Long-spined scorpionfish are not particularly colourful, nor conspicious, animals, so it's quite likely that the few that reveal themselves in shallow pools and intertidal pans are but the visible tips of an invisible population at large and at home in the fringes of local reefs and offshore islands, where the scorpaenids lurk in the shadow of brown fronds and lead a life of stealth and fiendish subterfuge. Like other members of its family, Paracentropogon longispinis is armed with long spines on the dorsal and anal fins which inflict a biochemical bite on would-be predators. Accessory spines on the gills and near the eyes, though lacking waspish glands, discourage swipes at the head with erections of stiff resistance.
Paracentropogon belongs to a subfamily of Scorpaenidae popularly known as sailbacks or waspfishes, in which the dorsal spines emerge before or just above the eyes. Tetrarogines, as the group is called, are often characterised by a steep or slightly domed forehead, and possess a lacrimal bone with strong spines, which they can rotate to defend against facial assaults. Apart from a superficial resemblance to an aquatic cockerel, not a few members of the subfamily have markedly more modest jaws compared to the maws of other common scorpionfishes such as Parascorpaena picta and Scorpaenopsis diabolus (or cirrosa?), as well as their infamous cousin, the estuarine stonefish. The latter three species habitually lie among rock and rubble, revealing little of their profile until they engulf a victim and displaying a chronic reluctance to budge even when prodded by fragile feet.
Lacking a wide belly on which it can rest, Paracentropogon usually lurks amid dense weed or lingers with its flanks oblique on pebbly flats, where its mottled colours and laterally compressed form shield it from sight for the most part. This scorpionfish, the smallest of the 25 species recorded locally and by far the most abundant, shares its home with other small fish, transparent carideans and juvenile swimming crabs, but appears rather ill-equipped to tackle its neighbours; the predators themselves are often no larger than a lagoon goby and have gapes that can scarcely wolf down a plump Drombus. As it turns out, Paracentropogon is of little threat to its piscine bedmates – these little hunters are now known to feed largely on small invertebrates, particularly amphipods, minute crustaceans with flea-like bodies that inhabit marine vegetation and attain pestilential numbers during seasonal blooms of near-shore chlorophytes.
There are a few ecological consequences to this dietary specialisation, as revealed by Kwik (2011), who compared the trophic relationships between Paracentropogon longispinis and a sympatric, slightly larger, cousin, Trachicephalus uranoscopus. The latter, which belongs to the same tribe as Synanceia, is a benthic predator that may be mistaken for a stargazer, as both species share the habit of burying themselves in loose sediment to suck in unwary prey. Post-larval stargazer waspfish, according to Kwik, feed on amphipods and other small malacostracans, but adults target gobies, with the common and toxic Acentrogobius nebulosus forming a significant portion of its diet. There is, hence, little interspecific competition for food between the two scorpaenids and room for co-existence on impacted shores such as Changi Point, where the two species and another, non-specialist, piscivore, Cymbacephalus nematophthalmus, occur in numbers. The two scorpionfishes also pursue quite different foraging tactics: Paracentropogon is an active stalker, whereas waspfish launch surprise attacks from concealed positions.
Kwik also found that despite low mortality from predation, as might be expected of such venomous, cryptic animals, long-spined scorpionfish lead brief, furious lives, dying at about 1.5 years of age and reaching their maximum length in 11-15 months. Waspfish are similarly short-lived, surviving for just 2.5 years. The trade-off to success in shallow zones, it appears, could well lie in the need to devote physiological resources, ample as they are, to rapid growth and maturity as well as the remediation of stresses inflicted by volatile environments where salinity, temperature, turbidity and terrain are in constant flux. This strategy is reinforced by the reproductive patterns of both species, which spawn only during the northeast monsoon, a period of plenty that is believed to boost larval survivorship. What remains to be seen, however, are the fishes' thresholds of resilience to these coastal pressures, which are mainly man-made and have likely contributed to the near local extinction of other native scorpaenids, and which continue to chip away, in meek deference to an implacable hunger for land and luxury, at what remains of the habitats of these small, short-lived scorpions.