Novel sex-related characteristics of the longsnout seahorse Hippocampus reidi Ginsburg , 1933

This paper presents novel sex-related characteristics in Hippocampus reidi, expressed as a prominent and pigmented keel (males only), and by a row of dorsolateral spots mostly (97.4%) found in males. While the keel was found even in the smallest males sampled, the dorsolateral spots appear to be associated with body size. Ex situ observations revealed that during courtship males highlighted both characters by turning pale their background body colour. Understanding such colour and behavioural patterns is relevant to increase our knowledge about sexual selection in a morphologically and biologically unique group of fishes.

Knowledge about sexual dimorphism is central for understanding the ecology, behaviour, and life history of a species, as well as for making morphological comparisons between populations (e.g., Kitano et al., 2007).Many sexual dimorphisms are secondary changes in males, which generally are larger, often more colourful and more conspicuous, and more vocal than females (Darwin, 1871).Such secondary sexual characteristics are often interpreted as promoting success in male-male competition or female choice, and hence increasing opportunities for mating (Iguchi et al., 1991).
Seahorses (genus Hippocampus, family Syngnathidae) have a social structure that involves close pair-bonding, and are distinguished by extreme morphological specialization for paternal care, represented by a specialized pregnancy structure, the brood pouch.Aside from the pouch, however, these fishes reportedly tend to show no or little sexual dimorphisms (Foster & Vincent, 2004) other than those expressed as differences in body proportions.Sex-related disparity in colour patterns have seldom been described for seahorses, although dichromatism appears to be present in some species (see Lourie et al., 2004).
This paper describes novel sex-related characteristics of the longsnout seahorse Hippocampus reidi Ginsburg, 1933, based on specimens from Brazil.Hippocampus reidi is a slender-body species with variable colour patterns, consisting of at least six distinct background colours (Rosa et al., 2002), often profusely spotted with dark dots and numerous tiny white dots (especially on the tail), and may also have pale saddles across dorsolateral surfaces (see Lourie et al., 1999Lourie et al., , 2004)).As most seahorse species studied to date (see Foster & Vincent, 2004, for a review), H. reidi was inferred to be monogamous and forming pair bonds (Rosa et al., 2007).Published data on sex-related differences in the species are limited to the brood pouch and height (Foster & Vincent, 2004;Rosa et al., 2007).
Daytime underwater censuses (50 X 1-2 m transects) were performed in depths ranging from 0.01-2.0m.For each sighted specimen of H. reidi, data on colour patterns, height (following Lourie et al., 1999), sex and reproductive state (following Foster & Vincent, 2004;Rosa et al., 2007), and life stage (following Castro et al., 2008) were recorded.To avoid the risk of classifying young males (which have no brood pouches) as females, all juveniles were placed in the category "undetermined sex".Specimens were carefully hand-picked, kept underwater throughout data collection and then returned to the same place where originally found.Seahorses were identified by a collar placed on the base of the tail (Felício et al., 2006) to ensure that throughout the study all individual data came from different specimens.At the end of the study, an extensive effort was made to remove the marks.
Additional information was obtained from 21 pairs of captive reared specimens of H. reidi kept at the Laboratório de Peixes -Ecologia e Conservação (LAPEC), Universidade Federal da Paraíba, Brazil, between August 2007 and October 2008.Seahorses were observed using focal-animal sampling, following Lehner (1996).Each specimen was observed for a total of 60 min, during which all behaviours were recorded and filmed.
In order to assess diferences in the occurrence of the colour traits between sexes, life stages and reproductive states, χ² tests were performed.Juveniles were excluded from these analysis.Additionaly, comparisons between males height and the occurrence of colour traits were made using t test.
Examination of 1,406 specimens of H. reidi (582 males; 599 females; 225 juveniles) in the field revealed the existence of a novel sexual dimorphism in the longsnout seahorse (Fig. 1), expressed as an exclusive pattern consisting of a prominent and pigmented keel in males, and by a row of conspicuous dorsolateral spots on the trunk, mostly found in males.Marks appear as circled dark spots arranged dorsolaterally between adjacent trunk rings, on both sides of the body, and varied in number and colour intensity.All male seahorses examined in this study had the characteristic keel, from the smallest (8.8 cm height) to the largest specimen (19.3 cm), consisting of a sharp and dark edge along the ventral portion of the trunk.Dorsolateral spots were found in 342 specimens, the vast majority of which (97.4%) were males (χ² = 452.32,d.f.=1, p < 0.001; Fig. 2).Despite strongly related to male specimens, dorsolateral spots were found in seven females (n total = 599) and two juveniles (n total = 225).The occurrence of dorsolateral spots in males was significantly higher in adult specimens (χ² = 55.32,d.f.= 1, p < 0.001), regardless of reproductive stage (χ² = 2.66, d.f.= 1, p = 0.10).A positive correlation was also found between the occurrence of dorsolateral spots and mean height of adult males (Fig. 3), as follows: specimens with markings were significantly larger than those which did not exhibit that pattern (t = -11.12,d.f.= 580, p < 0.001).
Ex situ observations of 21 pairs of Hippocampus reidi revealed that during courtship the background colour of both males and females became pale (Fig. 4).In the males, this change highlighted the dark colouration of the keel and accentuated the dorsolateral spots against the paler background colour.
In general, morphology of seahorses is conservative and sexual dimorphism is mainly demonstrated by the brood pouch in males, besides body proportions and height (see Foster & Vincent, 2004).Other sex-related differences have been suggested to occur in a few species (see Table 1).Sexual dichromatism regarding dorsolateral spots, however, was only recorded for H. trimaculatus (Lourie et al., 1999(Lourie et al., , 2004), H. hippocampus ("form microstephanus"), H. manadensis and H. takakurai (Kuiter, 2009), in which this character was found exclusively or more frequently in males (Table 1), as found in this study for H. reidi.
Differently from the dorsolateral spots, the prominent and pigmented keel was only found in males, corroborating the results of Barros (2005), who observed that females of H. reidi lacked a keel.Records of sexual dimorphism related to the latter characteristic -described as a sharp median ridge running down the ventral side of the trunk (Lourie et al., 2004) -are uncommon in the genus Hippocampus (Table 1).Males with prominent keels have been recorded in H. capensis, H. ingens, H. jayakari, H. histrix (Lourie et al., 1999(Lourie et al., , 2004)), and H. biocellatus (Kuiter, 2009).In H. abdominalis, however, females' keel was usually deeper than in males (see Lourie et al., 2004).
Our observation that H. reidi males highlight the dorsolateral marks and keel during courtship by turning their background colour into a paler pattern was anecdotally reported by Kuiter (2009) for a morphotype of H. hippocampus.Keel highlighting due to body colour brightening was also reported for males and females of H. fuscus during courtship (Vincent, 1990).Given that brightening of colours is a characteristic behaviour displayed during seahorse daily greetings and courtship (see Foster & Vincent, 2004), and that monogamy appear to be reinforced by these daily greetings (H.fuscus, Vincent, 1995;H. whitei, Vincent & Sadler, 1995;H. zosterae, Masonjones & Lewis, 1996), the display of the highlighted marks and keel observed in this study may be an important signal to communicate readiness to mate, although the occurrence of dorsolateral spots per se was not related to reproductive activity.Amplification of the normal contrast of colour patterns has also been recorded for females of the pipefish Nerophis lumbriciformis (Monteiro et al., 2002), while in Syngnathus typhle and Syngnathus abaster both sexes amplified the normal contrast, in the  roles (H.abdominalis, Wilson & Martin-Smith, 2007;Mattle & Wilson, 2009;H. guttulatus, Naud et al., 2009).Mate choices often depend upon coloration and strongly affect both population divergence and speciation (Andersson, 1994).Therefore, it is not surprising that the use of colour patterns as sexual signals has been demonstrated in many other fish species (see Kodric-Brown, 1998).
The permanent striped pattern present in the pipefish Syngnathus typhle (present in both sexes) is increased in females during nuptial dance and intrasexual competition (Berglund et al., 1997;Bernet et al., 1998).Different studies have shown that this behaviour has important functions, such as predicting female mating success (Bernet et al., 1998), determining female dominance (Berglund & Rosenqvist, 2001), intimidating rival females (Berglund & Rosenqvist, 2009) enhancing reproductive success (Berglund et al., 1997), and serving as an amplifier of female body size (Berglund, 2000).With regards to S. typhle males, Berglund et al. (2005) found that specimens which displayed more benefited from this by receiving more eggs.Whether the colour patterns described here are used during malemale competition or for signaling male quality to females, it is unknown.Nevertheless, it is likely from our results that the patterns displayed by H. reidi represent a trade-off context of courtship and competition (Berglund et al., 1997(Berglund et al., , 2005;;Bernet et al., 1998;Silva et al., 2010).Given that both males and females seahorses pale their background colour during courtship, males should have distinct attractive patterns to call the attention of females -commonly the choosy sex in this group (see Foster & Vincent, 2004), although some species appear to show some plasticity in sex between decreased crypsis and increased conspicuousness.Adult seahorses are presumed to have few predators (Foster & Vincent, 2004), and therefore the highlighted colour patterns described in this study possibly do not enhance predation risk during reproduction.Describing a novel dimorphism in H. reidi, we hope to stimulate further research on biology, ecology and behaviour of seahorses.