Taxonomy & naming
Pseudotropheus sp. "Crabro Blue" is not a formally described species. It has no scientific name of its own, no describing author, no year of publication, and no entry in Eschmeyer's Catalog of Fishes, because no taxonomist has published a description establishing it. It is an informal taxon recognised in the field and aquarium literature, principally through Ad Konings's documentation of the far-northern Tanzanian shore. Its trade name references the formally described species Pseudotropheus crabro Ribbink & Lewis, 1982 — the striking black-and-yellow-banded "bumblebee" mbuna, whose type locality lies further south on the Malawian shore — with "Blue" marking this population's differently coloured, blue-toned appearance. This population should be understood as resembling or allied to P. crabro rather than as that species itself; the relationship has not been established by formal study. Pseudotropheus in its modern, narrow sense (following the splitting-off of Maylandia/Metriaclima, Tropheops, Cynotilapia, and other former Pseudotropheus species into separate genera) remains a genus of robust-bodied rock-dwelling mbuna. Until a revision addresses this population directly it should be treated strictly as a Konings-recognised undescribed form, and no authority or date should be attached to it.
Morphology
Pseudotropheus in its modern sense comprises robust-bodied, moderately elongate mbuna with a blunt head and comb-like unicuspid teeth for algae grazing — a similar general build to the described P. crabro, which reaches roughly 3.5–4 in. This population is expected to fall in a similar size range, though no formal measurement exists for it specifically. Consistent with its trade name, breeding males are understood to show blue body colouration, contrasting with the black-and-yellow banded pattern of nominate P. crabro; the extent to which any banding persists in this population, if at all, is not documented in accessible references, so no specific claim is made about it beyond the blue ground colour implied by the name. Females and subordinate males are expected to be plainer, in the typical mbuna pattern of duller, more cryptic non-dominant colouration.
Habitat
This form is recorded from Chitande Island and Nkanda, both on the Tanzanian shore in the far northern part of Lake Malawi — a stretch of coast at the northern limit of the lake's rocky mbuna habitat. As with other Pseudotropheus, it is expected to be strictly rock-associated, grazing algal films from rock surfaces in the shallow-to-moderate depths typical of mbuna habitat. The water column here follows the lake-wide profile: warm, alkaline, and well-buffered, broadly mid-20s Celsius with pH on the alkaline side of neutral. As an undescribed, locality-based form its precise depth range and full distribution beyond the Chitande–Nkanda stretch of coast are not established.
Feeding
Pseudotropheus as a genus are specialised algae grazers, using comb-like unicuspid teeth to scrape algal films and associated small invertebrates ("Aufwuchs") from rock surfaces in well-lit water, the same feeding mode documented for the described P. crabro. No feeding study exists for this population specifically, but this genus-typical herbivore/detritivore mode is the reasonable characterisation absent contrary data.
Mating
Like other Lake Malawi mbuna, Pseudotropheus sp. "Crabro Blue" is presumed polygynous, with males holding and defending small territories on the rock face, courting passing females from these sites while driving off rival males. There is no pair bond. Females move between male territories, choose largely on colour and site quality, and spawn briefly on the rock before departing with the fertilised clutch in the mouth. No behavioural study specific to this undescribed population exists; the account follows the general pattern documented for Pseudotropheus and rock-dwelling mbuna more broadly.
Breeding
Pseudotropheus sp. "Crabro Blue" is a maternal mouthbrooder, as are all Lake Malawi mbuna. After spawning on the rock face, the female carries the fertilised eggs in her mouth for roughly three weeks, feeding little during this period, before releasing free-swimming fry with no further care from either parent. Clutch size has not been documented for this form specifically; by analogy with the similarly sized P. crabro, a modest clutch — likely a few dozen eggs — is a reasonable expectation rather than a measured fact.
In the aquarium
Pseudotropheus sp. "Crabro Blue" is an uncommon aquarium fish, traded mainly through specialist importers working the far-northern Tanzanian shore. Where it is kept, husbandry follows the well-established mbuna template.
Dense, stable rockwork built directly on the tank floor is essential, since this is a strictly rock-associated grazer needing abundant surface area and shelter. A footprint of at least 47 in suits an adult group. Water chemistry should mirror Lake Malawi's profile: temperature in the mid-20s Celsius, pH roughly 7.7–8.6, and well-buffered water, with prolonged soft or acidic conditions avoided. Diet should emphasise vegetable matter — a good spirulina-based flake or pellet as the staple, with algae wafers and blanched vegetables as supplements — reflecting its herbivore/grazer biology; protein-heavy foods should be limited, as herbivorous mbuna are prone to digestive problems on rich diets. As with other mbuna, males are territorial and often aggressive, so a single male with multiple females and adequate rockwork to break sightlines works better than a pair in a small tank. Because it references but is not the same taxon as P. crabro, keepers should not cross it with true crabro stock.
Conservation
Pseudotropheus sp. "Crabro Blue" has not been evaluated by the IUCN, and as an undescribed form without a formal scientific name it has no Red List assessment of any kind. That silence is a statement about taxonomy, not about safety. As a rock-restricted population at the northern limit of the lake's Tanzanian shore, it faces the general pressures typical of that comparatively less-surveyed coast: ornamental collection and the broader basin-wide stresses of sedimentation and fishing pressure. No conservation status should be fabricated for it. The honest statement is that it is unassessed and that its true range and status remain to be established by formal study.