Tuesday, June 4, 2013

Brachiolaria larva of the seastar Mediaster aequalis

Mediaster aequalis is a bright red asteroid, three to seven inches in diameter, found subtidally off the coast of Oregon. M. aequalis has a breeding season from late March through May and spawns large (~1.2mm) yolky oocytes that are opaque and bright orange (Birkeland et al. 1971). In order to get the oocytes and sperm out we biopsied adult M. aequalis. Pieces of dissected ovary were placed into a solution of 1-methyl adenine (1-MA) for an hour to induce oocyte maturation. Oocytes were then fertilized with a dilute suspension of sperm. Several of the fertilized eggs developed.

I took some pictures of 35-day old brachiolaria larvae. As in other species of seastars with large yolky eggs, Mediaster lacks a feeding bipinnaria larva, and develops via a non-feeding pelagic brachiolaria. As you can see here these yolky non-feeding brachiolariae have three brachiolar arms (stubby appendages at upper right). These are used for attachment to substratum during metamorphosis. The juvenile seastar develops at the opposite (morphologically posterior) end. 

The many small bumps (clearly visible in the bottom picture) cover the juvenile rudiment and develop into adult spines. The larger bumps of the juvenile rudiment are the developing rays of the seastar. Once the juvenile rudiment and the brachiolar arms are formed (which may be as early as 9-10 days) the larva is capable of metamorphosing. M. aequalis will only settle and metamorphose on a suitable substratum, such as a tube of the annelid Phyllochaetopterus. If no suitable substratum is available, larvae can delay metamorphosis for as long as 14 months (Birkeland et al. 1971). 

Birkeland, C., Chia, F-S., Strathmann, R.R. 1971. Development, substratum selection, delay of metamorphosis and growth in the seastar, Mediaster aequalis stimpson. Biological Bulletin. 141:1, 99-108.

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