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Facts about the Convict Blenny/Goby Fish

[Phylum: Chordata] [Class: Actinopterygii] [Order: Cichliformes] [Family: Pholidichthyidae]

Its common names are convict blenny and engineer goby. Yet, Pholidichthys leucotaenia isn't classified with any of the true blennies (Blennioidei) or gobies (Gobiidae).

So what exactly is the marine fish species that belongs to the family Pholidichthyidae, where are they found, what do they eat, and how big do they get?

Geographical Range of the Convict Blenny

The convict fish (scientific name Pholidichthys leucotaenia) is found in neritic zones of the Indo-Pacific region (west-central Pacific Ocean).

But, some of the best places for divers to spot them underwater are:

No matter whether you call them convict blennies or engineer gobies, Pholidichthys leucotaenia is a fish species that inhabits a particular kind of marine environment.

They tend to occupy areas around shallow coral reef structures and rocky coastal zones, living between an upper depth limit of three (3) metres and down to depths approaching thirty (30) metres (98 feet).

Here's the thing:

As juveniles, these small schooling vertebrates usually hover together in tight gatherings and look like a big shoal of orgiastic striped eel catfish fry (Plotosus lineatus).

This schooling behaviour is a strategy used to capture plankton and to try and confuse their natural predators in the wild, especially moray eels, lionfish, and groupers.

Whereas, the adults prefer to spend their lives excavating burrows and tunnels in underwater debris, especially in sand and rubble beds that are composed of broken corals and pebbles.

Convict Blenny Appearance and Size

The main characteristics of convict blennies are their eel-like body form and blunt head. It's a similar shape to the snake eel species of the family Ophichthidae.

However, the juveniles tend to be more slender and their general body colouration is bright and black. They also have a single white linear streak that runs all the way from the snout to the tail.

Moving on...

As the schooling habit wears off, adult convict fish become more robust and secretive. Like their closest relatives, the gobies fish, they become 'engineers' that spend most of the day maintaining and modifying their crevices and burrows.

At full maturity, Pholidichthys leucotaenia can grow up to fifty (50) centimetres long (20 inches). This is when the body colours become more subtle, often with broken vertical stripes of yellow or white barring patterns.

What Do Convict Blennies Eat?

There are about nine hundred different types of blennies and the vast majority of them are carnivorous fishes. The convict blenny follows this trend and they get a lot of nourishment from small invertebrates and zooplankton.

Convict Fish (Pholidichthys leucotaenia): Is it a Blenny or Goby?Even so, in the wild convict blennies are also known to eat:

Fun Fact: Despite requiring specific care, the engineer goby is reef safe and one of the popular fish species kept by hobbyists in the marine aquarium trade.

Pholidichthys leucotaenia Social Behaviors

Compared with many other marine vertebrates, Pholidichthys leucotaenia remains overlooked and understudied by many zoologists and wildlife biologists.

Even so, the social behaviour of the convict blenny/goby fish is one that particularly interests many of its admirers - including our team at Private Scuba in Thailand.

The cooperative and somewhat territorial characteristics reflect a complex social structure for a fish of this stature and relatively small size.

Burrowing Lifestyle with Group Dynamics

It's common to find convict blennies living in small colonies where a few individuals will share the same network of burrows. These dugouts and tunnels create communal living spaces and sheltered retreats for the group.

Group members use their mouths to excavate sandy debris and reinforce the den. Not only does this kind of group collaboration strengthen the cohesion of its members, it also helps to ensure a safe refuge and concealed place to hide from any perceived danger.

Juvenile and Adult Interactions

It is fair to say that juveniles and adults choose to coexist inside the same tunnel system. But, only the juveniles school together in open water near the entrance of the burrow during the hours of daylight.

In contrast, the adults are a lot less bold and conspicuous. Instead, they tend to stay inside the burrow more often and rarely wander far away even if they venture into open water.

Simply put:

This perceived division of roles suggests that the adults focus on burrow maintenance and the juveniles are active food foragers.

Moreover, some experts suggest this advanced level of social organisation may be a unique example of underwater eusociality.

How Do Convict Blennies Reproduce?

Accurate information about the mating rituals of convict gobies is limited. Nonetheless, marine biologists believe they form small monogamous groups within the colony.

When a dominant male pairs off with several females for spawning, it's most likely she will deposit the eggs inside the underwater chamber until they hatch.

Key points:

The environmental triggers for spawning events and for convict blenny reproduction are most likely a rise in water temperature and lunar cycles.

Like numerous burrow-dwelling species, the females lay adhesive eggs that stick to the inner walls of the cave. It's thought that the male takes on the primary role of guarding the eggs.

The male uses his fins to fan the eggs. This helps to provide oxygenation and remove unwanted debris - a process known to occur with damselfishes (Pomacentridae).

Threats and Predators

Because Pholidichthys leucotaenia (convict blenny or engineer goby) create habitats near coral reef ecosystems, a range of natural predators target the species, especially the lively schools of juveniles.

Some of the typical ambush predators, stealth hunters, and opportunistic killers that eat convict blennies include:

Interesting Fact: The IUCN Red List of Threatened Species assessed the population status of the convict blenny (Pholidichthys leucotaenia) in 2023 and cites them as being of "Least Concern" (LC).

Related Information and Help Guides

Note: The short video [9:25 minutes] presented by 'Pacificklaus' contains some amazing footage of the strange and unusual convict fish, recorded during a scuba dive at Negros Island in the Philippines.

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