Alepocephaliformes
Alepocephaliformes is an order of deep-sea teleost fishes, commonly known as **slickheads**. They are found worldwide in all oceans, typically at depths between. The group contains two families, approximately 39 genera, and roughly 140 species.
Historically, these fishes were classified within the Clupeiformes and later as the suborder **Alepocephaloidei** within the Argentiniformes. However, modern molecular phylogenetic studies have elevated them to their own order within the cohort Otocephala. They are now considered the sister group to the Ostariophysi.
Etymology
The scientific name is derived from Ancient Greek: a-, lepos, and kephale, referring to the characteristic scaleless heads found in this group.Description
Unlike many other deep-sea fishes that possess highly modified body shapes, Alepocephaliformes generally retain a standard fusiform fish shape. They are mostly 20 to 50 cm in length, though some species are smaller. The largest known member is Narcetes shonanmaruae, which can exceed in length and weigh over, making it the largest known deep-sea-endemic bony fish.The body is typically dark in color, ranging from dark brown to black, although some species are pale. As an adaptation to their deep-sea environment, the skeleton is poorly ossified, and the flesh is often described as jelly-like.
Diagnostic morphological traits defined by Norman Bertram Marshall and subsequent authors include:
- **Absence of a swim bladder**.
- Absence of an adipose fin.
- The dorsal fin is positioned far back on the body, usually originating behind the midpoint of the standard length, generally opposite the anal fin.
- The head is large, with a wide mouth and large eyes, and lacks scales.
- The pectoral fins are small and set relatively low on the body.
- There are typically 7 to 9 branchiostegal rays.
- The parietal bones are separated by the supraoccipital.
- The posttemporal bone is not forked.
- Members of the family Platytroctidae possess bioluminescent organs, whereas Alepocephalidae do not.
Phylogeny and taxonomy
The internal systematics of the order generally recognize two families: the typical slickheads and the tubeshoulders. Genetic studies indicate that Platytroctidae is the basal sister group to the rest of the order. The genera Bathylaco and Leptochilichthys, which were historically placed in their own families, are now nested within Alepocephalidae.The following cladogram illustrates the internal relationships of the order based on mitochondrial genome sequences:
External relationships
The placement of Alepocephaliformes has fluctuated significantly. Originally grouped with herrings, they were long classified as a suborder of the Argentiniformes within the Protacanthopterygii. This grouping was based on morphological similarities in the crumena organ. However, molecular evidence has consistently shown that they are not closely related to argentiniforms.Instead, they are now placed within the cohort Otocephala. Recent large-scale phylogenetic analyses position Alepocephaliformes as the sister group to the Ostariophysi, or basal to the combined clade of Clupeiformes and Ostariophysi.