Swordfish are recognized marine predators, easily identified by their elongated, flattened bill. While adult swordfish are a familiar sight in ocean documentaries and fishing tales, their early life stages remain less commonly understood. The appearance of a baby swordfish, particularly in its larval form, differs markedly from that of its imposing adult counterpart.
Distinctive Features of Larval Swordfish
Newly hatched swordfish larvae are small, typically measuring about 4 millimeters. Their body shape is more rounded and plump than the sleek, torpedo-like form of an adult swordfish. These larvae often exhibit translucent or lightly pigmented coloration, contrasting sharply with the darker, metallic hues of mature swordfish.
Their proportionately large eyes are prominent on their small heads. The characteristic “sword” of the adult is not yet developed at hatching; instead, the upper jaw may appear as a short snout. At this early stage, the lower jaw is often longer than the upper jaw.
Larval swordfish possess continuous dorsal and anal fins along much of their body. These fins are not yet separated into the distinct finlets and tall dorsal fin characteristic of older swordfish. Young swordfish are born with prickly scales, which are later lost.
Growth and Development of the Sword
As a larval swordfish grows, its appearance transforms, particularly concerning its distinctive bill. The upper jaw begins to elongate and flatten, gradually forming the recognizable sword. This elongation eventually surpasses the lower jaw, which was initially longer, marking a clear progression towards adult morphology.
The body also becomes more streamlined and elongated during this growth phase. Concurrently, the continuous dorsal fin separates into a large, rigid first dorsal fin and a smaller second dorsal fin, with similar changes in the anal fins. The translucent larval coloration gives way to darker pigments as they mature.
The bill becomes noticeable when the larvae reach about 10 to 12 millimeters. Swordfish grow rapidly, often reaching about 1 meter (39 inches) within their first year. This rapid development sees them growing at an average rate of 0.125 to 0.25 inches per day.
Early Life Environment
Larval and juvenile swordfish primarily inhabit warm, tropical, and subtropical oceanic waters. They are typically found in the open ocean, exhibiting a pelagic lifestyle, drifting with currents in the upper water column. This habitat provides necessary conditions for their initial growth.
During early life, young swordfish feed on available prey. Their diet initially consists of zooplankton and other fish larvae. As they grow, their diet expands to include small fish, squid, and pelagic crustaceans.
Survival rates for larval swordfish are low due to predation pressure from larger marine organisms. Their small size and reliance on surface currents make them vulnerable targets in the vast ocean. Warm temperatures facilitate faster metabolic rates and growth, advantageous for outgrowing predators and enhancing survival.