George W. Barlow

Patterns of parental investment, dispersal and size among coral-reef fishes

SynopsisCoral-reef fishes have been selected to produce propagules for dispersal because they live in a patchy environment, and the adults cannot migrate between patches. For large species (>100 mm SL) and widely separated patches, numerous propagules are needed, often with specialized pelagic intervals. Individuals of small species are confined to portions of the reef. They are unable to produce enough eggs for effective longrange dispersal, and so they keep their vulnerable eggs and young out of the plankton until they are well developed enough to seek out and settle onto the appropriate habitat before dispersing.Guarded demersal eggs, requiring a greater individual investment by the small short-lived species, further reduces their individual fecundity. These costs to fecundity, and the reduction in vagile young appear to account for the lack of postzygotic investment in larger longlived species.All coral-reef fishes are selected to disperse, usually with a young planktonic propagule interval. Both large and small species produce a mixed outcome, with some propagules returned to, or retained at the home reef, while others disperse more widely. The smaller the species the greater the proportion of propagules retained.

Do Midas cichlids win through prowess or daring? it depends

SummaryIndividual Midas cichlids (Cichlasoma citrinellum) show persistent differences in aggressive behavior toward dummy fish. The starting question was whether the level of such behavior can be used to predict the winner of a fight. In the first (long-term) experiment two fish were matched for size, color, and sex. After 24 h the opaque barrier separating the 2 fish was removed; they immediately behaved aggressively. Scores for aggression toward dummies did not predict the winners, nor did taking the initiative in escalation. But weight did foretell the winners, who averaged only 2% heavier than their opponents. The conventional display phase of such fights was brief (20% of total duration), and escalation was rapid. Both winners and losers sustained damage, but losers accumulated damage faster than winners. In the subsequent (short-term) experiment the fish were separated only 1–2 h. Now aggression scores predicted winners, and winners were the fish who escalated. Weight of fish had no effect. The conventional phase was relatively much longer, about half the length of the fight. Losers accumulated damage at the same rate as the losers in the long-term experiment, but the fights were shorter; winners suffered little damage. The fish had difficulty assessing one another. Fighting prowess was remarkably uniform when weight was factored out. Daring to escalate, in contrast, varied among individuals and correlated with aggression scores. Prowess (=weight) determined the outcome in the long-term experiment, which may find its parallel in nature in intraterritorial disputes. Daring to escalate determined the winner in the short-term experiment; this may be comparable to establishing a territory in nature.Prowess probably results from strong directional selection because it has low costs and high benefits. In contrast, daring is subject to bi-directional selection because both costs and benefits are high. Resource holding potential is conventionally viewed as deriving from prowess of self and opponent and value of resource; to that one must add the individuals inherent aggressiveness.

Competition between Color Morphs of the Polychromatic Midas Cichlid Cichlasoma citrinellum

The Midas cichlid, Cichlasoma citrinellum, is a polychromatic fish that occurs in Nicaragua. All of these fish start life as normally colored, cryptic individuals. In some populations a few fish change into conspicuously colored morphs, most frequently gold. When kept in unmixed color groups, golds and normals grow at the same rate; but when they are mixed, growth of the golds becomes faster and that of the normals slower. Golds dominate normals in contests over food, which accounts for their advantage.

Variation and Stereotypy in the Displays of Anolis Aeneus (Sauria: Iguanidae)

The reactions of a territorial male lizard (Anolis aeneus) to an intruder were filmed and analyzed. Five displays were given to the intruder: Fanbob, Multibob, Jerkbob, Gorged-throat and Fan. Fanbob consists of the species-specific signature bob and dewlap extension and is the normal display given as a challenge to intruders. Multibob, another bob pattern, occurs when a degree of anxiety is present. It closely resembles the Courtship Bob, so the male is in effect giving an appeasement display while courting a female. Jerkbob is used as an introductory display preceding Fanbob during territorial advertisement. Gorged-throat is a threat display, and Fan may be used instead of Fanbob when inhibitory factors prevent Fanbob from being expressed. Fanbob was analyzed to test whether it fulfills the criteria of the Fixed Action Pattern. The cadence of the signature bob was individual-specific and stereotyped, but other components of Fanbob varied greatly. Among these were presence and absence of introductory movements (related to intruder distance), degree of sideflattening and degree of dewlap extension (related to resident aggressiveness), and height of head during the display. Height and dewlap extension, while necessary and integral components of Fanbob, varied independently, and were correlated with different sets of stimulus and response variables. Fanbob was thus shown to fit none of the criteria of a fixed action pattern.

Predicting and assessing dominance from size and coloration in the polychromatic midas cichlid

Abstract To test whether gold morphs dominate normal ones, twelve groups were observed, each consisting of three golds and three normals paired by weight. Golds dominated comparable normals. Smaller golds also profited from sharing the colour of the dominant fish. Contest and approach declined across days, but attack and yield remained high. Standard deviation of rank was progressively greater with higher rank, but uncertainty of rank plotted as an inverted U. Behaviours important in dominance showed high dependence on type of fish but low dependence on day; these behaviours also had more variation attributable to the coloration factor. From a set of stated criteria, dominance is deemed best assessed here by an index: (attacks) ļo (yields + attacks); contesting (threat) is actually misleading.

Spawning, eggs and larvae of the longnose filefish Oxymonacanthus longirostris, a monogamous coralivore

SynopsisThe longnose filefish (Monacanthidae, Oxymonacanthus longirostris) was studied at a patch reef on Enewetak Atoll. It swims with undulations of its median fins and feeds almost continuously on coral polyps, using its beak-like mouth. The spot in the tail resembles an eye so that the tail looks like a head when it protrudes from the coral during feeding. There is slight dichromatism in that males have brighter pelvic flaps. Most of the fish were color tagged for individual recognition. They tended to remain within a given area, although some fish roamed throughout the patch. Most of the larger adults formed monogamous pairs; the smaller fish were variously alone or in groups of 1–5 individuals who appeared to know one another. Only pairs having the largest males, on the periphery of the patch, were clearly territorial. Aggression was a common event featuring a few distinctive displays, most of which were used in courtship as well. Spawning may be a daily event, occurring around 1600 h. The female probes and selects a tuft of blue-green algae; the male nuzzles the female and they spawn in the algae. The green eggs are about 0.7 mm in diameter, demersal, adhesive, and hidden in the algae; they hatched in 53.5 h, just after sunset. The planktonic larvae fit the monacanthid type B of Leis & Rennis (1983) and were initially 2.5 mm TL; by day 5 their yolk was gone.

Why is the coral-reef fish Valenciennea strigata (Gobiidae) monogamous?

Abstract Parental-investment theory predicts that monogamy should be rare, and empirical evidence supports this prediction. Monogamy has generally been explained by either (1) a need for biparental care, or (2) a uniform distribution of limiting resources. By contrast, monogamy has evolved in several coral-reef fishes without biparental care, and many coral-reef fishes may not be limited by resources. Monogamy in these fishes might be explained by either (3) a low population density that favors mate fidelity, or (4) an abundance of resources that allows all males to breed and thus lowers the cost-benefit ratio for females that mate guard. We tested predictions of these hypotheses in the coral-reef fish, Valenciennea strigata, except that (1) biparental care had previously been rejected. We found no evidence of resource limitation (2a): food densities within territories did not differ from the density of food in unused habitat adjacent to territories; potential nest sites also existed in this unused habitat. Similarly, (2b) if resources limit the population, territory defense might require the coordinated efforts of a pair; however, widowed fish maintained their territories and (3) remated rapidly. Finally, (4) all males maintained a nest, both sexes enforced monogamy by mate guarding, and females benefited from guarding a high-quality (large) mate. Females mated to large males fed more than when they paired with small males. The results support the hypothesis that abundant and uniformly distributed resources can lead to monogamy by lowering the cost of guarding a mate when mate guarding provides benefits.

Do gold Midas cichlid fish win fights because of their color, or because they lack normal coloration?

SummaryGold morphs (G) of the polychromatic Midas cichlid, Cichlasoma citrinellum, dominate those of normal coloration (N). The adults vary greatly in weight because they continue to grow after reaching sexual maturity; relative weight is an important determinant of dominance. The first objective here was to characterize dominance in relation to color and to relative size by staging a large number of encounters between fish of various weights. Logistic curves were fitted to the data to analyze the relationship between relative size and winning. The baseline curves for N vs N and G vs G (Fig. 1, Table 1) did not differ (Table 2). The shape of the curve for N vs G also was not different from that of the baseline curves. But the curve for N vs G was shifted such that N must weigh 116% of G for the probability of winning to be 0.50.The dominance effect of G stems directly from their color, not from the absence of normal markings or from genetically coupled aggressiveness. When G were matched against white morphs (W) the logistic curve was displaced such that W must be 117% the weight of G to enjoy a 50% chance of winning, just as with N vs G. In N vs W, the chance of either winning was 50% when they were equal in weight, thus just as in the baseline curves for N vs N and G vs G.Absolute size played a role. Large mature fish had a higher logistic coefficient, indicating greater sensitivity to differences in relative weight, than did small immature fish (Table 3).Data were also gathered from some of the contests on the consequences of winning and losing. Losers were often forced to the surface and suffered more splits in their fins. However, when G lost to N many G were tolerated near the bottom (Table 4). On average, G losers had fewer splits in their fins than did the N winners. W losers were tolerated near the bottom, and W winners and losers suffered litte damage, suggesting the colorless pattern W provokes fewer attacks (Tables 4–6).

Incompatibility and assortative mating in the Midas cichlid

Summary1.The Midas cichlid Cichlasoma citrinellum of Nicaragua is a polychromatic fish. In some populations about 10% of the adults of both sexes are colored as the common goldfish. To explore differential effects of coloration on pair success, all four combinations of sex and color were observed, first separated by a transparent screen, then together. The normal size relation was used, i.e., the female about 85% the weight of the male.2.Time to spawn showed no difference according to pair types with or without the screen, but the pairs that formed without the screen spawned more quickly than those with it. Failure to pair due to the male attacking the female, without the screen, occurred faster in pairs with normal than in those with gold females. The combination least able to form a pair was gold male x normal female.3.Mixed-color pairs were then tested with the females half or equal to the weight of the males. Equal-sized mates in gold male x normal female pairs resulted in pair success comparable to “normal” levels. Other studies indicate that gold color inhibits aggression. Apparently the combination of larger size and gold color makes the gold male too intimidating for the normal female, while making the female as large as the male compensates for the males gold color.

Colour and levels of aggression in the midas cichlid

Abstract Previous work established that gold morphs of the Midas cichlid ( Cichlasoma citrinellum ) dominate normal ones of about the same size. Left unresolved, however, was whether the gold morphs dominate because they are inherently more aggressive or because the gold colour inhibits aggression. The issue was clarified here by comparing levels of aggression within groups of golds only, normals only, and golds plus normals. At first the groups of golds were the least aggressive. But 2 days later the levels of aggression in the other groups fell to about that of the golds. We conclude therefore that the gold colour inhibits attack, but that this effect is only discernible in groups when they are establishing inter-individual relationships disappearing when the groups stabilize. We suggest that gold coloration inhibits attacking by stimulating fear responses.