Diagonal-banded Sweetlips

The Diagonal-banded Sweetlips, scientific name Plectorhinchus lineatus, is a striking member of the Haemulidae family, commonly known as grunts or sweetlips. This species is a favorite among divers for its bold patterns and often forms captivating aggregations on coral reefs. It is widely distributed across the tropical waters of the Indo-Pacific, ranging from the eastern Indian Ocean to the western Pacific, including the Great Barrier Reef, Ryukyu Islands, and New Caledonia.

Typically found at depths of 1–35 m, the Diagonal-banded Sweetlips inhabits both inner and outer reef environments, including coral slopes, clear lagoons, and seaward reefs. While juveniles prefer shallow, sheltered reefs, adults are often observed individually or in groups. These fish are primarily nocturnal, foraging for benthic invertebrates, crustaceans, mollusks, and small fish over sandy flats and seagrass beds. During daylight hours, they conspicuously rest, often sheltering under ledges, bommie overhangs, or near coral thickets, making them a reliable subject for underwater photography. The Diagonal-banded Sweetlips is currently listed as "Least Concern" by the IUCN Red List, indicating a stable population. This species can reach a maximum total length of up to 72 cm, though individuals around 50–60 cm are commonly observed.

Identification

The Diagonal-banded Sweetlips is easily identifiable by its distinctive coloration and body shape, though its appearance changes significantly from juvenile to adult:

• Adults:
  • Body Coloration: The body is typically light grey to silvery white, fading to a clean white on the belly.
  • Stripes and Patterns: The most prominent feature is the numerous oblique black stripes that run across the upper part of the body. These stripes can sometimes appear broken or even form spots on the lower sides.
  • Lips: Adults possess prominent, fleshy yellow lips that become moderately swollen with age.
  • Fins: All fins are generally yellow. The caudal (tail), anal, and dorsal fins are adorned with distinct black spots. A bright red patch or spot is typically visible at the base of the pectoral and pelvic fins.
  • Body Shape: The body is compressed laterally, giving it an oval profile.
• Juveniles:
  • Coloration: Younger individuals exhibit a starkly different pattern, featuring a few horizontal to slightly angled dark brown bands along their body. As they mature, these bands gradually break up and transition into the oblique stripes and spots seen in adults.
  • Habitat: Juveniles are typically solitary and found in shallower, more protected reef areas.

The Diagonal-banded Sweetlips (Plectorhinchus lineatus) is a captivating inhabitant of the tropical Indo-Pacific, offering divers a vibrant spectacle across a variety of reef environments.

Habitat and Depth:

• This species is primarily found in warm, tropical marine waters, typically associated with coral reefs and inshore areas.
• Divers can encounter the Diagonal-banded Sweetlips in a range of depths, generally from 1 to 35 m, though some observations extend to 50 m.
• Adults frequent deep inner to outer reef habitats, including coral slopes of clear lagoons and seaward reefs.
• Juveniles, however, prefer the safety of shallow, protected reefs.

Ecosystem Associations and Substrate:

• During daylight hours, these sweetlips are often observed resting, either singly or in aggregations, near prominent coral bommies, reef ledges, and under overhangs in gulfs, bays, and lagoons.
• As nocturnal feeders, they become active at night, venturing over open sand flats and into seagrass beds to forage for benthic invertebrates.
• The species thrives in waters with temperatures typically ranging from 23.5 to 30.5°C.

Geographic Distribution:

• Its extensive range spans the eastern Indian Ocean and the western Pacific Ocean.
• Key areas include the Ryukyu Islands and Ogasawara Islands in the north, extending south to New Caledonia and across much of Australia, including the Great Barrier Reef, Scott Reef, and Ashmore Reef.
• Other countries within its distribution include Indonesia, the Philippines, New Ireland, Maldives, Sri Lanka, India, Malaysia, Vietnam, China, Taiwan, Japan, Papua-New Guinea, Palau islands, Guam, Micronesia, Solomon Islands, Vanuatu, and Niue Island.
• Popular dive sites like Challenger Bay and Bait Reef — Stepping Stones are known locations where this species is commonly observed.

The Diagonal-banded Sweetlips (Plectorhinchus lineatus) is a striking and often-sought-after species for divers in the Indo-Pacific. Known for its distinctive patterns and intriguing behaviors, it offers excellent opportunities for observation and underwater photography.

Underwater Encounter Behavior

During daylight hours, divers will typically encounter Diagonal-banded Sweetlips in resting aggregations. These fish are diurnal in their resting habits, often found sheltering under ledges, in caves, or around coral bommies and thickets on reef slopes and channels. They may appear singly or in small groups, sometimes forming a "multicoloured wall" as they swim in a synchronized manner. When approached by divers, they are generally docile and can be quite shy, often remaining relatively still, making them reliable subjects for photography. Juveniles, however, are solitary and prefer shallow, sheltered reefs. Interestingly, juvenile sweetlips of other species are known to mimic poisonous flatworms or nudibranchs with an undulating swimming motion to deter predators, though this specific behavior for *P. lineatus* juveniles is not explicitly detailed.

Feeding Habits

The Diagonal-banded Sweetlips is a nocturnal carnivore, actively foraging for food at night. They primarily feed on benthic invertebrates, including crustaceans, mollusks, worms, and sometimes small fish. Their hunting grounds are typically open sandy flats and seagrass beds, where they search through the substrate for prey.

Social Structure

Adult Diagonal-banded Sweetlips can be observed either solitarily or in groups, often forming small schools or aggregations during their daytime resting periods. These aggregations provide a sense of security while they are inactive. Juveniles, in contrast, are solitary and inhabit more sheltered, shallower reef areas.

Notable Seasonal Patterns

Reproduction in Diagonal-banded Sweetlips involves spawning aggregations. These events have been observed off the coast of Palau, typically around the new moon. This oviparous species forms distinct pairs to spawn, releasing their reproductive fluids in open water. While specific seasonal peaks for these spawning events across its entire range are not extensively documented for divers, the new moon phase is a key indicator for potential observation of breeding behavior in areas where they aggregate. They are found in tropical marine waters, generally encountered at 1–35 m depth, in waters of approximately 23–27°C.