larger. These larger forms are similar to the ‘aspina’ forms recently recognised in the River Thames in England. These samples were dominated by cladocerans or cyclopoid copepods. In one locality spineless forms were found without spined forms. The crustacean zooplankton in this locality was also dominated by cladocerans. Guest editors: S. S. S. Sarma, R. D. Gulati, R. L. Wallace, S. Nandini, H. J. Dumont & R. Rico-Martínez Advances in Rotifer Research     

Developmental Polymorphism Induced by Intraspecific Predation in the Ciliated Protozoon Onychodromus quadricornutus1

ABSTRACT. The hypotrich ciliate Onychodromus quadricornutus is remarkable in its potential for voluminous size (up to 900 μm in length), its possession of a unique set of four dorsal spines or horns, and its capability to express two kinds of developmental polymorphism induced by intraspecific predation. Cell length frequencies in replicates of a well-fed clone show normal distributions; starvation followed by intraspecific predation, however, induces cells within a clone to transform into two size classes: small lanceolate cells and cannibal giants. Induction experiments indicate that a substance released by cannibal giants stimulates defensive spine growth in clonemates within 24 h. Giants can also induce spine growth in non-clonemates. Furthermore, O. quadricornutus cells exposed to the predacious ciliate Lembadion magnum also develop hypertrophied spines. Selection experiments show that conspecific giants prey on cells with undeveloped spines (< 20 μm in length) to a much greater extent than on cells with developed spines (>40 μm in length). Transformation of a population of similarly sized O. quadricornutus cells into two different size classes may function to increase the range of potential prey sizes available to the O. quadricornutus population; hypertrophied spines appear to function as an inducible defense against intermittent predators appearing in the system including conspecific giants. This is the first reported case of a defensive developmental polymorphism induced by intraspecific predation.     

Effect of Temperature on Fecundity,Life Span and Morphology of Long‐ and Short‐Spined Clones of Brachionus caudatus f. apsteini (Rotifera)

We investigated the effect of temperature (20, 25 and 30 °C) on fecundity, life span and morphology of the rotifer Brachionus caudatus f. apsteini. For each temperature, short posterior‐spined and long posterior‐spined clones of B. caudatus f. apsteini were individually cultured for up to six generations. The rotifers were fed Chlorella sp. at a density of 1 × 10⁶ cells ml^–1. Morphometric data (body size and spine length) were collected. Total number of offspring producing by a single female per life cycle at high temperature was higher than at low temperature. The duration of juvenile period, reproductive period, post‐reproductive period and life span of both clones of B. caudatus f. apsteini decreased with increasing temperature. All offspring of short posterior‐spined clone produce posterior spines at 20 and 25 °C, with an average length of 19.8 ± 6.6 and 11.9 ± 2.6 μm, respectively. In contrast, they cannot develop posterior spines at 30 °C, at which the average length of the posterior spine remnant was 6.4 ± 1.3 μm. On the other hand, all offspring of long posterior‐spined clone have long posterior spines with average lengths of 36.8 ± 6.1, 36.3 ± 5.2 and 36.6 ± 6.2 μm at 20, 25 and 30 °C, respectively. This study indicated that the production of posterior spines can be induced by low temperature and that short posterior‐spined and long posterior‐spined clones are genetically different. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Maternal age and spine development in the rotifer Brachionus calyciflorus: increase of spine length with birth orders1

1. Maternal effects have long been known to influence phenotypic plasticity in rotifers. Females in Brachionus calyciflorus and several other species produce long‐spined offspring when the predatory rotifer Asplanchna is present; B. calyciflorus also develops short spines when food concentrations are low. These spines protect against predation and decrease food threshold concentrations. 2. Some strains of B. calyciflorus develop long spines even in the absence of Asplanchna and other environmental stimuli. We demonstrate in this study that spine length in such cases is dependent on the age of the mother. 3. In strains from Florida and Georgia, offspring spine length increased significantly with birth order, sometimes to lengths formerly observed only in the presence of Asplanchna. Significant variation in this trait was found among and within clones of a strain. Offspring body size also increased with maternal age. This is the first time maternal age has been shown to affect rotifer morphology. 4. These birth‐order effects may have important ecological implications and explain phenotypic plasticity and polymorphism in body size and spine length in populations when predators are absent and food concentrations are high. They may be a bet‐hedging mechanism to assure adaptation to rapid changes in predation pressure or food conditions.     

GENETIC VARIATION AND POLYMORPHISM IN THE INDUCIBLE SPINES OF A MARINE BRYOZOAN

Of particular value in understanding the evolution of genotypes with broad phenotypic ranges (phenotypic plasticity) are the few examples of organisms with adaptive plasticity, such as those that develop a defensive morphology in response to cues from predators. We know little about the heritability of inducible defensive characters or the range of phenotypes available for selection to act on in the field. Membranipora membranacea is a colonial marine bryozoan that produces spines within two days of exposure to waterborne predator extracts. Surveys done in 1993 and 1995 showed that the population at Friday Harbor Labs, Washington, was polymorphic for inducible spine type and was composed of a constitutively spined type that produced spines in the absence of a predator stimulus, an unspined phenotype that did not produce spines irrespective of a stimulus, and an inducibly spined phenotype that would produce spines if exposed to the appropriate cue. In 1995, the frequencies of these types were determined through a laboratory common-garden experiment; 178 colonies were cultured from metamorphosis through approximately 30 days and then exposed to the cue. The inducible type was the dominant, comprising 80.3% of the population. The constitutive type made up 6.2% of the population, and the remaining 13.4 % was the unspined type. The frequency of the three types was similar to a preliminary trial of the experiment run in 1993. Experiments also showed that the lengths of the spines of the inducible type varied continuously among genotypes. To assess causes of variation in the inducible spine response and its clonal heritability, 16 clones were subdivided and grown in a common environment and exposed to a single dosage of spine inducing substance (SIS). Spine length showed high clonal heritability. The range of colony responses from a single environment varied from relatively unresponsive to highly responsive colonies with a very low threshold of response. Norms of reaction were quantified for spine lengths of inducible genotypes originating from two field environments by testing them in a concentration series of SIS. Both spine length and spine type varied with concentration of inducer. Within a clone, colonies were more likely to produce membranous spines than corner spines at higher concentrations. At low concentrations, only straight spines were produced. This study showed that populations of M. membranacea at Friday Harbor are a mix of inducible, nonspined and constitutively spined individuals. Even the inducible individuals showed high heritable variation in the length of spine activated, suggesting that there is considerable scope for the evolution of this character. A norm-of-reaction experiment further showed that the type of spine produced, membranous or corner, varied with the concentration of the cue. Factors maintaining the polymorphism and the broad range of genotypes could include high costs of defending the spined types coupled with a shifting biotic regime.     

Taxonomic significance of spine morphology in the echinoid genera Diadema and Echinothrix

Abstract. The spine morphology of all established species of Diadema and Echinothrix, including 2 color morphs of E. calamaris, were examined externally and internally via transverse sectioning to identify diagnostic species features and to assess the morphological relationship between species. Forty‐nine different morphological characters were measured and analysed using ordination by multi‐dimensional scaling (MDS) and cluster analysis. Specimens of Diadema paucispinum and D. setosum had very distinct spine structures. In D. paucispinum, the spines were more robust than those of other species of Diadema. This was evident in the spine's internal structure, with large, closely packed solid wedges, a small axial cavity, and rings of trabeculae throughout the spine's length. The spines in D. setosum were distinctive because of their length in relation to test size and the reduced flaring of their verticillations. The spines of other members of this genus were very similar to each other. Without careful sectioning, the spines from specimens of D. antillarum, D. ascensionis, D. mexicanum and D. savignyi were difficult to differentiate. The internal structures of spines for each species did, however, possess a combination of features that differentiated the species. Such features included the shape, orientation, and number of solid wedges, the presence or absence of spokes and rings of trabeculae between the solid wedges, and the presence or absence of tissue within the axial cavity. Individuals of Diadema palmeri also had spines morphologically similar to other species, however, the red pigmentation of these spines (in life and when preserved) made them easily distinguishable. The spine structures of the 2 species of Echinothrix were starkly different, while the white and brown color morphs of E. calamaris had morphologically distinctive ambulacral and interambulacral spines. The blunt, open‐tipped interambulacral spines, with reticular tissue present in the axial cavity of the white color morph, were easily distinguished from the pointed, closed‐tipped spines, with a hollow axial cavity found in the brown color morph. Such differences indicate that the brown color morph is either a subspecies or a separate species. Taken together the data show that each species has significant morphological differences in the structure of the spines. It is evident from our data that spine morphology is a useful tool to differentiate these commonly confused species.     

Costs of predator‐induced morphological defences in Daphnia

1. Inducible defences are advantageous because they protect the prey while limiting associated fitness costs. The presence of these costs is an essential component of this conditional strategy, since their absence would favour constitutive (fixed) defences. In some cases, however, these costs have been difficult to measure because of complex interactions between the defences themselves, resultant life history changes and the organism’s environment. 2. The pond‐dwelling water flea, Daphnia pulex, forms defensive neck spines in response to kairomones released by predatory larvae of the phantom midge, Chaoborus. This predator–prey interaction and the formation of these inducible defences have been well studied, but costs associated with the development of neck spines remain unclear. In this study, I address this problem by analysing the effect of Chaoborus kairomones on the life history responses (and fitness costs associated with these responses) of two clones of D. pulex that are from the same pond population, but differ greatly in their degree of neck spine development. 3. Both D. pulex clones exhibited the same predator‐induced shifts in life history: larger size at birth, reduced juvenile growth rate (producing a smaller size at maturity), delayed reproduction and a reduction in the number of neonates produced after the first clutch. Relative fitness decreased significantly and to the same degree (c. 10% reduction in r) in each clone. This observed fitness cost was not directly related to the neck spines per se since the cost was the same in both clones, despite their considerable differences in neck spine development. Rather, it appears to be indirectly related to this antipredator morphology via a combination of delayed reproduction and a set of life history trade‐offs (decreased growth rate, decreased reproduction after the first clutch) for increased neonate body size, which is necessary for neck spines to be effective defences. This suite of induced responses is probably a result of local adaptation of these two D. pulex clones to their common pond environment. 4. Costs of inducible defences do not always entail direct allocation costs associated with forming and maintaining a defence, but may also involve indirect life history responses that are specific to particular environmental situations. This local adaptation would explain the highly variable life history responses observed among D. pulex clones from different pond environments.     

Life-history responses to Chuoborus of spined and unspined Daphniu pulex

Water-borne chemicals released by the larvae of the predatoryphantom midge Chaoborus are known to induce morphological modificationsin its prey Daphnia pulex: these cladocerans develop neck spineswhich may carry several teeth. Some work has shown that thesemorphological variations enhance the prey's chances of escape.but since these neck teeth are not fixed defence reactions,they are thought to entail some form of cost, such as delayedmaturation and reduced fecundity. In this study. the relationshipbetween morphological and life-history changes in four clonesof Daphnia pulex reared in the presence and absence of Chaoborusflavicans was examined. Special emphasis was placed on the genotypiccomparison of the modifications. While all four clones showeda delay in maturation time in the presence of Chaoborus, theneck spine responses differed markedly among the genotypes:one clone never had any neck teeth, another always producedone single tooth, and two clones produced varying numbers ofteeth per spine (means 2.9 and 4. respectively). These resultsindicate that there is no general pattern of neck teeth productioncorresponding to delayed maturation. What there appears to beis genetic variability in two independent and possibly adaptiveresponses. However, the clone without neck teeth was the onlyone which showed no predator-induced reduction in fecundity.Another common morphological response to Chaoborus was thatjuveniles of all clones developed elongated tail spines.     

Morphological Form Variation in a Loricate Rotifer,Keratella tropica APSTEIN from a Perennial Pond,Janaktal, Gwalior,India

Morphological form variation in a loricate rotifer, Keratella tropica APSTEIN is decribed. Form variation in this rotifer involves appearance and development of the left postero-lateral spine. The right postero-lateral spine varies, too. It increases with increasing length of lorica and left postero-lateral spine. Three morphological forms, viz. reducta, asymmetrica and heterospina are recognized.     

Taxonomic problems with the species Keratella hiemalis

Keratella hiemalis was described by Carlin (1943) from Motalaström in southern Sweden. It was distinguished from K. quadrata by its low morphological variability and its occurrence at only low temperatures. Morphological characteristics are: straight lateral borders of carapace and spines, caudal spines of medium length, and triangular, first median facet. The species has been found by many authors in the hypolimnion of oligotrophic lakes. Some workers have applied the name Hiemalis to a similarly-sized species, K. testudo. This species occurs in high mountain and arctic pools and displays substantial morphological variability.Deceased.     

Crowding‐induced changes in growth,reproduction and morphology of Daphnia

• 1 Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life‐history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow‐through system in the presence of abundant food.
• 2 Water from Daphnia at 85 L^‐1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small‐bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species.
• 3 Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D. lumholtzi and D. ambigua.
• 4 Chemicals released by crowded conspecifics may provide an additional, density‐dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.

     

Evolution ofNaso thynnoides and the status ofN. minor (Acanthuridae; Actinopterygii)

Naso minor was described from a single specimen (Smith, 1966). Only one other specimen has since been reported (Randall, 1986). The species apparently differed fromN. thynnoides in the ratio of fork length to head length and eye diameter, the shape of the caudal peduncle spine, and in number of dorsal spines. Collections of 24 specimens of four- and five-spined individuals (putatively assigned to both species) from the Philippines revealed that the first three differences are not valid. However, spine number, the length of the nasal groove, the pigmentation of the basal plate of the caudal peduncle spine, and the morphology of the first dorsal-fin pterygiophore confirm the distinctness of the two species.     

Extreme cyclomorphosis in Daphnia lumholtzi

1. Daphnia lumholtzi, not previously reported in North America, was found in a small reservoir in East Texas in January, 1991, This species possesses extremely long spines and large fornices; an allometric study was performed to detect any temporal differences in specific growth rates of the spines relative to the body. 2. In nature, mature females attained 1.8mm body length, excluding spines, but when the head and tail spines are included, the total length reached a maximum of 5.6mm. 3. Differences in the growth patterns of the head spine and the tail spine relative to the body existed for D. lumholtzi from January to March 1991. Both the head and the tail spines grew at a faster rate than the body during all 3 months although the rates varied between them. The results contradict the invertebrate predation hypothesis (Dodson, 1974) in that D. lumholtzi's head and tail spines continue to grow during adulthood instead of stopping after the juvenile instars. 4. The head spines grew at a constant allometric rate over time while the tail spine grew faster as the temperature increased. Both varied significantly in length over the 3 months, with animals having the shortest spines in February and the longest in March.     

Cyclomorphosis in Daphnia: an adaptation to avoid invertebrate predation

Seasonal morphological changes in three Daphnia species were followed over a two-year period in two lakes that differ in invertebrate and fish pressure. Whereas the morphology of D. hyalina, the biggest of the three species, varied little from season to season, D. cucullata, the smallest, exhibited the most pronounced seasonal changes in head height/carapace length ratio. The pattern of seasonal changes of body proportions was similar in all size classes and isometric growth of the head was reported for D. cucullata. Unlike the head, tail spine length/carapace length ratio almost did not vary seasonally. Strong negative allometry of tail spine growth was observed. These results are consistent with the hypothesis that helmets and tail spines provide protection against invertebrates in the two smallest, thus most endangered species.