Identifying Juvenile Scolopsis bilineata and Scolopsis lineata

Introduction

Identifying juvenile Scolopsis bilineata and Scolopsis lineata can be tricky. While these species show distinct differences in color and pattern as adults, their juvenile stages are very similar and highly variable, making identification difficult.

Currently, there is a lack of resources that offer clear features for distinguishing juveniles of these two species. This guide aims to provide a non-official resource for differentiating these species in their juvenile stages. Note that this guide does not cover the color variations of S. bilineata found in the Fiji and Indian Ocean. The "phases" used here have not been scientifically determined and only aim to show how the pattern and coloration develop during the fish's growth. Click on the links to see example observations for each phase.

Scolopsis bilineata

Phase 1:

• Dorsal Fin Black Spot:Usually large, extending from dorsal fin spine 1 to spine 5 or less.
• Dorsal Fin Color: Mostly white and yellow.
• Markings Below the Eye: No mark or a faint dark mark.
• Lower Yellow Line: Top and bottom margins parallel or diverging, at least until it crosses the lateral line.

https://www.inaturalist.org/observations/148501977 (@wayneosborn)

Phase 2:

• Dorsal Fin Black Spot:Usually large, extending from dorsal fin spine 1 to spine 5 or less.
• Dorsal Fin Color: Mostly white and yellow.
• Markings Below the Eye: Dark mark below the eye becomes more prominent.
• Lower Yellow Line: Top and bottom margins parallel or diverging, at least until it crosses the lateral line.

https://www.inaturalist.org/observations/215705998 (@ellapennington)

Phase 3:

• Dorsal Fin Black Spot: Usually large, extending from dorsal fin spine 1 to spine 5 or less.
• Dorsal Fin Color: Mostly white and yellow.
• Markings Below the Eye: Dark line running from below the eye to the pectoral fin.
• Lower Yellow Line: Top and bottom margins parallel, at least until it crosses the lateral line.

https://www.inaturalist.org/observations/87643233 (@pl_stenger)

Phase 4:

• Dorsal Fin Black Spot:Usually medium, extending from dorsal fin spine 1 to spine 4 or less.
• Dorsal Fin Color: Mostly white and yellow.
• Markings Below the Eye: Dark line running from below the eye to the pectoral fin.
• Lower Yellow Line: Top and bottom margins parallel or slightly converging.

https://www.inaturalist.org/observations/64029471 (@roxylaz)

Phase 5:

• Dorsal Fin Black Spot:Usually small, extending from dorsal fin spine 1 to spine 4 or less.
• Dorsal Fin Color: Mostly white and yellow.
• Markings Below the Eye: Dark line runs from below the eye, to the pectoral fin, then gradually curves upward.
• Lower Yellow Line: Top and bottom margins parallel or converging.
• White Spot: A white spot starts developing posteriorly.

https://www.inaturalist.org/observations/142144018 (@jeanpaulboerekamps)

Scolopsis lineata

Phase 1:

• Dorsal Fin Black Spot: Medium, extending from dorsal fin spine 1 to spine 4 or less.
• Dorsal Fin Color: Mostly translucent and white.
• Markings Below the Eye: May or may not have a faint dark mark.
• Lower Yellow Line: Top and bottom margins parallel or converging.

https://www.inaturalist.org/observations/218049596 (@davisdamaledo)

Phase 2:

• Dorsal Fin Black Spot: Medium, extending from dorsal fin spine 1 to spine 4 or less.
• Dorsal Fin Color: Mostly translucent and white.
• Markings Below the Eye: May or may not have a faint dark mark.
• Lower Yellow Line: Top and bottom margins converging.

https://www.inaturalist.org/observations/147658198 (@uwkwaj)

Phase 3:

• Dorsal Fin Black Spot: Medium, extending from dorsal fin spine 1 to spine 4 or less.
• Dorsal Fin Color: Mostly translucent and white.
• Markings Below the Eye: No line below the eye.
• Lower Yellow Line: Top and bottom margins converging.

https://www.inaturalist.org/observations/6889051 (@markusgmeiner)

Phase 4:

• Dorsal Fin Black Spot: Small, extending from dorsal fin spine 1 to spine 3 or less.
• Dorsal Fin Color: Mostly translucent and white.
• Markings Below the Eye: No line below the eye.
• Lower Yellow Line: Top and bottom margins converging until closing the yellow line near the lateral line.

https://www.inaturalist.org/observations/147594813 (@johnsear)

Phase 5:

• Dorsal Fin Black Spot: Small, extending from dorsal fin spine 1 to spine 3 or less.
• Dorsal Fin Color: Mostly translucent and white.
• Markings Below the Eye: No line below the eye.
• Lower Yellow Line: Top and bottom margins converging until closing the yellow line before the lateral line.

https://www.inaturalist.org/observations/204187704 (@raphetnature)

Summary of Key Differences

In some cases, juvenile S. lineata and S. bilineata may exhibit features that overlap. Careful observation of all features is essential for accurate identification.

• Dorsal Fin Black Spot: S. bilineata has a larger black spot extending up to spine 5, while S. lineata has a smaller spot extending up to spine 4. If the black spot reach spine 5, S. lineata is ruled out.
• Dorsal Fin Color: S. bilineata has a mostly white and yellow spiny dorsal fin, while S. lineata has an almost transparent and white spiny dorsal fin.
• Markings Below the Eye: S. bilineata develops a black line running from below the eye to the pectoral fin, while S. lineata does not. However, both species can exhibit a faint dark mark below the eye.
• Lower Yellow Line: The lower yellow line in S. bilineata usually has parallel or slighly diverging margins, while in S. lineata the margins converge.
• White Spot: In a later phase S. bilineata develops a white, horizontaly shaped spot posteriorly, while S. lineata does not. Note that adult S. lineata develops verticaly shaped pale bands at various location on its body.

References

• Russell, B.C. (1990) - FAO species catalogue. Vol. 12.Nemipterid Fishes of the World. (Threadfin breams, Whiptail breams, Monocle breams, Dwarf monocle breams, and Coral breams). Family Nemipteridae. An Annotated and Illustrated Catalogue of Nemipterid Species known to Date.
• Boaden, A. E., & Kingsford, M. J. (2013) - Distributions and habitat associations of the bridled monocle bream Scolopsis bilineatus (Nemipteridae): a demographic approach.
• iNaturalist observations of Scolopsis bilineata and Scolopsis lineata.